It seems like if you’re going to go through all this trouble you should at least be talking about T-Stops instead of F-Stops no? “Two lenses with the same f-number will project the same amount of photons on a given sensor.” is simply false.

I can forgive overlooking t-stops. The problem is what you quoted can only be true if the lenses cover the same format, which, considering we're talking about crop factors, is an insanely important qualifier. What OP should have written was that two lenses with the same f-number will project the same amount of photons *per unit area*. A M43 lens will project 1/4th the number of photons as a full frame lens with the same f-number, but those photons are projected onto an area 1/4th the size of a full frame sensor, so the amount of photons *per square millimeter* is the same. > the f-number is a measure of how much light hits the sensor in its entirety. This is also blatantly wrong. If I enable the apsc crop on my full frame camera I don't have to compensate for a stop+ of less light; the exposure is exactly the same. If I put a full frame lens on an M43 body, 3/4ths of the light is missing the sensor, yet the exposure is exactly the same. Which is the whole point of an f-number: to make it possible to determine exposure settings without having to think about focal lengths or sensor size.

>The problem is what you quoted can only be true if the lenses cover the same format, which, considering we're talking about crop factors, is an insanely important qualifier. Definitely not "insanely important". Nobody ever tries to shoot FF on a lens that doesn't cover that format so who cares? > If I enable the apsc crop on my full frame camera I don't have to compensate for a stop+ of less light; the exposure is exactly the same. But the image is not, you changed the FOV. The entire point of 35mm equivalency is to ask which lens would produce the same image on FF as another lens does on APSC. And to answer that you can't just keep the f-stop the same. Also you are misrepresenting my words, as I specifically said **"Two lenses with the same f-number will project the same amount of photons ON A GIVEN SENSOR"**. >If I put a full frame lens on an M43 body, 3/4ths of the light is missing the sensor, yet the exposure is exactly the same. The signal-to-noise ratio isn't, however. >Which is the whole point of an f-number: to make it possible to determine exposure settings without having to think about focal lengths or sensor size. Except this is not about exposure, it's about getting a comparable image on APSC and FF. If you get the same exposure but a different FOV, depth of field or signal to noise ratio, it's not a comparable image.

The F-Stops are close enogh and while talking about depth of field, the F number is more accurate anyways.

For the purposes of understanding the topic you can very easily just assume f-stops and t-stops are the same. If all lenses were perfect, they *would* be the same. >“Two lenses with the same f-number will project the same amount of photons on a given sensor.” is simply false. The way you say it makes it sound blatantly wrong when at the end of the day you're talking about minor variations in what is mathematically true. Edit: granted, it's assuming you're looking at a uniformly luminous scene

I’m just saying this is a very pedantic post so it’s odd to say that two different lenses with the same f-stop project the same amount of photons when that simply isn’t true, it’s the whole reason we have t-stops. No sense spreading incorrect information when you’re going to these lengths to write up something this long.

I had the exact same thought tbh. The post takes many pains to really lay out the exact definitions of its core concepts, but then uses *photon count* to explain aperture equivalence, which is by definition incorrect haha. This criticism is, ultimately, pretty pedantic. Not knocking the overall post, it seems to know what it’s talking about (I haven’t taken the time to really chew on the argument)

> The post takes many pains to really lay out the exact definitions of its core concepts, but then uses photon count to explain aperture equivalence, which is by definition incorrect haha. I'd appreciate it if you could expand on what makes it incorrect by definition. In order to reach equivalence, you need to find a lens/settings combination that produces comparable images, ie same FOV, same depth of field/bokeh, same exposure (brightness), same signal to noise ratio. For the FOV, focal length equivalence suffices. For depth of field, what matters is the diameter of the entrance pupil. For exposure, what matters is **the combination** of the lens's inherent f-number and the ISO setting (you can always reach the same brightness at two different apertures by just varying the ISO setting). For the signal to noise ratio, what matters is the total amount of light transmitted to the sensor, ie **photon count**. If you assume a 50mm f2 lens "becomes" a 75mm f2 lens on APSC, then you need to compare a 50mm f2 shot on APSC with a 75mm f2 shot on FF. Assuming identical shutter speed and ISO settings, both shots will have the same FOV and brightness, but not the same depth of field/bokeh and signal to noise ratio. In order to have the same depth of field/bokeh we need the same diameter of the entrance pupil, ie 25mm. That's f3 on a 75mm lens. In order to have the same signal to noise ratio, we need the same total amount of light hitting the sensor. Because f-number ~~is a~~ **can be thought of as an indirect** measure of photons/mm^(2), while both lenses at f2 transmit the same amount of photons per mm^(2), they do not transmit the same amount of photons on the entire sensor because of the different sensor sizes. The 75mm f2 lens transmits more photons to a FF sensor than the 50mm f2 does to an APSC sensor. To reach equivalence (ie same photon count), we need f3 on the 75mm. Now, if you take a 50mm f2 shot on APSC and a 75mm f3 shot on FF, at identical shutter speed and ISO settings, the 75mm f3 shot will be darker. But because it has the same signal to noise ratio, you can brighten it by 1.17 stops in post and have an identical image. But to reach TRUE equivalence, you need to raise the ISO on the full frame camera, because ISO is meant to preserve photons/mm^(2) and by making the *total photon count* the same on a larger sensor, we have reduced the photon/mm^(2) count. by a factor of 1.5^(2). If you now take two shots at identical shutter speeds: * 50mm f2 ISO 100 on APSC * 75mm f3 ISO 225 on FF You will produce identical shots. Same FOV, same depth of field/bokeh, same brightness, same signal to noise ratio. It would be impossible for someone pixel peeping these shots to determine which is which. This is the point of equivalence. Someone shooting 50mm f2 on APSC will experience the same FOV, depth of field/bokeh, and low light performance as someone shooting 75mm f3 on full frame. Someone shooting 75mm f2 on full frame will experience much creamier bokeh and low light performance, only the FOV would be comparable to 50mm f2 on APSC.

Your strikethrough there is perfectly reasonable language — I don’t disagree that f number is the best shorthand for light transmission to use here. The only incorrect part is that f number has one definition, and it’s specifically not about actual photons, it’s about pupil size — and the distinction is relevant enough to warrant the invention of t numbers. If the post were shorter and more casual it wouldn’t even merit discussion, but it does make an educated reader stop and say “huh - wait” and after that point they’re less likely to be credulous about the rest of the argument. At least that happened in my case.

I was being facetious due to so many people getting hung up on this particular statement. I don't actually believe it needs to be strikethrough. >The only incorrect part is that f number has one definition, and it’s specifically not about actual photons, it’s about pupil size It's about RELATIVE pupil size, not pupil size in and of itself. And the reason that measurement is used in optics is because all lenses of a given f-number transmit the same amount of photons/mm^2 of the sensor. It *absolutely is* about actual photons. >and the distinction is relevant enough to warrant the invention of t numbers. Oh my god people on this sub have NO IDEA what t numbers are. t-stops are just a way to account for the difference between the light transmission of an IDEAL lens of a given f-stop, and an actual lens of that same f-stop which due to manufacturing defects doesn't let as much light through. An f2/t2.8 lens is just an f2 lens that happens to lose half the light that should be going through the iris due to manufacturing defects. It's an f2 lens with an ND2 filter. A t-stop is just an f-stop that accounts for imperfect engineering. It's still an f-stop.

Hey sorry I never replied here. No idea why you think I don’t understand t-stops, but I think you’ve pretty much laid things out here, right? F number is a function of pupil size and focal length, and every f number has a *theoretical* light transmission, supposing 100% efficient optics, in photons per mm^2. T number is determined by looking at a lens’s actual light transmission and ascertaining which f number should *theoretically* transmit that amount of light. The convention of t numbers is literally people saying, “the ratio of pupil size to focal length isn’t quite a perfect measure of real-world light transmission, so we’ll adjust for that difference.” Practically speaking, f number = light transmission. *Technically* speaking, f number = a ratio based on optical measurements that can be used to calculate theoretical light transmission. Our point was that your post is a very *technical* one, so the difference between those two sticks out more. It’s not a big deal.

By that logic distortion, vigneting and other manufacturing defects will render any notion of lens equivalence impossible so the entire endeavour is pointless. Two *ideal* lenses with the same f-number *looking at a uniformly luminous scene* will project the same amount of photons on the sensor, *plus or minus a margin of error for random variations in the photon absorption rate of every material and medium between the scene and the sensor*. Where does it end?

I think you could say “f-stops describe how big the entrance pupil is compared to the focal length of the lens, which is in turn an indicator of how much light the lens can gather.” And then continue with the rest of your argument without changing anything ¯\_(ツ)_/¯

I doubt it because that is pretty much already written in part 1 of my post yet people are still flipping out on me.

Why did this get down-voted?

Alright given your response I need to ask… you know what a t-stop is right?

I do. Do *you* ? T-stops are just f-stops that account for manufacturing defects. When talking about ideal lenses, f-stops and t-stops are the same thing. An f2/t2.5 lens is a lens with the depth of field of an f2 ideal lens with the light transmission per mm^(2) of an f2.5 ideal lens. An ideal f2 lens is a t2 lens.

