ISO matters!

As I mentioned in the glossary post, ISO refers to the level of sensitivity to light of the image sensor or the film. The higher the ISO, the more sensitive it is. It basically comes down to this: The higher the ISO, the more noise you will see in your images. It becomes a tradeoff you need to make depending on the available light. ISO starts at 100 for most cameras, and goes up from there. At ISO 200, your sensor grabs twice as much light in the same amount of time as it would at ISO 100. And at ISO 400, it would grab twice as much light as at ISO 200, and four times as much as at ISO 100.

So in that sense, the math is pretty easy(assuming a constant aperture, for simplicity). If you were taking a picture that needed 1 second to expose properly at ISO 800, the exposure times would look like this:

  ISO     Shutter Speed/exposure time(in seconds)
  100     8
  200     4
  400     2
  800     1
  1600    1/2
  3200    1/4
  6400    1/8
  *assumes constant aperture
 
Ideally, you would always use be able to use ISO 100, or high ISOs would not give you any noise, but this is not the case. So the available light will dictate what ISO you should use. If there are moving subjects, like people, the shutter speed will need to be at least 1/50, and probably more like 1/100 to get crisp shots, so you dial that in, choose a wide aperture(since available light is the limiting factor), and then just see what ISO lets you expose the image properly. You will need to see what high ISOs look like in your camera, and determine if that is something you are okay with or not. 

Here is a real quick test I took, where the ISO increased in each shot, in order to demonstrate how ISO affects an image. The shutter speed had to be faster each time, to compensate for the ISO going up and collecting more light. 

As you can see, the noise level by 3200 are quite high, when compared to ISO 100. So this should illustrate why you don't want to shoot at high ISOs if you don't have to. But the available light dictates that. If you are taking night pictures, and you will be using a tripod, you may be able to drop the ISO to a low level(and minimize noise) and just increase the exposure time(use a slower shutter speed).

One thing that you will want to avoid, is using a high ISO when you didn't need to, because this needlessly degrades your image quality. If you are outside, shooting portraits or landscapes, and you're getting shutter speeds of 1/1000 but are using ISO 800, that is technically "wrong". Because for landscapes and portraits, you do not need such a fast shutter speed, so you can slow that down quite a bit, and then use that extra light to reduce your ISO. This happened to me numerous times when I was shooting in low light the night before, and the next day I would run out and then be taking pictures of whatever it may have been, but forgot to check the ISO was at. Just something to watch out for. 

Here are some shots at "high" ISOs. Just to show you that you shouldn't be afraid to crank it up when you need to. 

ISO 2000, 50mm, 1/100, f/1.8

ISO 3200, f/3.5, 8mm, 1/60

ISO 12800!(by accident!), f/3.5, 8mm, 45 seconds

In the end, like everything else, ISO is just something you will need to experiment and see what works for the type of photos you are taking. The levels of noise even at high ISOs like 1600, 3200, etc. using modern cameras are becoming more and more controlled and yielding very acceptable images. So don't be afraid to crank up the ISO when necessary. A sharp picture with a little noise that is better than a noiseless blurry picture, or no picture at all.