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Are Camera Manufacturers Cheating You in Regards to ISO And Aperture?

By Paul Faecks on May 31st 2014

Have you ever wondered how camera brands manage to build a zoom lens with a constant aperture from wide angle to tele? Well, I have. Almost all camera manufacturers have cameras with lenses that just sound too good to be true. Tony Northrup makes a case in the video below, how camera brands fool you. Tony backs up his theory with multiple formulas that make absolutely no sense to me (I’m a complete math-nobrainer…), but Tony does a great job explaining it in an understandable way. This video is a bit long at 37 minutes, but it does give you something to think about.


Every photographer should be familiar with the crop factor and how it influences the framing  of your image. If you aren’t, we have some great articles on this topic here and here.

Here is what Tony Northrup argues: Whenever you use a lens with any given focal length, you have to multiply that focal length by the crop factor because the focal length that is written on your lens is usually equivalent to full-frame 35mm sensors. Nothing new.

BUT, the aperture is the focal length divided by the diameter of the opening at the front of the lens. So whenever you have to change the focal length because of the crop factor, the aperture has to be changed as well.

Let’s take a look at an example: if you have a 100mm lens with a 25mm front opening you have a lens with a maximum aperture of f/4. Makes sense, doesn’t it?


When we change the 100mm in our calculation above, we of course have to change the aperture as well, because the aperture doesn’t describe the size of the front opening, the aperture describes the RELATIONSHIP between focal length and front opening.

And here is the problem, according to Tony: Many camera manufacturers “forget” to change the maximum aperture in their advertising material.


Take a look at this one: the Lumix 12-35mm f/2.8 is advertised as the equivalent to a 24-70mm f/2.8 lens. Let’s just do the calculation:


If you divide 70 by 12.5, you don’t get 2.8. Instead what you’re getting is 5.6. That proves that the Lumix 12-35mm lens would be a 24-70mm f/5.6 lens equivalent to 35mm Full-frame.


Have you ever thought about this? Do you agree with Tony and his claim?

[via Tony Northrup, images via screenshots]

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Paul Faecks is a portrait- and fine art photographer, based in Berlin. If you want to check out his latest work, you can do so by following him on Instagram or by liking his Facebook Page

Q&A Discussions

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  1. John

    Hi, didn’t have time to read every comment so I’m not sure if this was mentioned. 35mm equivalent only relates to angle of view captured at the sensor. The focal length or magnification doesn’t change no matter what camera you put a lens on. That said, the aperture is as stated on lens. If anyone needs more explanation of this drop a line.

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  2. Richard

    Here is a reason why manufacturers are not cheating us
    No one disputed the fact that the D.O.F. is different between full and crop sensors but exposure is the same
    If you pick up a 250mm F1.8 lens
    What is the first thing that pops into your heads?
    WOW shallow depth of field
    WOW fast lens

    Just today SLRlounge has an article about the “Fastest lens” Not the shallowest depth of field

    Everyone thinks of the speed and the speed does not change

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  3. Ed Hubert

    Total bunk, because to get the same amount of light in to the sensor you have to move back, which means you DOF will have to change to get the same exact picture. So your f-stop has to change. If you stay at exactly the same distance the you are cutting the amount of light down. This really is not that difficult to figure out because two of the things that control your DOF are lens length and distance from subject. There is no way to get the same exact picture unless you change one if you change the other.

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  4. Samarth Singh

    This guy’s theory caught my attention but within1 minute I judged that its wrong. Read carefully Mr. Researcher:

    It doesn’t matter whether you put a 12-35mm lens, in a Micro 4/3rds, APSC or a full frame or Medium format camera….
    In each case, it will remain a 12-35mm only!! Does the internal focal length changes by changing the camera body? No, it doesn’t.

    And in each of these cases, the light reaching the sensor (per unit of area) is the same…. Hence, Aperture doesn’t change.

    Conclusion: By changing camera body, the frame coverage changes (the bigger- the wider), but aperture remains same.

    We calculate Full frame equivalents mostly to help the film photographers understand the frame coverage. Full frame equivalents are not actual focal lengths, they’re convenience numbers.