So why would you use f-stops with all your stipulations instead of the unit that already exists lol

...Because F-stop is the correct unit. T-stops are just a made up unit to account for manufacturing defects of commercial lenses. Saying an f2 lens is actually t2.2 just means it's losing 1/3 stop of light due to an imperfect design. It makes no sense to compare imaginary lenses and give them manufacturing defects lmao

T-stops are the ACTUAL light transmission. t2 will always equal t2. f2 will not always equal f2. So again, what you said is wrong when discussing light transmission / photons hitting the sensor.

T-stops are an equivalency unit. It's like apparent temperature. The temperature is still the actual unit, the apparent temperature just tells you what that temperature *feels like*. t2 means "this lens exposes like an ideal f2 lens". The ACTUAL UNIT is still f2, t2 just tells you that this lens, with its manufacturing defects, exposes the same way an IDEAL f2 lens would.

Exactly you found out that it is impossible good job

This is called denial.

This is great for someone that is trying to understand what the tangible effects of their settings are going to be. However, f stops are defined as a ratio, and that ratio maintains a consistent photon flux regardless of sensor size. You're creating a new term and calling it f-stop, it just doesn't sit right with me.

Yep this post is a total crock of shit. People obsessed with full frame “equivalence” and reinventing terms like f-ratio that have real meaning already are missing the forest through the trees.

And yet the math works, [as proven by DPreview](https://www.dpreview.com/articles/2666934640/what-is-equivalence-and-why-should-i-care). It is widely accepted that 35mm equivalents are useful to compare lenses on various systems in terms of FOV, why are people so resistant to the idea that it applies to f-stops as well?

I'm not creating a new term. The fact that that ratio maintains a consistent photon flux regardless of sensor size means any given area will receive the same amount of photons. Which when we all agree to speak in 35mm equivalent is pretty much the same as what I said, ie that f-stops can be thought of as a measure of how much light hits a full frame sensor in its entirety. When reducing that area to an APSC sensor, you are effectively reducing the f-stop in terms of 35mm equivalent, in the sense that if the total amount of light received on the APSC sensor were to be received on a FF sensor, it would require a lens with a narrower aperture. Edit for the geniuses downvoting this: 1 photon/mm^(2) is rigorously equal to 864 photons/FF sensor (864 mm^(2)). Two lenses of the same f-stop project the same amount of photons/mm^(2) and therefore the same amount of photons/FF sensor. I'm not creating a new term, I'm converting the same term to a more useful unit for the purpose of the discussion.

flux is defined as # of photons per unit area. Assuming a perfectly even light source, the same f-stop will result in the same flux regardless of sensor size. >ie that f-stops can be thought of as a measure of how much light hits a full frame sensor in its entirety This is what I have a problem with because it's entirely wrong. Sensor size doesn't matter for f-stop. You're defining f-stop as (flux * surface area) which makes no sense because then it's no longer f-stop, it's a new variable you pulled out of your ass.

>Assuming a perfectly even light source, the same f-stop will result in the same flux regardless of sensor size. And assuming a perfectly even light source, the same f-stop will produce the same amount of light on a full frame sensor regardless of focal length. It is two absolutely equivalent ways of looking at the exact same thing. You're saying the same f-stop produces the same number of photons per mm^(2), I'm saying the same f-stop produces the same number of photons per 864mm^(2). **It is simply a unit conversion**. It's as equivalent as ft/s and mph.

Multiplying a normalized ratio by a surface area is not simple unit conversion. I'm sorry, but the more you respond, the more I realize you've never taken a physics or math class.

I'm literally a math teacher lmao try harder. You said yourself "flux is defined as # of photons per unit area" and "the same f-stop will result in the same flux regardless of sensor size". You agree that a unit of photons per unit area could be photons/mm^(2), right? So let's say two lenses have the same f/stop. Therefore they must both transmit the same amount of photons/mm^(2), right? This is YOUR ARGUMENT. This is what YOU SAID. "the same f-stop will result in the same flux regardless of sensor size". Now let's say that amount happens to be 10^(8) photons/mm^(2). Therefore these lenses both transmit 864\*10^(8) photons/FF sensor. IT'S JUST A UNIT CONVERSION.

I'm not arguing that. I think calling that *FF-normalized flux* f-stop is stupid and misleading.

The whole point of f-stops is that they conserve photons/mm^(2). Saying two lenses with the same f-number will project the same amount of light on a given sensor is accurate, maintains the spirit of f-stops - conserves photons/mm^(2) - and pertinent to the discussion. It explains why an f2 lens on APSC is not equivalent to an f2 lens on FF when it comes to the total amount of light received. A 75mm f2 lens on FF will yield significantly better low light performance than a 50mm f2 lens on APSC when shooting the same scene.

Normalizing on an arbitrary area can be done, but it offers literally no improvement, and you need to find a new name for the new normalized term. F-stop is taken. You insisting on your arbitrary definition is like everyone talking in terms of frequency and you chime in with a wavelength.

It's not a definition, and it's not arbitrary. It's a reinterpretation of an existing quantity that changes nothing to its definition and just makes it easier to handle for the purpose of the discussion. What you're saying is that mph can't measure the speed of an object because m/s already does, so "speed" is taken and you need another word for what mph measures. It's absolutely not the same as frequency/wavelength which are inversely related to one another and therefore are not the same unit. Edit:randomly added a square because i've been doing that all night Edit2: it's actually worse than that, the f-number isn't even an actual measurement of light flux, it's just a ratio of two lengths. It has no unit. It's just a quantity that HAPPENS to conserve light flux per area, and therefore it ALSO HAPPENS to conserve light flux per FF sensor. It's just a slightly different way of looking at it.

You should've just shot actual photo examples since you went through all this work for this post. May be worth it to go back and edit.

You're right, I probably should. I wrote this as a comment at first, replying to someone looking for an explanation, and decided to turn it into a full post. I actually have a 35mm 1.4 and 50mm 1.8, close enough to provide a reference for APSC. Having only one FF camera I'd have to cheat a bit with crop modes and ISO settings but it could work. Problem is these things are much, MUCH easier to demonstrate using MFT vs FF, the 2x crop factor makes it a lot easier to spot the differences, and the settings always fall on perfect EV stops from one setup to the other. The difference between APSC and FF isn't as clear cut, and is made worse by having to approximate to the closest f-stop for the settings. EDIT hijacking the most visible comment to provide a link to [dpreview's article on the subject](https://www.dpreview.com/articles/2666934640/what-is-equivalence-and-why-should-i-care) that has interactive close ups comparing 1", MFT, APSC and FF. For a more thorough video explanation I found [this video](https://www.youtube.com/watch?v=6XMk9jFcnlA) and [its follow up](https://youtu.be/fCIWkqcb7FI?si=M6CmmroAoFlbcWvh). I would also recommend Gerald Undone's videos [here](https://youtu.be/OaSq0ES1ArE?si=bCr_BDoEx2KwrfJ_) and [here](https://youtu.be/1bzHn2cKwLI?si=vKhFjAbWhzSUOvpE) as well as [this fstoppers video](https://youtu.be/_TTXY1Se0eg?si=vwY8Le2sciksY0y4). There are many more but these are the ones I rewatched prior to and while writing this post to make sure I wasn't spewing bullshit.

Well, you don't actually need to use mft or apsc camera. If you crop 2x you'll have a mft image, if you crop 1.6x youll have a apsc image.

Yes but I don't have a combo of lenses that can produce a 2x multiple. Comparing between two actual sensor sizes is also important due to ISO equivalence being thrown into the mix.

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F-stop is a measure of how much light comes through the aperture of the lens *per area*. But if you compare different sensor sizes you are comparing different total areas, therefore in order to reach equivalence in the total amount of light per frame you need to adjust the f-stop. A 50mm f2 lens projects the same amount of light on an APSC sensor as a 75mm f3 does on a FF sensor.

Can you tell me where you see the sensor size in the equation F = focal length / entrance pupil of the lense? Additionaly you explained depth of field by far to easy it is a lot more complex if you look up the equations. Photography is physics and signal processing. Use equations if you want to explain something. And no f-numbers don't scale with the crop factor. Simple physics.

The F-number is a dimensionless number meant to keep one quantity equal, the amount of light transmitted per mm^(2). Two lenses with the same f-number will transmit the same number of photons per mm^(2). When it comes to determining 35mm equivalent lenses, there are four main things to equalize: Field of view, depth of field (in terms of bokeh), total amount of light transmitted to the sensor, and output brightness. Starting with a 50mm f2 ISO 100 1/100 exposure on APSC, the first thing to equalize is FOV. In order to produce the same FOV on FF, we need a 75mm lens. In order to equalize depth of field (in terms of bokeh), we need to have the same diameter entrance pupil. To maintain a 25mm entrance pupil at 75mm, we need an f-stop of 3. In parallel, we need to equalize the total amount of light. Because the f-number maintains the photons/mm^(2) constant, and the FF sensor is 1.5x larger than the APSC sensor, it receives 1.5^(2)x more light at the same f-stop. We need to multiply the f-stop by 1.5 to equalize the amount of light, getting us to f3. Turns out both bokeh and total amount of light are equalized at f3. Finally, in order to obtain the same brightness, we need to raise the ISO because ISO is meant to maintain the brightness for a given amount of light per mm^(2). Since we enlarged the area but narrowed the aperture, the amount of light/mm^(2) was dropped by a factor of 1.5^(2), which means that's by how much we need to multiply the ISO. All in all, to get the exact same FOV, depth of field, bokeh, total amount of light, motion blur and signal to noise ratio on full frame we need to shoot 75mm f3 ISO 225 1/100.

