Changing ISO on a digital, what happens

[Grandpaw]

I know when you change the ISO on a digital camera it changes the sensitivity of the sensor but can anyone give me a simple grandpaw version of what is going on, Jeff

[Len J]

I think it increases the gain.

It amplifies the signal coming off the sensor.

Len

[Franglais]

You remember tape cassettes? Usually you record on one and it comes out fine. Sometimes you make a mistake and you don't record it stongly enough. When you playback the sound is faint. Never mind. You just increase the volume (gain) on your amplifier. Now it sounds just fine - except that in the quiet bits you notice there is more background hiss i.e. noise.

Camera sensors are just the same. At their nominal ISO setting you have a big fat signal. As you increase the ISO the basic signal gets fainter so the camera amplifies it (increases the gain). But this also increases the background noise.

What's amazing is that it works so well. Much better than when trying to push film.

[Greg McCary]

I would think it would just make the camera meter faster or slower. Just like a film camera?

[Grandpaw]

You remember tape cassettes? Usually you record on one and it comes out fine. Sometimes you make a mistake and you don't record it stongly enough. When you playback the sound is faint. Never mind. You just increase the volume (gain) on your amplifier. Now it sounds just fine - except that in the quiet bits you notice there is more background hiss i.e. noise.

Camera sensors are just the same. At their nominal ISO setting you have a big fat signal. As you increase the ISO the basic signal gets fainter so the camera amplifies it (increases the gain). But this also increases the background noise.

What's amazing is that it works so well. Much better than when trying to push film.

Thanks folks for the quick replies. This was a simple explanation that makes since! My kind of example, Jeff

[another view]

I would think it would just make the camera meter faster or slower. Just like a film camera?

That's what a film camera does because it knows you're putting in more (or less) light sensitive film. With a digital camera of course there's no film so changing the light meter's reading is only half of it - the other half is amplifying the sensor's signal.