    Its an under-researched article and I can challenge this guy for a debate on this topic.

    What do you think guys? Am I wrong?

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  5. Bill Taylor

    So, every single camera (with the same sensor size) ever made has exactly the same noise? Grab a really old full frame and a brand new 5DmkIII and compare the noise levels.

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  6. Bill

    Hi Guy’s I’ve watched the Video and sort of under stood it. So I got my A700 and my A900 and did the test with a 50mm. The A700 is 1.5 crop so I took the same photo on tripod with same aperture, shutter and ISO on the same subject and yes the background blur was different. I did Tony s Math and altered the aperture on the A700 and guess what I got the same image with same background blur. I think Tony s point is if the companys give you an 35mm equivalent then it should include the aperture because F1.7 on my A700 is not the same on my A900.

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    • Romain Menke

      Yes and no….

      You did not get the same background blur because you moved the camera back and adjusted the focus.
      This has nothing to do with the camera type, sensor size, or how manufacturers advertise gear. 35mm equivalent is only to give an idea of the field of view, so people can sort of imagine what kind of lens they are buying. Keep in mind there are many different crop factors and the focal length of 35mm lenses is the most known way to describe the field of view of lenses

      To get a similar background blur you can compensate by changing the aperture. (like Tony says) If Tony had started and stopped there I would be fine with all this. But he didn’t, he kept on going about false advertising (wrong), total light gathered by the sensor (inventing a new term doesn’t make it a thing), field of view,….

      The bottom line is simple:
      when buying a camera and choosing a certain crop factor you have to do this for a raison. Expecting a point a shoot to perform like a FF DSLR is just ****(insert mild insult here)

      However the lenses do perform just the same no matter the sensor type or size. (the exception here are old, analogue era, lenses with a CMOS sensor, they give more noise)

      You just crop a piece out of the image circle. Understanding this will allow you to make better buying decisions and make you a better photographer. Tony’s funky math will not help you in anyway.

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  7. So funny

    Ouch ! Tony is very confused.

    I started guessing there was something wrong when Tony put the formula for SNR= P / √P. Anyone who put a formula like this in a presentation without realizing that P / √P is in fact equal to √P must have flunked his math through college…or stopped math in middle school. That person is not qualified to give a presentation on concepts he does not begin to understand.

    I had to stop half way during the SNR part because I could not take the nonsense anymore. ISO and SNR do not depend on the sensor size but on the size of the sensor cells (or pixels), it doe not matter what the sensor size is. “ISO is proportional to pixel size” is a much easier concept that the nonsense dished out by Tony -and- as a bonus, it is true. So instead of making something simple complicated and false, just keep it simple and true.

    Tony, a little knoledge is a dangerous thing. I recommend you speak with people who have a better understanding than you do on the subjects you covered and reshoot the video.

    Seriously SLR lounge, it harms your reputation to keep this video online. You are defending Tony but should worry instead about the people who will view the video and become as confused as Tony is.

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    • Stan Rogers

      To be completely fair (and without desiring to stir up any additional controversy, or any desire to appear to be agreeing with the premise of the article and linked video):

      To get a complete picture of the noise component, you also have to consider quantization noise. That comes in two flavours: depth quantization (how many bits are used to represent the photon collection at the sensel and the statistical accuracy of that representation); and spatial quantization (how many pixels are used to represent the image, and the relationship those recorded pixels have to the image itself, which is affected by colour filter matrices, anti-aliasing filters, and so forth). Of necessity, a smaller sensor will either have greater spatial quantization noise (fewer sensels of about the same size for the “same” image) or greater depth quantization noise (a similar number of smaller sensels, giving a greater statistical sampling error per pixel while effectively raising the electrical noise floor). So while it is possible to state categorically that *when pixel-peeping* (that is, when considering only the per-pixel error) it is possible for a small sensor to perform exactly on par with a larger sensor that uses the same technology, when the entire image is considered, a smaller sensor will always produce greater noise than a larger sensor of the same generation.