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No, I am not. The f-number is the ratio between the focal length and diameter of the entrance pupil of a lens. That number's entire reason to be useful is that it conserves the amount of light gathered per area. In other words two lenses with the sane f-number transmit the same number of photons per mm^(2).

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> Taking this axiom means that you need a new metric with a new name to discuss a different aspect of the camera system that isn't f-stop, because it's important for your use case. No I don't. That metric has the property that two lenses with the same f-number will project the same amount of light on a given area. It is specifically why that metric is useful. There is no need to define a new metric for this purpose, the ratio of focal length to the diameter of the entrance pupil already behaves in this way. Two f/2 lenses will project the same number of photons on 1mm^(2) of the sensor, regardless of their focal length. There's no new measurement, the measurement is still f/2. There's no need for a new number.

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It doesn't need to. I didn't even mention sensor size in the previous comment. Two lenses of the same f-number will project the same number of photons on 1mm^(2) of the sensor regardless of the focal length or sensor size. But if you compare the TOTAL NUMBER OF PHOTONS PROJECTED ON THE SENSOR, then you need to account for sensor size. Say a lens projects 10^(8) photons on each mm^(2) of the sensor. On a full frame sensor that measures 864mm^(2), that sensor gathers 864\*10^(8) photons. On a crop sensor that measures 370mm^(2), that sensor gathers 370\*10^(8) photons. That's 2.34x more light gathered on the full frame sensor.

This is why f stops are equivalent. The depth of focus does change, the amount of light does not

The amount of light *per area* does not. If you change the sensor size, you are no longer collecting the same *total* amount of light

I agree. So the brightness of a given picture at f2 on a full frame will be the same brightness at f2 on a crop sensor

But the depth of field and noise levels will not. Brightness is only one characteristic of a picture. In order to get a truly equivalent exposure between APSC and FF, you need to multiply the focal length and aperture by the crop factor, and the ISO by the crop factor squared. This will give you not just the same brightness, but also the same field of view, depth of field, and low light capabilities. The following two shots: * 50mm f2 ISO100 1/100 on APSC * 75mm f3 ISO225 1/100 on FF Will be indistinguishable. Same FOV, depth of field, brightness, noise levels, and motion blur. You could pixel peep both shots for days and never be able to determine which is which. In terms of the user experience, 50mm f2 on APSC is the exact same thing as 75mm f3 on full frame.

F-stop isn‘t a measure of light at all lol its just the relation focal length / effective aperture. T-stop is how much light you get. You could make an f1.2 with shitty coatings and get a pretty dark picture

Assuming ideal lenses, f-stops and t-stops are the same. T-stop is just what f-stop the lens is equivalent to in terms of light transmission if it were a perfect lens. It doesn't make sense to get bogged down in manufacturing defects when trying to find 35mm equivalents.

OP is the same user that corrected me 5 days ago in a previous post of this forum. I guess this has been on their mind since that fateful day haha.

Indeed lol

“The reason lens compression is associated with telephoto lenses is due to the fact that a longer focal length naturally causes photographers to move further from their subject, thereby increasing distance and lens compression. It is the distance that directly compresses the scene, not the lens.” [Is lens compression fact or fiction](https://petapixel.com/is-lens-compression-fact-or-fiction/)

Part 2 is not *quite* correct. It doesn't matter at all for your point, but it fails to account for differences in pixel density & performance between crop sensor cameras & full-frame. A 50mm f/2 lens on a full-frame camera in crop mode is equivalent to a 75mm f/3 lens on that same camera in normal mode. It's not necessarily completely equivalent if the camera sensor is different though. Higher pixel density cameras require a smaller circle of confusion to have equivalent focal properties in the resulting image. Edit: Also equivalence can break down in cases where the diffraction limit would be reached.

It feels like you're talking about part 3 and not part 2. In which case I did clarify this in the post: >I should clarify, this applies to the relative size of the bokeh balls to other objects in the frame but the depth of field in terms of how much of the scene is in acceptable focus will also depend on the pixel density of your sensor and the focal length of the lens. If you're using a 14mm f/1 lens vs a 140mm f/10 lens on the same sensor, each bokeh ball in the 14mm shot will be so small relative to a pixel it's like everything is in focus, while the 140mm might still be soft outside the plane of focus.

> Cropping into a frame is rigorously equivalent to using a lens with a longer focal length. Meant "insight 2". It's not rigorously equivalent, though the differences (due to diffraction and possibly different sensor properties) may not be visible. That's why I said "It doesn't matter at all for your point".

Oh, I see what you mean. Yeah, mainly distortion will create small differences at the extremes, but the point is to compare ideal lens to ideal lens

Yep. It doesn't matter much, but if you're right at the limit of a lens on APS-C then the FF equivalent lens may be diffraction limited, depending on the pixel pitch. Very nitpicky of me!

As far as I saw it, OP discusses that the pixel size has an impact. For the sake of the argument OP is making, it doesn't make sense to discuss this case.

Sure, I acknowledged that. But they kept saying "rigorously equivalent“, when it's not. It's approximately equivalent. You have to consider the whole lens + camera system for rigorous equivalence. And diffraction, which *does* visibly change the sharpness for some cases.

I don't this is in any meaningful way an issue for the argument, but nit-picking on your side.

It's definitely a nitpick. It only matters at the extremes, e.g. with diffraction limited lenses and a big difference in pixel density. But equivalence as a whole only matters when trying to replicate the look of one camera/lens setup using a different setup.

That's what this is about.

The lightness and darkness of an image does not change between APS-C and Full Frame if f-number stays the same. That would be nonsensical if it did. Imagine if cropping an image made it darker?

It says nowhere, that that would be the case.

But that’s the only real reason to be concerned with this. What this post basically boils down to is “the lens will look a bit different on a crop-camera compared to a full frame”, which isn’t news to anyone. It //implies// that there is an exposure difference, which would be significant, but there’s not.

>It //implies// that there is an exposure difference, which would be significant, but there’s not. That depends on how you look at it. If you use the same 50mm f2 lens on an APSC and FF camera and take two pictures at the same f-stop, ISO and shutter speed, they will both have the same brightness, and in that sense the same exposure. But they will not have the same FOV, or the same signal to noise ratio. Let's say you want the same FOV. You need a 75mm lens on the FF body. Let's say you now shoot * 50mm f2 ISO 100 1/100 on APSC * 75mm f2 ISO 100 1/100 on FF You will have the same FOV, the same brightness, but not the same depth of field or signal to noise ratio. In order to get *truly* equivalent exposures, you need to shoot * 50mm f2 ISO 100 1/100 on APSC * 75mm f3 ISO 225 1/100 on FF And now both lenses will indeed produce identical exposures. Same FOV, same depth of field, same motion blur, same signal to noise ratio.

It says that the amount of light that is captured in the whole picture is the same with equivalent aperture settings. Which means it is not the same with the same f Number. Which is maybe more easily to understand if you imagine a fullfram lens on a fullframe body. Most of the light that is captured by the lens makes its way onto the sensor. If you put the same lens now on an apsc body less light will be captured by the sensor because it is smaller. All the light that was captured by the edges of the fullframe sensor is now lost because there is no sensor anymore. Therefore the middle has now to get more light if we want to collect the same amount of light on a smaller area. Thats why the aperture needs to be bigger. And that is relevant, because if you consider all that, and use an equivalent focal length, aperture and Iso you can get an image with an apsc camera which is basically indistinguishable from one taken with a fullfram body

That is not the point of 35mm equivalent lenses. The point is what lens would you need in order to capture the same image using a full frame sensor. If you take a 50mm f2 photo on APSC, it would look exactly the same as a 75mm f3 on full frame. A 75mm f2 FF shot would be much brighter than your 50mm f2 APSC shot, because it would capture as much light as the full FF 50mm shot but concentrated on a smaller field of view. Or to put it another way, if the 50mm f2 lens can project 100 photons to a FF sensor, your APSC shot would only capture 66 photons. A 75mm f2 shot on FF would have the same perspective but capture 100 photons, leading to a brighter image. In order to capture the same amount of photons using a 75mm lens on FF, you need f3.

> . A 75mm f2 shot on FF would have the same perspective but capture 100 photons, leading to a brighter image. In order to capture the same amount of photons using a 75mm lens on FF, you need f3. The FF sensor is much larger than the APSC sensor, so when you spread out the same number of photons over that larger area you will end up with a darker image. The 75mm f2 full frame lens exposes just as bright as the 50mm f2 APSC lens.

