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.