      Now, taking off my pedant hat, I’m happy to say that the point is nearly moot. When its images are down-sized to typical web dimensions or printed and viewed from a reasonable distance for the size of the print, there isn’t a single current-generation “serious” camera that can’t get the job done at ISOs up to 6400. (I’ll leave the non-i/”thousand series” Canons out of the discussion.) That’s four stops better than my “OMG low-light emergency, live with the grainstorm” film and a stop better than my “you’re kidding, right? you seriously want me to shoot this?” double-nought spy film (P3200 rated at 3200) from the Late Acetate Era. Full-frame 35mm sensors or the new crop of CMOS MF cameras/backs will give you a cleaner ISO 6400 image, certianly, and most will go higher before they become surveillance-quality. And it’s really, really nice to be able to go there, and there are circumstances (particularly in the sport and concert genres) where it can be pretty much necessary, but really, we’re talking about taking pictures under circumstances where an out-of-frame passer-by answering a cell phone or lighting a cigarette is going to TOTALLY DESTROY YOUR LIGHTING. That really is the level of sensitivity we’re talking about.

      So yes, the Four Thirds sensor is almost exactly the same size as a 110 film frame, and there is an extent to which you can expect 110-like performance. You’ll be using shorter lenses for the “same” images, so DoF at a given f-stop will be smaller at infinity. And there will be significantly less bellows draw for any given subject distance, further increaasing DoF. (Head shots with my 8×10 were macro photography — more than half life-size — and had the same DoF problems you’d have shooting, say, a dandelion head with a full-frame 35mm format camera. Apertures of f/22 to f/32 barely got the job done, and the ears would still be falling out of focus.) But no 110 camera was ever going to give you a nice 13×19 print even if you hand-stripped Tech Pan into a cartridge and could override the ASA tabs.

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  8. Jorge

    The premise is wrong therefore the conclusion is wrong too. The entrance pupil is not the physical diameter of the front element in a system of multiple elements but the diameter of a virtual image located behind the first element.
    As an example I’ve mentioned the Nikkor 6mm f2.8. This lens has a front element with a diameter of 237mm so if we use the formula as seen in the video the f number will be 0.02 and not 2.8 and clearly that’s wrong.
    If n=f/D then D=f/n so this lens have an “entrance pupil” of 2.14mm and not 237mm.
    In a zoom with fixed f number in all the range the diameter of the virtual image (the entrance pupil) is variable and that’s how they keep the constant aperture in all the range.
    I really like SLRLounge but I think you should have to interview an expert in optics with a physics degree.
    And please stop using the crop factor. That’s only to comparing the angle of vision of a system to a 35 mm frame size and just for that.

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    • Romain Menke

      I did not know that bit about the diameter of the virtual image.
      So something informative did come from this post ;)

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  9. Hrvoje

    HOnestly, i haven’t even watched the video, because from the article it is obvious that it’s all bunch of bull…
    Please let me point out one simple starting fact that makes the whole theory flawed. When a manufacturer gives you a 12-35 f/2.8 lens for a 2x crop factor sensor like 4/3rds format, anyone with a right mind and basic optics knowledge knows that it means it gives you FOV (FIELD OF VIEW!!) equivalent, for that sensor size, of 24-70 on 35mm format, not that it is ACTUALLY 24-70mm lens. Physically, TRUE focal range of that lens is still 12mm to 35mm, and those are the numbers you have to take into account when calculating the aperture ratio.
    I follow SLR Lounge on a regular basis, and i can’t believe that you posted something so idiotic… Kudos guys :-( Paul Faecks, it’s not about math, it’s about basics man :-( Don’t write about stuff you know too little about, please.

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  10. Romain Menke

    Since Tony is actively responding to this post I will put my thoughts here.

    First of all , I understand what Tony is saying:
    -to get the same result with camera’s with different sensor sizes you should change aperture settings, iso, and focal length of the lens

    -when camera manufactures make comparisons between their lens for a small sensor and a 35mm equivalent they are lying to people who think they will get an actual 35mm (sensor size) lens

    The 35mm equivalent however is the way to express the field of view in a way that has become a standard in photography.

