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ISO has nothing to do with your exposure – Here’s how digital ISO really works

by 27 Comments

Everybody knows the exposure triangle by now, right? ISO, aperture and shutter speed. Once you know your scene’s exposure value, you can just balance out the three to get a good exposure. And as you adjust one up or down, you need to adjust another in the opposite direction to compensate. Well, what if one of them isn’t really anything to do with exposure?

That’s the argument put forth by Chris Lee of the YouTube channel pal2tech, and it’s a compelling one. Back in the days of film, it was a little different, and your ISO really did reflect the sensitivity of the film stock to light. These days, though, with digital cameras, not so much.

 

With digital cameras today, your sensor only has one native level of sensitivity. Everything after that is just electronic jiggery-pokery caused by the analogue-to-digital converter inside your camera. In any given scene, regardless of whether your camera’s set to ISO100 or ISO3.3 million, with your aperture at the same size and your shutter at the same speed, the same amount of light will always be hitting the sensor. And regardless of the ISO, the sensor is always sending out the same signal to the rest of the electronics.

From there, ISO is simply an amplification of that signal before it’s converted to digital ones and zeroes. And the “extended” ISO range? Well, that’s just maths (and not even clever maths), after your image has been converted to digital. It’s the ISO equivalent of “digital zoom”.

It’s a very misunderstood thing in the world of digital photography, and many still describe ISO as the “sensitivity” of the sensor, when it really isn’t anything like that at all. When it comes to digital cameras, ISO is essentially “gain”. In video cameras, this ability has actually been called “gain” for years. With photographers coming from decades of film, where ISO was a real measurable thing, the name just kind of stuck when we all made the move to digital, because it eased the transition. We’ve just kept it ever since.

Chris admits that adjusting the “sensitivity” is still a useful way to think about the practical effect that altering your ISO has, albeit incorrect from a technical standpoint. Chris says he’ll be delving a little deeper into this in a future video on “ISO Invariance” (another hotly debated topic). So, be sure to subscribe to his channel to keep an eye out for that one.

[via PetaPixel]

Related posts:

A comprehensive visual guide to equivalent exposure Default ThumbnailBack to Basics – ISO and exposure wrap up Making sense of aperture, shutter speed and ISO with the exposure triangle Learn the exposure triangle easily with a little help from Kool-Aid

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John Aldred is based in Scotland and photographs people in the wild and animals in the studio.

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  • Astro

    It is exactly the same as turning up the speaker volume. You get louder sound but louder static, too. It doesn’t alter the original recording at all.

    • Richard Jackson

      Volume up to 11 for the win!

  • Good video. I’ve always said that about ISO. ISO is the same as film. The faster, sensitive film has bigger silver halide grains which degrades the image overall. You only use it in a pinch. Exposure is only aperture and shutter speed. ISO only comes into play when you run out of aperture and shutter speed combinations to properly expose the image.

    • John Wojciechowski except when it comes to ISO invariant sensors, which there are few of. But there are sensors that scale performance really well, in terms of noise, at higher iso.

    • David Sargent, same as film. No two film makers ISO ratings are alike, even from the same manufacturer. Photogs has to buy bulk film by lot numbers to get consistency.

  • It does not really matter how it works, but that it works in the same way to get a photograph that you would not get set to iso 100.

  • Why? Don’t you?

  • Isn’t this want Tony Northrup was talking about in a video last year? Pretty sure he got flamed hard for it😂

  • Remember when Tony Northrop said the same basic thing last year and got totally lambasted for it? Some one should say sorry. Just sayin.

  • Daniel

    We’ve had “equivalent” focal length. Now we can have “equivalent” ISO.

  • Move the ISO setting and watch the exposure change.

    Stop trying to get clicks with silliness.

  • Man, he needs to calm the fuck down

  • It is basically the same as saying: lets forget about ISO and just save files in 20stops RAW files and pump up the ‘iso’ in post to the right levels. But stating you should only use Fstop or shutterspeed for exposing a picture is stupid. Fstop and Shutterspeed determine the look of your picture. Those things are locked for art directional reasons, the only thing left is to get the right exposure is ISO.

    • Richard Jackson

      And that’s exactly what happens in video, where your fps is often locked, leaving only two other aspects available to adjust your image exposure.

