ISO is another way that you can control the overall exposure of your image. And like F-stop or aperture and like shutter speed, it's just a series of numbers.
Traditionally ISO, or as it used to be called by some camera manufacturers, ASA, is a standard that relates to the quality of the image. So back in the day, and still to this day, you'll get stuff like this. This is a roll of Kodak Portrait 800. This is ISO 800. So in theory, the grain quality in this film at 800 is going to be, for all intents and purposes, the same as the quality on this camera when I'm shooting at ISO 800.
So what exactly does changing your ISO do? Most cameras will start at ISO 100 and jump up to 200, 400, 800, 1600, 3200, and some cameras will even go as high as 6400 and you'll find a couple that will go even higher than that. Basically ISO controls the overall quality, like I said, of your image. ISO 100 is more reserved for bright daylight, maybe outdoors. Whereas something like ISO 6400 or 3200 is more reserved for indoor locations, shooting in low light.
For example, if I were shooting a concert, I'd probably shoot it around 32, 6400. At ISO 6400, you need a lot less light to expose the sensor. However, you'd trade off quality of the image. So you're going to have a lot grainier of a photo than say if you were shooting at ISO 100 would be much, much crisper.
Most people when they are setting their camera's exposure, they'll set their F-stop and their shutter speed first and then they'll address their ISO accordingly to make sure that they're getting a fast enough shutter speed and the appropriate F-stop. As cameras get better and better, this number here gets less and less grainy. And it's actually gotten to the point where digital cameras can shoot in much better low light situations than traditional film cameras and even more than that digital cameras can now basically see better than the human eye in low light conditions.
And this isn't the world's most interesting photo. It was shot for a test I was doing on the Nikon J1 camera. But here we go, this is ISO 6400 and you can see if you zoom in, that it has a lot of grain, a lot of loss of quality. And this is also ISO 3200. These are the two highest ISOs on the Nikon J1 camera. And then these are the two lowest. This is ISO 200 and ISO 100. Again, I had to change the aperture and the shutter speed to get these to all be the same exposure, but again you can tell things look a lot smoother here than they do here. And that's just an important thing to keep in mind with ISO, that when you do crank it up in low light, you are going to see some quality loss.
So that's the basics of ISO and how it relates to your camera and your overall quality of your image.