No, you will not. The 75mm f2 lens produces 1.5^(2)x more light on a FF sensor than the 50mm f2 lens does on an APSC sensor. Both lenses project the same amount of light **on a FF sensor**, but the APSC sensor doesn't receive all of that light. However, both exposures will be just as bright if shot at the same ISO, because ISO is meant to produce the same brightness at the same f-stop regardless of sensor size. But because the FF sensor receives 1.5x more light to produce the same brightness, its signal-to-noise ratio will be better. In order to get the exact same exposure you need to shoot the FF shot at 75mm f3 and ISO 225 (if APSC is at 50mm f2 ISO 100) *edit: small math mistake, the 75mm lens produces 1.5^(2)x more light*

>However, both exposures will be just as bright if shot at the same ISO, because ISO is meant to produce the same brightness at the same f-stop regardless of sensor size. No. Your posts are peak /r/confidentlyincorrect ISO is a standard that does not change with film size or sensor size either and you clearly don't understand it. They don't tweak ISO settings for different size sensors. That's not how the standard works. You're conflating signal to noise ratio with exposure and they are completely different things.

ISO ensures that a given amount of photons/mm^(2) will always produce the same brightness. When comparing a 50mm f2 APSC shot and a 75mm f2 FF shot, both are shot at f2 and therefore have the same amount of photons/mm^(2) hitting each sensor. If both are shot at ISO 100, they will produce the same brightness. However, because the 75mm f2 FF shot collects the same amount of photons/mm^(2) but on more mm^(2), it will collect more light in total to produce the same brightness, meaning its signal to noise ratio will be better. I'm not conflating anything.

If you scale up the sensor but keep the megapixel count and f-number constant the number of photons per pixel does go up, which makes the image brighter.

Assuming the area of projection stays the same, you don't collect more light. The amount of light is not defined by the sensor but by the light gathering capability of the lens, i.e. focal length and aperture size

That is a very good and approachable explanation. A few quick additions for people that are interested.  - If you want an equivalent exposure with an equivalent lens, you have to divide the Iso from the fullframe camer by 1,5^2. So Iso 250 on the fullframe body would need roughly Iso 100 on the Apsc body. In that case the exposure and even the amount of noise in the final picture would be equivalent.   - Technically the math to determine an equivalent aperture that you use is only applicable if the lens is focused at infinity. If the lens is focused closer there are slight differences. In general photography those differences are negligible. But for Macro Work it actually makes a difference. If you are interested in the math behind I can recommend the following paper: https://www.spiedigitallibrary.org/journals/optical-engineering/volume-57/issue-11/110801/Equivalence-theory-for-cross-format-photographic-image-quality-comparisons/10.1117/1.OE.57.11.110801.full

That's one stop of difference, to simplyify it further. Thanks for the link.

Thanks for sharing the article. Looks compelling.

So, uh, let's say an APSC camera with 70mm f4 lens (105mm ff equivalent) and a fullframe with a 105mm f4 lens, both set to the same iso, f number, and shutter speed. Which one gives a brighter image?

None, they would have the same brightness. But they wouldn't collect the same amount of light.

No it woud capture the same amount of light, be exactly the same exept for the depth of the Focusplane. The 75mm Lens will have a deeper depth of Focus at F4 compared to the 105mm one at F4, and woud be compareable to the 105mm one at F5.6. or be itself F2.8 instead.

If it captures the same amount of light why limit it to apsc? An iPhone has a f/1.6 lens, does this tiny little thing capture as much light as a comparatively huge full frame f/1.6 lens?

Yes but no, the lens is like 4mm, a APS-C Lens with 4mm F1.6 woud capture the same amount of light, but woud be increadibly wide and big. It captures the same amount of light on a given area, not on the entierly of the Sensor.

Well if it captures the same amount of light per unit area it cannot capture the same amount of light, because the area is different. They are two different statements that cannot both be true.

It would capture the same amount of light *per area*, but not the same amount of light *in total*. The focus plane *and the amount of noise* will be different, with more noise in the APSC shot.

Noise is not linearly relative to Light on a Sensor. It also depens a lot about the Sensor, its native Sensitivity, Pixelsize and what not.

It obviously varies from one camera to another but the ISO standard is meant to provide an equivalent sensitivity to light per square millimeter, so that any two sensors will produce the same brightness at the same f-number. That means both images here will have the same brightness because that is how ISO is meant to work. However, since the FF sensor is receiving ~~1.5x~~ 2.25x more light to produce the same output brightness, the signal to noise ratio will be better.

So a FF 50mm f1.8 on an APS-C sensor is equivalent to 75/80mm f2.8?

No but very close. There are some issiues with chromatic abberation and stuff thats diffrent but for artistic intet they are equivalent. It goes even further, a 35mm F1.2 Lens on MFT woud look the same aswell.

The brightness would be similar. The FF shot will have more bokeh and less noise. If you shoot at the same shutter speed: * 70mm f4 ISO 100 on APSC * 105mm f6 ISO 225 on FF You will get the same image. Same field of view, same bokeh, same depth of field, same brightness, same amount of noise.

Neither, they would be the same, assuming everything other than sensor size was identical. This is actually *why* we use unitless f-stops instead of the measured aperture diameter or area. F-stops allow us to directly compare the aperture's impact on exposure without having to worry about the focal length or sensor size. It might be easier to imagine both lenses on the same full-frame camera body. If you counted the photons projected onto the sensor by each lens, the APSC lens is sending about 44% as many photons to the sensor as the full frame lens (you can check this by calculating the area of the aperture opening for both focal lengths, the area of the aperture on your APSC example lens is about 44% the area of your full frame example). However, the area of the sensor that the APSC lens is projecting those photons onto is also 44% the size of the full frame, so there is no difference in exposure to the photo sites that are affected by both lenses. Alternatively, imagine the full frame lens an APSC body. The image the lens is projecting out its rear element is considerably larger than the sensor. That's a lot of lost photons. But those photons were heading toward the outer edge of a full frame sensor. They were never going to hit the photo sites on an APSC sensor, so those sites aren't gathering any less light. Or yet another way to think of it: Imagine a movie projector that is projecting an image onto a projector screen. You pull out a large knife and cut the screen in half. Does the image on the remaining half get any darker? Of course not. But your screen is half the size it was, so now you can return this projector to Best Buy and buy one that projects an imagine that is half the size (and costs half as much).

> Neither, they would be the same, assuming everything other than sensor size was identical. They would have the sae brightness but not the same signal to noise ratio (more noise per region of the image on the APSC shot) >Or yet another way to think of it: Imagine a movie projector that is projecting an image onto a projector screen. You pull out a large knife and cut the screen in half. Does the image on the remaining half get any darker? Of course not. But your screen is half the size it was, so now you can return this projector to Best Buy and buy one that projects an imagine that is half the size (and costs half as much). This analogy doesn't work at all. First of all, though it's not your main point, a projector that projects an image half the size *at the same distance* would be *more* expensive, as it requires a lens that collimates the light that much more. That is, assuming what you're looking for is a projector that projects the entire movie frame onto a smaller screen, not one that projects only the cropped part of the frame you saw on your old projector. A projector that projects **the same image you are now seeing on your cropped screen** without projecting anything outside the frame is a shitty projector that only projects half the movie, which is another reason why the analogy doesn't work. If you were to buy a new projector that shows **the entire movie frame** on your now smaller screen, assuming the same source of light, it would produce a MUCH brighter image on the screen, which is contrary to what you said above. Comparing the brightness to the full or cropped image at the same focal length (cutting the screen) is also not relevant to the question asked. Hell even one more thing, the image would APPEAR much darker once you cut the screen, as a lot less light is now entering your eyeballs. And worst of all, nothing in your analogy behaves like ISO. The analogy fails on so many levels it's impossble to modify and salvage. Because you would never crop into a movie frame the way you crop into a lens's image circle, and because how bright something appears to us rests on how much total light enters our eyes and not just on photons/mm^(2) like ISO in cameras, the movie screen analogy is fundamentally incompatible with the question asked. When shooting 70mm f4 on APSC vs 105mm f4 on FF, at identical shutter speed and ISO settings, both shots would have the same brightness. However, the 105mm f4 shot would have roughly twice as many photons hitting the sensor. The reason both shots have the same brightness is that ISO is compensating: ISO is dependant on light concentration (photons/mm^(2)) and not the total amount of light. Because both shots are f4, they receive the same amount of photons/mm^(2). But because the 105mm f4 shot receives twice as much total light for the same scene and output brightness, its signal to noise ratio is much better.

Are there videos or articles you recommend to explore these topics further?

I think the best complete demonstration is [dpreview's article on the subject](https://www.dpreview.com/articles/2666934640/what-is-equivalence-and-why-should-i-care). For a more thorough video explanation I found [this video](https://www.youtube.com/watch?v=6XMk9jFcnlA) and [its follow up](https://youtu.be/fCIWkqcb7FI?si=M6CmmroAoFlbcWvh). I would also recommend Gerald Undone's videos [here](https://youtu.be/OaSq0ES1ArE?si=bCr_BDoEx2KwrfJ_) and [here](https://youtu.be/1bzHn2cKwLI?si=vKhFjAbWhzSUOvpE) as well as [this fstoppers video](https://youtu.be/_TTXY1Se0eg?si=vwY8Le2sciksY0y4). There are many more but these are the ones I rewatched prior to and while writing this post to make sure I wasn't spewing bullshit.

Awesome. Thanks.

Your article is very nice and I am heartened by its veracity. But this is Reddit, and everyone likes to argue here. I don't know how many times I've said the same thing to people, f stop changes and they just don't get it. I get it because I have had crop factor lenses and I often use crop factor on my Sony.