    But all the math, conclusions and the intent behind all this is so incredibly flawed, that Tony should not be allowed to speak on the internet.

    First of all:
    -ISO = light sensitivity of a specific unit

    in film this was a single grain, in digital photography this is a single pixel
    Tony’s calculations for light gathered are based on the sensor size and not the pixel size and thus wrong.
    A 20mp digital back will gather a lot more light per pixel than a 36mp full frame nikon or sony.
    But an 80mp digital back will gather less light per pixel than a 5mp full frame camera.
    Sensor size is irrelevant.
    Why do we use ISO?
    Because it is practical to know how much light is needed to saturate a pixel or grain.

    -Aperture scale = a scale divided in F-stops that halves the amount of light going through the lens with each stop.

    -Maximum Aperture = the front element diameter / the focal length

    These things are completely unrelated to what is behind the lens.
    Neither the lens nor the light passing through it will behave differently because of the sensor size, or if there is a sensor or film, or anything at all.

    -So what is this thing about bokeh and depth of field and why does it change when you change sensor size.
    Well it doesn’t.
    You however will change the way you photograph.

    When shooting a portrait with a full frame camera and a 200mm lens you will position yourself and the subject in relation to the background so you get a nice blurred out background and the face entirely in the frame.

    When you put the same lens on a camera with a smaller sensor, you will have to move back to get the person in the frame.
    In doing so you reduce the distance between the person and the background in relation to the distance between the camera and the person.
    You focused farther away and depth of field increases when you put your focus more towards infinity.

    So the lens didn’t change, you moved your feet.

    -So what about field of view.
    To understand field of view you need to know where it came from.
    Field of view was the relation between the image circle and the subject circle.
    (lenses project circular images)
    A telelens will gather a small circle in front of the lens and disperse those rays to create a (relatively) large image circle
    A wide angle lens will gather a very large circle in front of the lens and converge (compress) those rays into a (relatively) small image circle

    This relation defines an optical distortion. Choosing a lens is also choosing a specific optical distortion.
    Again the lens doesn’t care what you do with the light.
    It will distort the image equally no matter the sensor you will use.

    Back in present time field of view now means the relation between the sensor size and the subject circle.
    This gives the wrong impression that the lens changes when you use different sensor sizes.
    In fact the old standard still stands, the optical distortion is still true no matter the sensor size.
    What changes is how much of the image circle you crop.
    This is also why it is called a crop factor.

    It doesn’t effect ISO, it doesn’t effect noise, or depth of field, or bokeh, light gathered per pixel or anything else!

    So why can’t you take full frame like images with a crop sensor camera?
    Well because you want to get the same framing (crop of image circle) with the same lens (field of view and aperture) from the same distance (depth of field/bokeh and perspective).
    This is impossible. If you would make a 35mm equivalent lens for an APS-C camera with a big enough aperture to allow the same depth of field, you would still get a different optical distortion.

    One last thing:
    SNR (signal to noise ratio) = Signal / Noise

    Tony’s equation is just weird and I would like to know where he got it from.

    So a true buying guide:

    -if you like optical distortion of wide angle lenses, buy a camera with a smaller sensor and corresponding lenses.
    -if you like the optical compression of tele lenses, buy a camera with a bigger sensor (a large format camera with a digital back and a stitched image gives amazingly compressed images)

    -if you like noise, buy a camera with very tiny pixels (technology advances, newer is worse)
    -if you dislike noise, buy a camera with big pixels (technology advances, newer is better)

    -if you like funky math start reading about Quantum mechanics, lots of math that makes no sense but is actually correct

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  11. Gonzalo

    The last picture is funny, why would you divide 70mm by 12.5 when the lens you are talking about is not 70mm but 35mm?
    Tony may think he knows a lot about physics, but definitely he has serious problems with basic orthography; remember what the parenthesis symbol mean? Then go back to the slide he is accusing of cheating, and see that the facts are 12-35 and f2.8. The 35-70 equivalency is between brackets, and I don’t need to explain what than implies, do I?
    Anyway, this ridiculous discussion would finish if we would stop talking about 35mm (arbitrarily adapted as a measure) and, instead, would talk about angle of view, in which case we would not need absurd conversions that mean nothing to photography, and only to physics.

