Nathan recently wrote to me with a question:
Can you tell me what actually happens inside a DSLR camera when you change the ISO settings. For that matter what does it do in a film camera.

Interesting question!

Let's start with the film because that's what the digital version of ISO is trying to replicate. Film coatings are, as you know, sensitive to light. The more light that hits them the more the chemicals react. But the chemicals can be mixed in different ways so that they are more or less sensitive to light. So a film that is very very sensitive to light will go from unexposed to completely over-exposed much quicker than a less sensitive film. In film a higher ISO number means more sensitive film. More sensitive film means faster shutter speeds and better ability to get sharp shots in low light. Higher ISO also usually means grainier photos. The difference in grain between 100 ISO film and 400 ISO film is clearly visible.

On some film cameras there is a dial that allows you to manually set the ISO. The idea is that you set the ISO to match the film you're using but some people use that creatively. On other cameras the ISO is set automatically by reading the metal stripes on the side of the film canister. Basically the ISO setting on a film camera is used together with a light meter reading to calculate shutter speed and aperture in order to get a correct exposure.

All this stuff with chemicals and the sensitivity of the film is what a dSLR is trying to emulate with the digital sensor. However, there's just one sensor in your camera. You can't swap it out for a more sensitive one when you need more sensitivity. So what the camera does instead is to amplify the electrical signals that come out of the sensor. I'll explain how that works..

First of all you have to think of the sensor as a grid of dots or pixels. For the sake of simplicity forget about colour - just think in black and white. What each little pixel in your sensor does is catch photons. Photons are little parcels of light. Each pixel can hold a certain number of photons. Let's call that number 100. It's WAY larger than that but 100 is a nice simple number to work with. So in our simplified black and white world each pixel of the sensor can hold up to 100 photons before it can't hold any more.

So when you're working at low ISO, with no signal amplification, light comes into your camera as photons and starts getting collected in the little sensor pixels. For a well exposed picture a bright spot might collect maybe 80 photons while a dark spot might collect only 20. That's fine because both are well within the number of photons our pixels can hold. So they send a signal back into the camera telling it how many pixels they collected during this exposure.

So next the sun goes down, it's twilight and let's say it's only half as bright as it was. If we shoot again with the same settings we now get only 40 photons in that bright spot and 10 photons in that dark spot. But our sensor pixel can hold up to 100. 40 is less than half of 100 so our picture is going to come out dark and muddy. So if we amplify the signal by doubling it we get back to 80 photons for the bright spot and 20 photons for the dark spot. That gives us a nicely exposed shot again.

Each stop of light you remove halves the brightness, in essence halving the number of photons being collected. So going from ISO 100 to 200 means amplifying the signal by 2x. Going from ISO 100 to ISO 400 means amplifying the signal by 4x and so on. More amplification means more grain.

So that's the simple picture of how it works. Of course the world isn't so orderly and there are variations in how much light reaches a pixel. So two pixels that should be the same brightness might through random chance get a slightly different number of photons. Maybe one bright spot might get 81 photons while another gets 79. That's a small difference if you're not amplifying the signal. But those small differences make a huge difference if you're doubling or quadrupling the signal. That's why you see more grain on a high ISO picture.