I've gotten a few requests to go back and cover the exposure triangle a bit more in-depth, so I'm starting with ISO.
ISO is the measure of how sensitive your camera is to light. Turn it up, you effectively get more light (notice I didn’t say literally. You don’t actually increase the amount of light hitting your sensor. Instead, you increase your sensor’s sensitivity to the light). The tradeoff of increasing the sensitivity is that you signal-to-noise ratio is increased. This manifests itself as noise or grain in your images. Raise your ISO too high, and your image quality suffers. At ISO 100 your images will basically be noise-free, but taking your ISO up to 12800 (shown on our cameras as Hi 1.0) will make them unacceptably grainy.
What does this mean for you? Basically, you want to shoot with the lowest ISO possible while still exposing your photos well. If you don’t have to shoot with a higher ISO, don’t. But, sometimes you need to raise your ISO because you’re in a low-light scenario. It’s better to sacrifice grain for sharper images than to have less grainy images that are either too dark or too blurry because you kept a low ISO and shot with too slow of a shutter speed.
These are the Whole Stops (depending on how you have your camera set up, you will have either 1 or 2 additional values in between each of the values below. Our cameras are set 2 third stops in between each whole stop)
100 – 200 – 400 – 800 – 1600 – 3200 – 6400 – 12800(Hi 1.0) (these numbers reflect the ISO range of our school cameras)
Each 1 stop increase DOUBLES the light your sensor receives
Each 1 stop decrease HALVES the light your sensor receives
ISO 200 is effectively twice as much light as 100. ISO 400 is effectively four times as much light as ISO 100. ISO 800 is effectively sixteen times the light of ISO 100. The converse of this is also true.
Stops in photography are how we measure light. A stop is a universal measure of light across the different ways we control our exposure (ISO, Shutter Speed, Aperture, Flash Power, etc.). We’ll get into more into stops later.
Take a look at this article for examples and more details.