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    • Tony Northrup

      I don’t follow your comment on the equation, but do understand that my changes keep total light gathered constant, and that in turn keeps depth of field and total image noise constant. It all works out quite beautifully and simply.

      And, in the name of science, I encourage you to repeat my tests. If I’m wrong, you’ll get drastically different results. But you might just get the same results that I did.

      And I totally agree that we should use angle of view instead of focal length. I also think we should measure total light gathered rather than light intensity.

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  12. Alex

    The focal length IS equivalent (When used on a MFT body) in terms of magnification from the same distance to a 24-70 Lens on a full frame camera.

    The Aperture IS F2.8, it allows the same intensity of light to fall on each mm² of the crop sensor as a 24-70 F2.8 would do on a Full frame camera.

    The aperture size is not the same, but the ratio between focal length and aperture size IS the same, and as such we are not being lied to, they are stating facts. It’s simply down to us to realise that using cropped sensor cameras with shorter focal lengths (to maintain equivalence) and consequentially narrower apertures (actual, not by ratio to focal length), will result in wider depth of field and more noise.

    Short of reducing the sensitivity of MFT cameras (Drop ISO 100 to ISO 25) and then re-labeling lenses that are really F2.8 lenses as F5.6 (or making F1.4 zooms and labeling THEM F2.8), this is honestly nonsense, and could not be implemented in a way short of madness.

    I wash my hands of this weirdness.

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    • Tony Northrup

      You’re right that light intensity stays the same for a given f/stop number regardless of sensor size, and I describe that in my video. But check my results, and you’ll see that depth-of-field, total light gathered, and total image noise all change, and that’s the point of my video.

      I don’t argue that MFT lenses should re-label their native aperture. I just argue that if you put the focal length in 35mm terms, you need to adjust the aperture as well, because if you convert only the focal length, then you’re overstating the lens capabilities.

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  13. Bernard

    I am sorry but this is a carload of bull.

    Before throwing such filth around, this Tony fellow should take the time to study optics. Which I have, incidentally, up to the PhD level… And which lens designers have.

    The crop factor ” is a plague. It has been abused and misused for so long it’s funny it hasn’t taken a vacation yet.

    Crop factor is dsimply a way to relate the field of view of different sensor sizes. NOTHING MORE. It doesn’t change the property of the lens one bit. For Pete’s sake.

    It also doesn’t change the fov, the iso relation, or the capacity of your battery pack.

    I like slr lounge, but seriously you guys should consider getting someone learned in optics to act as editor. Saying you “suck at math” (which isn’t something to be proud of) isn’t an excuse.

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    • Tony Northrup

      Crop factor isn’t simply a way to calculate how field of view changes; it’s a way to achieve similar results with different lenses and sensors. As I’m on a photography trip through Europe at the moment with a 5D Mark III and GH4, I’m relying on the math quite constantly.

      Crop factor is an abstract concept used for comparisons to the commonly cited 35mm format. Of course it doesn’t physically change your lens. I certainly don’t argue that, I just provide information based on my own experiments and research that can help you make more accurate comparisons.

      With that said, I’d happily address any errors you might bring up, but you only seem to be arguing that nobody should use crop factor… and that’s fine if you don’t need it, but many of us do find it useful.

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  14. Jorge

    According the way he use the formula the Nikkor 6mm f2.8 should be f0.02

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  15. Alfons

    What i gathered from Tonys video, is that the ISO on crop cameras are boosted to at higher number, and labeled as the correct number for exposure purposes.

    The comparison to water that instructors often use, can be used here as well. If a smaller sensor has the same number of pixels as a larger one, each pixel has to be smaller. Because of that each sensor pixel, is hit with less photons in the same amount of time than on a larger sensor. Because of that a smaller sensor is noisier, and Tony did in fact show that if you take ISO and aperture crop into account, the noise level is the same for crop and full frame.

    Manby I’m dense, but it makes total sense to me. Our crop cameras won’t suddenly take worse (or better) pictures because of Tonys video, so no need to get defensive about it.