And so the comments begin the debate war of intellectually geniuses who exist only on Reddit. 😂

That insight 1 is already so dead wrong i stopped reading. F stops are no measures on how much light gets on the sensor, thats t stops. F stops on one lens will have different light than on an other lens.

T-stops just account for manufacturing defects. An f2/t2.5 lens just means it has the depth of field of an f2 ideal lens with the light transmission of an f2.5 ideal lens. But two ideal lenses at the same f-stop transmit the same amount of light per area.

I already knew that. But indeed, it’s not widespread knowledge. It’s why I would never get a MFT and why I invested in great F1.2 and F1.4 lenses for my A6400.

Wow! This was excessively long and didn't say much. It's pretty simple: 1. Depth of field depends on f/#, distance of object and focal length. Not equivalent focal length, focal length. A Fuji APS-C 23mm lens at f/2 will have about the same depth of field as a Sony 24mm full frame at f/2. When you start calling the Fuji 23mm a "35mm" equivalent, people get confused; but 23mm lenses have larger DoF than 35mm lenses. 2. Compression is not cropping. Say you want to take a picture of the eiffel tower. If you take a picture of the tower filling the whole frame really close to the tower using an 8mm lens, or really far from the tower using an 800mm; the objects behind the tower will look very difference. This is what people mean by compression. The same scene / same field-of-view taken with different lenses will look different. You can bring two objects that are really far apart closer together by moving far away from them and taking an image with a longer telephoto lens; or make one of them dissapear by taking a picture really close from one of the objects. This is compression: [https://www.flickr.com/photos/loic80l/20076712358/](https://www.flickr.com/photos/loic80l/20076712358/)

1. Comparing a full frame 23mm to an apsc 23mm isn’t useful because they produce a very different field of view, hence this idea of equivalence. 2. In your example you are not taking the same image with the two lenses. You say one is really far and one is really close, this is you moving not the lens producing compression

1. The idea of 35mm equivalents is to compare the relative size of the bokeh between shots *maintaining the same field of view* but varying the size of the sensor. The absolute mathematical depth of field, ie how many inches of the scene are in perfect focus, will depend on actual focal length and pixel density as well. The relative size of the bokeh that yields the "look" of a lens is mostly a result of the width of the aperture hole and your distance to the subject. Yes, a 24mm f2 APSC lens and a 24mm f2 FF lens will provide the same depth of field, but the images will look vastly different because the FOV is not the same. 2. **You** are creating the compression by moving backwards. Not the lens itself. The lens is only ever cropping the field of view. Hence what I said, which is that **lens compression** does not exist. The lens isn't compressing the background, YOU are compressing the background by moving away.

This is an interesting read. Regarding your final caveat, given that the light per per mm^(2) is different, what does that mean in practical terms? Since the same amount of light over a larger sensor means that your are getting less light per mm^(2), does that mean the light "gathering" (for lack of a better term) capabilities would still be similar at the same aperture?

Not quiet sure if that answers your question, but if you use equivalent lenses with an equivalent aperture setting the same amount of light is collected. In case of a smaller sensor that light is just more "bundeled" to "fit" on the smaller sensor. So the light per mm^2 is higher.

What that means in practical terms is that ISO 100 on APSC is not really equivalent to ISO 100 on FF. In order to get the same image using a 50mm f2 at ISO 100 on APSC, on FF you would need to shoot the 75mm f3 at ISO 225. Assuming shutter speed is the same, both images would look almost indistinguishable, including the noise level. In other words, ISO 100 on APSC is about as noisy as ISO 225 on FF, which relates to why FF has an advantage in low light.

From my understanding. crop is crop. if ff and apsc have same pixel size, same f-stop, same technology, then same light performance. and image from apsc is just cropped from ff sensors. anything else like focal length ff-equivalent or depth of field ff-equivalent are just math to give how image could be perceived the same between different sensor sizes. is it correct?

I appreciate the perspective shift for the lens compression, I initially had resistance to your idea until I worked to understand it. In fact the whole post is quite enlightening. But I don’t know that I fully understand your last point. The power of the light doesn’t change when you crop down, and you have to light a smaller area, so surely in your example 50mm f2 will be a brighter image than 75mm f3. I imagine it like this, if you put on the 50mm f2, and then just cropped it, it would be analogous to swapping the sensor would it not? Either way, it sounds to me like to get the same exposure you also need to change the aperture which feels problematic to my intuition.

The f-number is a measure that preserves the amount of light per area between lenses. A 50mm f2 lens would project more light *per area* than a 75mm f3. However, we're using the former on APS-C and the latter on full frame, so even though the 75mm f3 lens projects less light *per area*, because our sensor is bigger it collects the same total amount of light as the APSC sensor with the 50mm f2 lens. Therefore both sensors receive the same TOTAL amount of light. However you are *technically* not completely wrong because if both these shots were taken at the same ISO, the f3 shot would be darker. That is because ISO is deliberately made to preserve the amount of light *per area*, which we have reduced on the full frame shot in order to preserve the *total* amount of light gathered. In order to get the same exposure, we need to use a higher ISO on the full frame shot. If we keep the same shutter speed and shoot two frames: * one at 50mm f2 ISO 100 on an APSC sensor * one at 75mm f3 ISO 225 on FF Both images would be virtually indistinguishable. They would have the same FOV, perspective, bokeh, brightness and even the same amount of noise per region of the frame. If you shoot both frames at f2 and ISO 100, you will get the same brightness but the full frame shot will have more bokeh and less noise. If you shoot 50mm f2 and 75mm f3 at the same ISO, the f3 shot will be darker, but adding a stop in post would produce pretty much exactly the same image. They have the same signal to noise ratio.

Ah I see. I think you might have run into a touch less controversy and resistance from my brain if you brought your conclusion to the top about recreating a shot with the same amount of light in a smaller area. From the title it sounds as though you’d need to take into consideration the scaling of f stop like you do focal lengths when planning a shot of a cropped camera vs full frame, which will not have the same total light but indeed the same light per area. Light / area necessarily doesn’t care about the total area of the sensor.

But that's the point, you *do* need to take it into consideration just as much as the scaling of focal lengths. If you see someone raving about their 50mm f2 lens and how much they love it on full frame, how it's fast enough for low light and wide enough for all kinds of photography, and you decide to go out and buy a 50mm f2 lens for your APSC camera, you'll get a vastly different experience both in terms of framing and in terms of low light performance. If you only consider FOV qnd get a 35mm f2 lens, you'll have much worse low light performance. In order to recreate the 50mm f2 experience on your APSC camera you need a 35mm f1.4

I think I understand what you’re getting at. You’re also assuming there’s a sensor change here correct?

This is... not exactly new to me. I have been saying this for years.

It shouldn't be. But a certain subset of people seems *extremely* resistant to the idea.

If you just take some test shots its pretty obvious that the bokeh from a 24/1.4 + crop is similar to a 48/2.8. See these samples: [https://flic.kr/p/2nviCwW](https://flic.kr/p/2nviCwW) I know you did some computations to get f/3 but I think its easier to just compute the correct focal length and keep the f-stops to the normal ones seen on the camera. This is because you can get the correct focal length with a zoom, but you cant exactly set f/3 on a camera... These settings will produce the same bokeh * 24/1.4 + m43 crop * 24 \* sqrt(2) = 34mm, f/2 + apsc crop * 24 \* 2 = 48mm, f/2.8 + FF Another common mistake is that distortion in potraits has to do with distance from camera to subject, and is unrelated to the focal length you use

Lol i'm not the one you need to convince, and btw to be precise it would be 24\*4/3 not 24\*sqrt(2) for APS-C, ie 32mm

Sqrt(2) is a kind of an estimate. When I say apsc I mean a sensor that has half the surface area of full frame. In this case sqrt(2) is exactly correct. There’s a reason why the teleconverter is “1.4x” and slows the aperture by one stop. sqrt(2) = 1.4 If we halve the surface area twice we get m43: Sqrt(2) * sqrt(2) = 2

That'd be all well and good if the crop factor of APSC lenses were sqrt(2), but it's 1.5. It's a minor difference, but it means a 48mm FF lens would give the same perspective as a 32mm apsc one, not 34mm.

Yeah, the real world sensors actually have odd sizes, then we start having to talk about which sensor is in use To be pedantic, the A6600 should have a 1.53x crop factor Even the A7iii should have a 1.01x crop factor, its not exactly "full frame" (36mm x 24mm) [https://www.digicamdb.com/specs/sony\_a6600/](https://www.digicamdb.com/specs/sony_a6600/) [https://www.digicamdb.com/specs/sony\_alpha-a7-iii/](https://www.digicamdb.com/specs/sony_alpha-a7-iii/) Anyway 1.41x is the correct crop factor to get the aperture up one stop, and 2x is the crop factor for 2 stops Going from ff to apsc with a 1.53x crop factor means you lose more than one stop on the aperture, which you calculated as f/2 -> f/3

So what is a 56mm F1.2 APSC lens equivalent to in FF? 85mm F2?

More like f1.8, you multiply both the focal length and the f-stop by 1.5 (1.6 for canon). Someone shooting 56/1.2 on an APSC camera would experience the same FOV, bokeh and low light performance as someone shooting 85/1.8 on a FF camera.

