ISO ASA DIN
We have talked about aperture and shutter speed as 2 different ways to control the exposure of a photo. Now we will talk about the final factor the ISO or in basic terms the sensitivity of the film or digital sensor. With the ISO the higher the number the more sensitive the film or sensor. That means an ISO of 100 needs twice as much light for correct exposure as an ISO of 200 and so on up the scale.
So you ask, “Is that all it controls?” Well no, just like the shutter speed controls motion blur and the aperture controls depth of field the ISO also controls another factor of the image. When the ISO goes up the more grain or noise you will see in the picture. If you are using film the higher the ISO the more grain, which is normally something you want to avoid. That is not to say that grain can sometimes be nice and some photographers even use it to add texture but too much usually is just a distraction. A big difference between film and digital is that digital grain is called noise and it is almost never pleasing. That means that too much noise always looks bad.
The photo above of the dancers was taken with an ISO of 2500, the aperture was opened up to f4.5, which was the widest for that lens, and the shutter speed was all the way down to 1/40 of a second, which as you remember is really slow for freezing action. If I had a faster lens, like an f1.8 I could have used an ISO of 800 and shutter speed of 1/80 of second which would have been much better for this image. Not only would it be easier to freeze the motion at 1/80 of second over 1/40 of second but there would have been substantially less noise. Take a look at the blow up below to see what I mean by too much noise and this is after the use of software to reduce it.The lesson from all of this is the lower the ISO the better unless you have no light and then you can push it up but try to keep it under control.
On another note, newer cameras are doing an incredible job of reducing noise with larger sensors and better overall control of noise. This image was taken with a Nikon D300, which has 12 megapixels and actually does pretty good in low light.