  • I think the guy is simply playing with semantics. To simply say that aperture and/or shutter speed are the ONLY things affecting exposure is only partially correct. For example, if I was asked to take photos of a high school football game (notoriously bad lighting) I couldn’t simply set the ss at 1/1000 and aperture at f/2.8 while paying no attention to the ISO. If I were to use an ISO of, say, 400, the image would come out horribly underexposed (e.g. it would be so dark as to make the image basically useless.) On the other hand, if I cranked the ISO to 6400 or higher the image would be usable in the sense that one could see the players and the histogram would not show under or over exposure at either end of the scale. People looking at such an image would conclude that the exposure was correct even though there was a lot of noise in the image. In the old days of film they simply called photos using high ISO/ASA film as being grainy.

    All you have to do is put your camera in manual mode, set your desired aperture and shutter speed, and then watch the exposure meter change as you increase and decrease the ISO. If the camera’s exposure meter (or even an external meter) is designed to help you set the proper exposure then one can only conclude that ISO is one of the three variants that affect exposure. After all, when using an external meter, you have to input shutter speed, aperture AND ISO in order to determine a correct exposure.

    My conclusion is that even though this guy may have some technically correct information, the fact is that, to the vast majority of us, adjusting the ISO is critical to us getting the correct “exposure.”

  • The triangle works ,use it please

  • I think he’s arguing semantic. Of course, ISO doesn’t control how much light hitting the sensor. However, for most people “exposure” means the brightness of a photo and ISO being included into “exposure triagle” because it is a tool that can increase or decrease the overall brightness. It’s great to know how exactly ISO works but trying to correct others for misunderstanding the terminology is a waste of time.

  • Owen Harvey

    It’s all about the end exposure.
    If you max out your shutter(motion blur) and max out your Aperture(shallow) and your still 5 stops under, ISO is the only way to go. It is an absolute essential. It is as important as shutter and aperture and deserves it’s place alongside them.

    The only point I could take from this video, is him saying that shutter and aperture are the pre setup for light hitting the sensor, and the ISO is for once the light has hit the sensor. OK, it’s an interesting thought nugget, but that is about it.

  • Stephen Weller

    “… jiggery-pokery…”? As a (retired) engineering technologist and an aspiring amateur photographer, I am deeply offended by your nonsensical butchery of the English language.

  • That was a great performance. It’s nice to hear someone say that there actually is a difference between digital and analog. What he says sounds logical enough. It might not be true for older lenses but in most cases it is not a problem to underexpose an image -3EV on K3 and then adjust it in post (if you shoot raw). I guess that one benefit would be better dynamic range

  • Of course it doesn’t. Same way in using a higher ASA rated film. The film doesn’t control light. Aperture and shutter speed does. So while, yes, the ISO plays a role, the only thing ISO does is allow you to use different aperture/shutter combinations to create a correct exposure. Not sure why or what part of what you’re saying people don’t get.

    • Where I disagree with this video is that taking ISO into account is one of the three elements that one has to take into account in order to get the correct exposure. Take sports as an example (and there are many, many more); the generally accepted rule of thumb for many sports is that one should use shutter speeds around 1/1000 and aperture around 2.8. If I were to shoot a night time high school football game using these settings, but having an ISO of 200, the resulting image would be way too dark. The vast majority of people would state that the image is under-exposed. While I understand that sensitivity to light is amplified as the ISO increases, and doesn’t change the actual exposure, one must still take into account what ISO to use in order to get a properly exposed image.

      No matter what this guy says, everyone uses shutter speed, aperture and ISO (to adjust for ambient light conditions) when making their settings. I think this guy is just trying to say “Hey look how smart I am.”

      • Timothyf7

        Actually, he is simply saying that ISO is not really a part of exposure. It is a computer process that alters the exposure after it is completed. Yes, it has always been part of the process, but what he was arguing was that it isn’t a true part of exposing a scene to the sensor.

    • Graham Martin you obviously take Iso into account. What this guy is simply saying just in a few short words, is that ISO has nothing to do with the actual exposure. In that regard he is correct.

    • Steve Schizas , I understand what he is saying and, in that respect, he is correct. What I don’t get is why he devoted an entire video to the subject. Even if people think that ISO changes the exposure, who cares? As long as they know how to set the ISO, isn’t that all that matters?

  • Simo

    so Tony was right after all

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