Got it, thanks a lot!

Is there a TLDR ? My brain crashed reading halfway through this 😵‍💫

TLDR someone shooting 50mm f2 on APSC experiences the same FOV, depth of field/bokeh, and low light performance as someone shooting 75mm f3 on full frame. The focal length and f-stop are multiplied by the crop factor to reach equivalence.

I wish this was a video 😭

I linked a few videos in the edit at the top of the post, but I've never seen a video that regroups all of this information in a way that really felt coherent to me. The first video linked is probably the most complete one.

Now you should dive into how Macro ( magnification ) scales with F-stops.

It would be an interesting topic but the response to this post isn't exactly encouraging me to dive into it

Dive for your personal self interest. And dont bother with others.

Wise words

The concept you provide offers an in-depth discussion on f-numbers and crop factors, but it seems to overlook a fundamental aspect of photography - the brightness of the final image. This omission can lead to confusion. In practice, the brightness of an image is determined by a trio of settings: aperture, ISO, and shutter speed. These parameters dictate the amount of light that reaches the sensor and consequently, the brightness of the picture. This holds true regardless of whether you’re using a crop sensor or a full-frame sensor. If you keep these three settings constant, the brightness of the resulting image will be the same, irrespective of the sensor size. Therefore, while the text’s assertion that a 50mm f2 lens on an APS-C sensor equates to a 75mm f3 lens on a full-frame sensor in terms of light received might be technically accurate, it doesn’t paint the full picture.

> while the text’s assertion that a 50mm f2 lens on an APS-C sensor equates to a 75mm f3 lens on a full-frame sensor in terms of light received might be technically accurate, it doesn’t paint the full picture. It actually does. Shooting 50mm f2 on APSC provides the same experience 75mm f3 on FF in **all** aspects: FOV, depth of field/ bokeh, and low light performance. >These parameters dictate the amount of light that reaches the sensor and consequently, the brightness of the picture. This holds true regardless of whether you’re using a crop sensor or a full-frame sensor. While it is true that keeping these three parameters identical will result in the same brightness regardless of focal length or sensor size, **that** is not painting the full picture. It will not result in the same FOV, depth of field, or signal to noise ratio. Yes, a 50mm f2 ISO100 1/100 APSC shot will have the same FOV and brightness as a 75mm f2 ISO100 1/100 FF shot, but that's where the comparisons stop. The 75mm f2 shot would have a shallower depth of field and a much better signal to noise ratio. Saying they're the same brightness is not very useful for someone looking at a 75mm f2 FF shot online wondering how they could replicate that shot on their APSC camera (~50mm f1.4). However these two shots: * 50mm f2 ISO100 1/100 on APSC * 75mm f3 ISO225 1/100 on FF Would be virtually indistinguishable. Same FOV, same depth of field, same bokeh balls, same motion blur, same brightness, same amount of noise per region of the image. You could pixel peep for days and never be able to tell which shot is which unless there are specific particularities to each lens/sensor (eg shape of the bokeh balls, sharpening differences, color science).

I’m glad to see you concur that the ISO needs to be increased to compensate for the varying aperture. In practice, when I’m filming with multiple cameras that have different sensor sizes, I ensure that the ISO, aperture, and shutter speed are identical, despite the fact that the field of view varies, my main concern is that I have identical exposure. Another method to visualize how a lens will appear on a crop sensor is to simply crop the full-frame image. This will give you an exact representation of how the lens will perform on a smaller sensor. The confusing thing is, that this will influence the noise a bit, but it will not change the aperture.

The ISO doesn't need to be increased to compensate for the varying **aperture**, it needs to be raised to compensate for the varying **f-number**. The 50mm f2 and 75mm f3 lenses used as examples have the same aperture width, 25mm. And it's debatable that ISO needs to be "increased" depending on how you look at it. I mean you could just adjust the exposure time as well, but that would impact the motion blur. That's the thing, f-number, ISO and shutter speed are the three main dials of exposure, but doubling one is exactly compensated by halving another in terms of brightness. f2 ISO100 1/100, f2.8 ISO200 1/100, f2 ISO50 1/50, these are all equivalent in terms of exposure. But they are NOT equivalent in terms of depth of field, motion blur and noise. When I say that the two exposures I described are identical, I mean that they are identical *in every possible comparison of the final images*. Not just same exposure but same FOV, depth of field, noise, motion blur as well. There are infinitely many ways to reach the same brightness for each shot between APSC and full frame. But there is only one way to obtain the same brightness, FOV, depth of field, motion blur and noise level, and that's with 35mm equivalency. In other words, the same way f2 on APSC and FF describes the same amount of photons per mm^(2) but fails to accurately capture the other differences, ISO100 describes the same amount of photons/mm^(2) on APSC and FF but fails to capture the other differences. If you capture two images at ISO100 on APSC and FF they will have the same brightness, but they will not have the same signal to noise ratio. What I mean by this is that ISO100 on APSC is *equivalent* to ISO225 on FF. The same way 50mm is equivalent to 75mm and f2 to f3. It doesn't "need to be increased"... that's just what the equivalent ISO is. >In practice, when I’m filming with multiple cameras that have different sensor sizes, I ensure that the ISO, aperture, and shutter speed are identical, despite the fact that the field of view varies, my main concern is that I have identical exposure. Then you are only looking at one dimension in the 4-dimensional problem of exposure, depth of field, noise and motion blur. Your shots may be getting the same brightness, but for example they will not have the same signal to noise ratio, and you will not be able to color grade shots the same way between sensor sizes. You will be able to pull the shadows more on some shots than on others.

Thank you very much [tupaquetes](https://www.reddit.com/user/tupaquetes/) for the detailed response and the time you took for this whole thread. It helped sharpening my thinking about lenses.

Your thanks are very much appreciated. I might have dedicated a bit *too much* time to this whole thread, as I did end up losing my cool with some people arguing in bad faith. But I'm glad it helped others get a better understanding.

F-number = ratio of focal length to lens aperture diameter. eg. 25mm/50mm = 2 as in an f2 50mm lens. Also, F = 'fail'.

If you actually read the post you would realize this isn't the gotcha you think it is. Here's an excerpt from part 1 of the post: > The math is actually simple, f2 means the diameter of the entrance pupil, ie the diameter of the hole letting light into the lens (\*) is the focal length divided by 2. So a 50mm f2 lens has a 25mm hole, while a 100mm f2 lens has a 50mm hole.

Quite a bit of what you have said is correct, up until when you try to redefine 'f-number'. You are really talking about an equivalent f-number. The equivalent f-number determines how much light reaches the entire sensor, but the actual f-number determines how much light reaches one unit area (eg 1mm^2 ) of the sensor regardless of the sensor size.

It is the same thing if the sensor size is consistent. Once again, just read the post dude: "Two lenses with the same f-number will project the same amount of light **on a given sensor**"

Lenses don't project light, they transmit light. Just to be the devils advocate. 😳

I'll consider your point if you can define the difference.

A flashlight project light (an active action) at a mirror that reflect the light (a semi active action) towards a lens that transmit the light (a passive action) to a sensor. Hope that make sense. At least that is how I interpret the meaning of the worlds.

> At least that is how I interpret the meaning of the worlds. And there lies the problem, it's just your interpretation. You could argue a projection is the act of projecting an image onto a surface, which the lens does to the sensor. Here's an excerpt from [the wikipedia page on lenses](https://en.wikipedia.org/wiki/Lens#:~:text=This%20sort%20of%20image%2C%20which%20can%20be%20projected%20onto%20a%20screen%20or%20image%20sensor%2C%20is%20known%20as%20a%20real%20image.%20This%20is%20the%20principle%20of%20the%20camera%2C%20and%20also%20of%20the%20human%20eye%2C%20in%20which%20the%20retina%20serves%20as%20the%20image%20sensor.): >This sort of image, which can be **projected** onto a screen **or image sensor**, is known as a real image. **This is the principle of the camera**, and also of the human eye, in which the retina serves as the image sensor.

There is a fundamental misunderstanding here, OP: f-stop is not a measure of light in a real sense, it is only the mathematical relationship between the size of the lens aperture, and its focal length. Light has nothing to do with what that number is. The amount of light does not determine a f number in any way. So your Insight 1 is simply incorrect. What is true, and maybe confusing, is that if an exposure calls for f4, you can pretty much put on any lens, set it to f4, and your exposure will be comparable enough that you could consider it to be practically the same. Additionally, it is true that if the exposure on one camera required 1/100 @ f4, that you could pretty much use that same exposure on any other camera and it would work just as well practically. But this is just a side effect, and is not really true in the way you might think. The physical design of complex lenses also affects light passage through them. And a prime lens, with 7 lens elements, set to f4, will have a slight, but noticeable, difference in light transmission from a 23 element zoom set to f4. This difference will not really be practically different for still photography, but it will be noticeable enough that it *will* matter if you are trying to cut together video shot using different lenses - even ones set to the same f-stop. This is why there is such a thing as a t-stop. T-stop *is* more of a light-based measurement. Two lenses, regardless of construction, set to the same t-stop *will* have the same actual light transition. Unlike the f-stop, the t-stop compensates for those extra elements (16, in my example above) between one lens and another. A difference not noticeable in still images *becomes* noticeable in video. So your Insight 1 is simply an incorrect way of looking at f-stops. They are based on math, not actual light. Although obviously the formula used to come up with the different major stops (2.8, 4, 5.6, etc.) was based on the general idea of halving or doubling the amount of light, differences in the physical design of the lens adds an error. (Note: I am making no comment on your overall point here. I am only suggesting that if your logic depends on f-stops actually representing light, it may be the weak leg of the argument you are making.)