    “A crop sensor is noisier because the pixels are closer together” – maybe, but on a full frame or medium format, the pixels are right next to each other as well – i don’t see any gaps in my photos ;)

    My guess is a crop is noisier because each pixel is smaller, and thus gather less light…. and needs to be boosted, so we are able to get the right exposure.

    My interpretation of what Tony said is that if the ISO wasn’t boosted, you would have to crop the aperture. I saw the video when it first came out, so this is all from memory ;)

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    • Tony Northrup

      You’re right-on, though the linked video doesn’t cover pixel density (because it don’t matter when measuring total image noise and depth of field).

      However, because it was such a common comment, i did address the effect of pixel density in the follow-up video:

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  16. Jorge

    First of all sorry for my english, is not my first language.

    According to my old physics book the f number is n=f/d where f is the focal length and d is the diameter of the “entrance pupil”. In a multiple elements lens this diameter is NOT the diameter of the first element but the diameter of a virtual image inside the optic system itself and can be greater than the diameter of it. He says “the opening at the front” and that’s wrong. That would be right if we were talking of a single element and that’s not the case of a modern lens. And why is he applying the crop factor to calculate the f number? That’s simply wrong.

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    • Shi Zheng

      crop factor changes your field of view, and therefore affect your focal length.

      100mm with a opening of 4mm is f25 ON A FULL FRAME SENSOR

      on a crop sensor it has a field of view of 150mm!
      so 150mm with a 4mm opening is f36!

      even if multi-element lenses have a virtual image, its a inherent trait of that lens, and if you change between the full frame and a 1.5x crop, that virtual image stays the same, so having a 1.5x crop on the focal length has a 1.5x affect on f-stop

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  17. Des Carter

    The size does make a difference if you use the same focal length and put a full frame sensor equal distance as a crop sensor your full frame sensor will be cropped too.

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  18. Richard

    So tell me
    If I were to take apart my camera, cut the sensor in 2 and put it back….does the F stop change?

    If I were to take smaller film and use that in my film camera……does the F stop change?

    No to both

    F stop is the size of the opening over the distance from the opening to the sensor

    F25 on a 100 MM lens is 4mm wide and 100 mm away from the sensor 4/100 or 1/25 or F25

    The size of the sensor plays no part what so ever

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    • Tony Northrup

      Richard, I’d suggest re-watching the video. This is the “same settings, same exposure” argument that I cover in this response video:

      What you say is totally true, and understanding this is a prerequisite to understanding what I’m trying to teach. So, you’re half-way there, and the rest will probably make sense if you re-watch the video.

      The key element to understand is the difference between light intensity and total light gathered. The light intensity stays the same, and thus the settings and image brightness stay the same, because f/stop number and ISO are both based on light intensity. However, the total light gathered changes, and in the video I demonstrate that total light gathered has far more impact on your final image quality than light intensity.

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    • Shi Zheng

      the sensor size plays a part in your field of view (focal length)!

      100mm with a opening of 4mm is f25 ON A FULL FRAME SENSOR

      on a crop sensor it has a field of view of 150mm!
      so 150mm with a 4mm opening is f36!

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  19. Des Carter

    I also agree with Tony on this. Another way to test is with flash. I use a flash meter to determine aperture on crop frame sensor and my pics are usually under exposed. I do the maths and get the right exposure. 50mm gives me 75mm equivalent but F2.8 is not the same, so have to drop to F1.7 to get close to F2.8 equivalent. When using Flash my expose never seems to be bang on and I believe this is why.

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    • Tony Northrup

      I appreciate the support, but I don’t think your flash meter problems are related. That’s measuring light intensity and exposure settings, which don’t change for different sensor sizes.

      Lenses transmit different amounts of light, so if you’re also changing the lens, that could be the issue. All lenses have some light loss; but some have more than others.

      The only other explanation i can think of for your problem is that your crop sensor camera might be a Fuji, because Fuji under-rates their ISOs to make their noise levels seem better than they actually are. I cover this at the end of the video. The in-camera meter adjusts for it automatically, but any external meter will cause the camera to under-expose.