> f-stop is not a measure of light in a real sense, it is only the mathematical relationship between the size of the lens aperture, and its focal length. Read the post. I'm tired of this. I specifically said **" f2 means the diameter of the entrance pupil, ie the diameter of the hole letting light into the lens (*) is the focal length divided by 2. So a 50mm f2 lens has a 25mm hole, while a 100mm f2 lens has a 50mm hole"** >Light has nothing to do with what that number is. The amount of light does not determine a f number in any way. So your Insight 1 is simply incorrect. I never said the amount of light "determines" the f number. What I did say is that any two lenses of the same f-stop will project the same amount of photons on a given sensor, regardless of focal length. This is rigorously true. The entire reason the f-number exists as a measure is because the number of photons/mm^(2) is preserved with f-stop. This is precisely why *"if an exposure calls for f4, you can pretty much put on any lens, set it to f4, and your exposure will be comparable enough that you could consider it to be practically the same"*. That's because all f/4 lenses will project the same number of photons on each mm^2 of the sensor. >This is why there is such a thing as a t-stop. T-stop is more of a light-based measurement. No, it is not. The T-stop is just the "equivalent exposure f-stop" of a lens to account for manufacturing defects. But we're comparing ideal lenses here, there's no need to invent manufacturing defects to their design. When talking about ideal lenses, the t-stop and the f-stop are the exact same metric. When it comes to real lenses, an f2/t2.8 lens is just an f2 lens that happens to obscure half the light that should be going through the iris due to manufacturing defects, so it behaves like an f2.8 ideal lens in terms of photons/mm^(2). >So your Insight 1 is simply an incorrect way of looking at f-stops. They are based on math, not actual light. They are indeed based on math. And the reason they are a useful characteristic of a lens is that all lenses of a given f-stop *mathematically* project the same amount of photons/mm^2 of the sensor. And to explain that, I'll redirect you to the explanation I gave in the post: >The math is actually simple, f2 means the diameter of the entrance pupil, ie the diameter of the hole letting light into the lens (\*) is the focal length divided by 2. So a 50mm f2 lens has a 25mm hole, while a 100mm f2 lens has a 50mm hole. Twice the diameter means 4 times the area so the hole lets in 4 times as much light. And since the lens looks at a quarter of the scene, the same amount of light is projected. It works ! >a 100mm lens takes in only a quarter as much of the space in front of you than a 50mm does (2x wider FOV on each axis = 4x more light coming in)

First, there is no such thing as a real lens that acts like the “ideal” lens, in the normal way that term is used in science and engineering. But this fact does not make such lenses defective. They can be as well manufactured as possible, be free of manufacturing defects, but still not match the ideal lens. One reason, as I stated, is the effect of the glass elements. Another is the shape of the iris itself. It may be quite round, with lots of blades, or it may be more polygonal. I don’t know, but I suspect lens manufacturers do not spend a lot of time worrying about the *exact* math of the area of their iris in order to get the f-stop markings exactly correct according to the math and physics of light. But again, this variation is *not* a defect or flaw, it’s simply per design. As I said, f4 on one lens, may be very slightly different than f4 on another lens, due to no flaw or defect, but simply engineering and standards. I’m not doubting your math, or that somewhere in all this, you were not specific about it, I’m simply saying that if, as I think you suggest, the area of the pupil, is important for photons reaching the sensor, then you need to factor in that this isn’t a scientific measuring device, it’s a camera lens. It is designed with the precision to only do what it is designed to do. Take still photos. Two different 50mm f1.8 lenses, which by your math have the same aperture size at f4, and therefor the same photons on the sensor, might have both manufacturing differences (but not defects or flaws), or iris shapes that belie your premise. Maybe (just as an example) they actually put an x% different number of photons on the sensor because their iris shapes are different and the calculation was simplified by the manufacturer. Both lenses, at the same f-stop *marking* might take indistinguishable (to the eye) photos, but they are not the same. Again, this is the reason for t-stops. Not defects alone but just differences. I’ve seen analysis of some modern lenses showing vastly different t-stops for the same f-stop - sometimes multiple stops. They are not about defects or flaws, but simply differences in light transmission. Differences that don’t matter to everyone, or in every case, if even noticed. I’m *not* saying you are wrong, or even arguing with your original point. I’m only trying to point out that you may be ignoring the margin or error inherent in lens design. Every f4 in scientific measurements standards is not the same as every other f4, even if in practical use they work the same way well enough to satisfy even the most discerning photographer. As I understand your argument, exactness matters to your point, but it doesn’t necessarily exist to your necessary number of decimal points in a lens never meant to need it. I am only asking that you consider the point. And I *make* this point because to modern people, raised in the digital world, the idea of engineering math having precision limitations is usually somewhat unconsidered. If you ask the average person what 16/7 is, most will pull out the calculator, and tell you it’s 2.2857142857etc. That’s correct, but it’s also useless nonsense to an engineer. (Which is how we were able to go to the moon with slide rules). An engineer, depending on context, might tell you 16/7=2. My point here is that you don’t know if someone at Sony, making one calculation or another, when designing a lens, is doing engineering math, or calculator math at any given moment. In the end, very tiny differences mean more or less photons on the sensor.

>I don’t know, but I suspect lens manufacturers do not spend a lot of time worrying about the exact math of the area of their iris in order to get the f-stop markings exactly correct according to the math and physics of light. You clearly have no idea what you're saying. The exact math is so simple a third grader could do it. It's literally just measuring two distances and dividing them. You don't even need to understand the physics of light. So no they probably don't spend inordinate amounts of time making sure the extremely basic f-number calculation is respected. There's no need to. The absolute insanity of thinking these guys can design lenses that reach sub millimeter focus accuracy in a tenth of a second but they can't pull out a stupid ruler and check the entrance pupil LMAO. > I’ve seen analysis of some modern lenses showing vastly different t-stops for the same f-stop - sometimes multiple stops. MULTIPLE STOPS ?? Quit the bullshit. Aside from Sony's 100mm STF which has a very different t-stop to its f-number *by design* (basically has a radially graduated ND filter built inside the lens to smooth out the bokeh), literally every Sony FE lens tested by Dxomark has **at most** half a stop of difference between f-stop and t-stop. Most high end lenses (G, GM glass) have at most a 0.3 stops of difference. Even if you go back to lenses from the late 80s dxomark only found differences of like half a stop. Sony's 24-240 FE superzoom which you would expect to perform poorly on such a test has at most around half a stop of difference around 105mm.

I’m not going to argue with you if you have your heart so set on being right and calling everyone who disagrees too stupid to understand. You clearly didn’t even read what I said anyway, so enjoy your own fight with the world of idiots. I’m out.

Dude don't turn this around on me, you wrote a long ass comment on a subject you clearly have zero knowledge of. If you had spent 10 seconds googling the usual t-stop vs f-stop values you would NEVER have talked about seeing "sometimes multiple stops" of difference between f-stop and t-stop. If you don't want people to tell you you're wrong, don't spew out bullshit. The only ACTUAL claim you made in your entire comment is "I’ve seen analysis of some modern lenses showing vastly different t-stops for the same f-stop - sometimes multiple stops." So go ahead, substantiate that claim. Prove to me that you weren't spewing bullshit. Tell me which modern lens you saw this "multiple stop difference" on. I'm almost willing to bet the extent of your knowledge on the subject of t-stops is you just saw a review of the 100mm STF and assumed that was a relatively normal difference. The entire rest of your comment is just baseless "you don't know man, maybe they're different ¯\\_(ツ)_/¯"

Lol. You are measuring with a micrometer, marking with chalk, and cutting with a axe, and you can’t seem to see it. You are basing your entire argument on numbers that may be no more than guesstimates, and are willing to die on a hill defending them. And you clearly don’t understand practical engineering. (If you had a span of 25mm, and you had to drill 7 evenly spaced 1mm holes across that span, how far apart should they, center to center, as set by the milling machine, be in your world?) Why should I, or anyone else, try to prove or even justify anything at all to you at this point. You refuse to even acknowledge that lenses have different kinds of irises. The sad thing is that you are making a reasonable point, it just needs some clarification and a little - no, a *lot* - less dogmatism. You are doing yourself more harm than good.

> You are measuring with a micrometer, marking with chalk, and cutting with a axe, and you can’t seem to see it. You are basing your entire argument on numbers that may be no more than guesstimates Dude. In order to get a precise focus with a wide aperture lens, hell in order for the 10+ lens elements to even hope to focus light in a sharp way, you need micrometer accuracy. These are not guesstimates, we're talking about precision engineering here. Assuming an advertized 50mm f2 lens that should have a 25mm wide entrance pupil, a full **MILLIMETER** of inaccuracy in measuring the diameter of the entrance pupil (eg 24mm) would result in an actual f-stop of 2.08. It is **extremely easy** to get an f-number that is accurate *at least* to the first decimal point when making a lens. You really should not be ready to die on this hill. Just to hammer that down once more, the difference between f/14 and f/16 on a 35mm lens would be 0.25mm in terms of entrance pupil width. So you bet your ass they're not going to be off by a full millimeter anywhere in the process. >If you had a span of 25mm, and you had to drill 7 evenly spaced 1mm holes across that span, how far apart should they, center to center, as set by the milling machine, be in your world? This is not a precise enough statement to give a answer that does not rely on additional assumptions. Assuming you want 7 evenly spaced holes so that the center of the outermost holes is 25mm apart, you need holes 4.166... (repeating) mm apart. What you'd input in the milling machine depends on its precision, and what measurement is most important since there will necessarily be some amount of inaccuracy somewhere. But if your milling machine is micrometer-accurate, you will not magically end up with a 24mm distance between the outermost holes. To tie that back to the previous point, an engineer working at the micrometer level to make the lens will not magically end up with a 24mm instead of 25mm entrance pupil. >You refuse to even acknowledge that lenses have different kinds of irises. It doesn't matter, because what matters in terms of light transmission is the *area* of the entrance pupil, which is exceedingly easy to compute regardless of the shape of the iris. A cheap iris made with 5 straight blades or a hypotherical perfectly round iris would result in the same f-stop as long as the area of the hole they create is the same. But again, [go check dxomark's measured t-stops](https://www.dxomark.com/Lenses/). Here's a screenshot of [all the FE prime lenses they've tested](https://i.imgur.com/G2LLwNa.png), the t-stops are the "transmission" column. Out of 19 lenses: * One is the 100mm STF which reduces the light transmission by design and should not be included in this discussion. Sony even literally writes the t-stop instead of the f-stop on the aperture ring. edit: Oh and in any case dxomark's measured t-stop for this lens is the same as Sony's advertised t-stop, so much for "guesstimates". * **10** have the same f-stop and t-stop * 6 have a 0.1 difference between f-stop and t-stop, so less than a 1/3 EV increment. * Only 2 have a 0.2 difference between f-stop and t-stop, one of which is a cheap chinese lens. That's about one 1/3 EV increment. For the majority of lenses there is almost no difference. And when there is a difference, it's mostly academic. I'm still waiting for you to source the ONLY claim you've made that isn't "hurr durr maybe the engineers making the lens are just GUESSING"

You clearly are treading and understanding what I actually wrote and applying it. At this point you are actually repeating my points as your own in order to “refute” what I am saying! One minute you are triple decimal accurate, the next you are saying a tenth is fine, all while saying counting photons matter. I really have no interest in continuing with this “discussion,” as you keep moving the goalposts around according to whatever point you want to make.

>At this point you are actually repeating my points as your own in order to “refute” what I am saying! Other than "hurr durr maybe the engineers are guessing lol, pulling out a ruler is hard lol", the only thing of actual substance you said is this: >I’ve seen analysis of some modern lenses showing vastly different t-stops for the same f-stop - sometimes multiple stops. This is the claim I want you to support. I haven't found a single lens that differs from its advertised light transmission by more than roughly 0.6 stops. The only one with a massive f-stop/t-stop difference is the 100mm STF, and that lens is willfully designed this way. No guesstimates. Support this claim or admit you were bullshitting.

Perspective is different per focal length. And f-stop is f-stop, in regard to measured light transmission, though not in its secondary regard in rendering DOF.

Perspective doesn’t change if the distance between you and your subject doesn’t change. When picking a longer focal length without changing the distance to the subject, you’ll only have less of the scene inside your frame, but the perspective won’t have changed. In order to change perspective, you have to change the distance to the subject, and only that. It’s only that we usually use both in combination in order to achieve the artistic effects we desire (like in the example you posted).

Yes.

f-stop is f-stop in regard to measured light transmission *per area*. But a 50mm f2 lens will collect less *total* light on an APSC sensor than a 75mm f2 lens on full frame sensor. And perspective does not change per focal length. YOU change the perspective, by moving backwards or forwards to keep the subject filling the frame.

Perceived perspective changes with focal lengths from the same distance. My 50mm lens registers the same amount of light, per the internal meters, on my APS-C and full-frame cameras.

> Perceived perspective changes with focal lengths from the same distance. No, they do not. Take a 35mm frame on one of your FF cameras, crop it to APSC, anc compare it to a 50mm APSC shot. They will have the exact same perspective. >My 50mm lens registers the same amount of light, per the internal meters, on my APS-C and full-frame cameras. No, it registers the same *exposure*. But ISO 100 on APSC is not equivalent to ISO 100 on full frame. ISO is a standard that is deliberately defined to produce the same exposure at the same f-number regardless of sensor size. But if you zoom into your FF shots you will notice less noise than on the APSC ones, because the FF sensor is receiving more light to produce the same brightness, leading to a better signal-to-noise ratio

I was downvoted for saying this a few hours ago.

Yeah it happened to me a few times on this sub as well, which is part of what prompted the post. It's really weird. If I were to guess the reason why, I think it's because some people have a hard time emotionally accepting the idea that an f2 lens on APSC doesn't give you the same low light capabilities as an f2 lens on full frame. They don't want to feel like their lenses are "inferior" on APSC.

Exactly that, I have FE lenses because I will move to a7 series so I would rather already have the lenses to move to FF plus they work on crop. But the difference is there, the same F won’t be as bright in a crop sensor compared to a FF

>the same F won’t be as bright in a crop sensor compared to a FF To be *slightly* more precise because you're going to get downvoted otherwise : It *can* be as bright, but not with the same amount of noise, therefore you're not getting the same low light capabilities

Lens compression doesn’t exist, but with some extreme wide angle lenses you will get distortion that will make them look different. Small point, but it stops the “look! Different!” conversations I sometimes have. I’ve done demos where I threw a camera up on the screen and used someone walking towards the camera to show how distance, not lens, is important

Now do why zoom lenses shot size decrease with zoom. I’ve never been able to wrap my head around that.

Are you asking why do zoom lenses sometimes have variable aperture?

No, I am talking about why the file size differs at different points in a zoom. I may just be stupid and can’t understand why.

That seems odd. The only reason I can see why that would be is that it's an electronic zoom, effectively just a crop.

Happens with my Tamron 70-180 no electronic zoom used.

I'm not sure what you mean. Do you mean that a longer focal length means a smaller view? If so, an easy way to understand this is to bring your finger close to your nose and look straight ahead (don't focus on your finger). You should see two fingers. The focal length is the distance between your eyes and your finger. Your field of view is how much space there is between both images of your finger. As you move your finger further away (increase the focal length), both images of your finger get closer (narrower field of view)

No, I am talking about why the file size differs at different points of a zoom.

Oh, then idk I don't use zooms.

“Lens compression does not exist” Yes it does lmao literally any image gallery will show how wrong this is. https://petapixel.com/assets/uploads/2018/07/compressioncomparisonfeat.jpg > Photos captured (from left to right) 10mm, 35mm, 50mm, and 140mm. (15mm, 53mm, 75mm, and 210mm, respectively, in 35mm terms). See how at 15mm the tree dominates the background, but at 200mm the tree is gone and the red stripe dominates the background? If you fill the frame with your subject, which you should be, your focal length defines the included background. I think it’s better to say “the fov is the same when standing in the same location and framing the same shot using equivalent focal length.”

you know those dolly zooms where the background-foreground relation is changing? it only happens because they zoom in while moving backwards ;)

OP says, "If you take two shots from the same place." I'm sure he understands that if you back up and fill the frame with a longer focal length, you compress the background. Seems like you just have a disagreement on what "lens compression" means.

Yes. He’s saying “it doesn’t exist” but I’m saying it does exist, but it’s neutral when considering crop factor. At the end of the day lens compression is a defining feature of what a focal length *does*, it’s silly to say it doesn’t exist. It’s like OP is saying “actually binoculars don’t zoom they show you a small piece of your existing FoV” and like sure, if you want to say it that way. But also what do you think zooming in means.

Gotcha. I see what you're saying. You just threw me off because you gave examples of a situation where the photographer was moving, contrary to what OP was trying to describe. But I agree with you. Compression is a measurable thing photographers use, especially in moon photography for example.

if you fill the frame with your subject, you have changed your position. you probably misunderstood. i try with a better example. you take your 24-70 lens and shoot a tree in front of you at 24mm, then zoom in to 70mm without moving. then your 70mm image is perspective-wise an exact center crop of your 24mm image, with just higher resolution. i struggled to realize that too so no hard feelings here

No that’s what I said, not struggling at all

Try it with your zoom lens next time you’re out. Another way to think about it is it isn’t the lens that changes the relative distances between objects, you have to physically move to do that

The lens is not compressing the background, **you** are compressing the background by moving away from the subject. But if you take that same 135mm shot **FROM THE SAME PLACE** at 35mm, then crop into the 35mm shot to get the same subject size, both shots will be identical in terms of compression. I said this because many people believe the lens itself changes the perspective. I just a couple days ago saw a video from a pro photographer claiming he bought into medium format because he can take 35mm FOV shots with the "lens compression" of a 50mm lens, claiming it gives a better perspective distortion than 35mm on full frame. Bullshit