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  20. Rich

    By this man’s logic, some Large Format lenses should be labelled as F/1.0

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    • Tony Northrup

      If you wanted to put them in 35mm terms, maybe, though most medium and large format lenses aren’t as fast as the faster 35mm lenses… More because of the popularity of the 35mm format than anything else.

      I address this in my follow-up video in “The Medium Format Argument” section:

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  21. Rich

    SLR Lounge seems desperate for hits, by resorting to posting seemingly controversial issues – that do not exist.

    Conspiracy theory territory.

    Even the first argument quoted does not seem to understand what

    “35mm EQUIVALENT” means.

    The quality of articles on SLR Lounge is beginning to dwindle.


    Concerned reader.

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    • Paul Faecks

      Hey Rich!
      No, we’re not just desperate for hits.
      And these calculations have nothing to do with conspiracy theories.

      Have a nice day

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    • PeterF

      Mathematical formulas are a conspiracy? My word Rich, are people not supposed to have brains at all and just accept what is told them? Explain HOW the math is lying and WHERE the conspiracy is because if you can’t do that all you are doing is making yourself look like a fool.

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  22. Bokeh Monk

    Tony is exactly right about all this . . . It’s not rocket-science but it is optical physics. I’m somewhat gratified that I studied photography at college. I’d recommend everyone read Michael J. Langford’s “Basic Photography” and “Advanced Photography” the textbooks we were required to ‘prove to understand per exams’ in order to graduate. BTW the ‘Basic Photography’ volume alone will explode most heads out there with it’s depth into the science of photography!

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  23. Rob-L

    “aperture doesn’t describe the size of the front opening, the aperture describes the RELATIONSHIP between focal length and front opening.”

    The f-number describes light transmission. The mathematical formula for calculating an f-stop is constant and has nothing to do with sensor size. Think about it. Have you ever seen a light meter ask for what size sensor you’re using? If you supply an ISO and shutter speed, it will tell you the f stop to use for a proper exposure.

    The only time you need to convert the f-stop among various sensor sizes is for comparing depth of field. But since the f-stop number itself only describes light transmission, no one is “lying” when they list the f-stop of a lens’ specs.

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    • Jesse


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    • Paul Faecks

      What you say is true to an extend.
      The official Wikipedia definition for f-stop is:
      “In optics, the f-number (sometimes called focal ratio, f-ratio, f-stop, or relative aperture[1]) of an optical system is the ratio of the lens’s focal length to the diameter of the entrance pupil.”
      What you mean is probably the T-stop.

      You pointed out that the lightmeter doesn’t ask for a sensor size and well, thats true. But Tony Northrup explains in his video why that’s the case:
      The ISO describes the light that is being recorded in a specific area of your sensor. Which means: a bigger sensor gathers more light.

      But camera manufacturers of crop factor cameras compensate for that, that’s why smaller sensors usually have more noise: because their area is smaller which means they receive less light.

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    • Tony Northrup

      Hi, Rob. It seems like you missed much of the point of my video, but I’m glad we agree on the focal length and depth of field aspects. They key part you missed is that both ISO and f/stop define *light intensity* rather than *total light gathered*. Light intensity defines light per square inch of sensor, which of course causes the total light gathered to decrease with smaller sensors.

      Another aspect my experiments and research determined was that light intensity has no real impact on your final image, but total light gathered controls visible image noise in the final picture, and relates directly to depth of field.

      Once you acknowledge those facts, everything else falls into place nicely.

      Also, I created a follow-up video to respond to common criticisms: Specifically, there’s a section on the “but my settings don’t change” argument.

      I’ll add that this isn’t based on my opinions, but rather my experimentation. I’d encourage you to repeat my experiments and see if you don’t get the same results. I’m open to the idea that I made mistakes (in fact I did correct a mistake not related to my core thesis in the follow-up video), but I do need to see some real evidence. Thanks!

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  24. Peter Tellone

    Most Flawed Psuedo-scientific theories I have ever heard…had to stop 1/2 way though

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