The ISO controls light sensitivity of the sensor, and together with the shutter speed and aperture it determines the exposure of an image. ISO numbers range from 50 to some hundred thousand depending on the camera you have. The higher the number the more sensitive the sensor is. This means that the more light available the lower your ISO should be. On the other hand, when it is darker and there is not a lot of light, you want to use a higher ISO to let the sensor take advantage of the light that is available.
You want to change the ISO to be able to best capture different scenes. If you only change your shutter speed or aperture you might be missing out on some great potential. And your shutter speed might be too long to be able to hand hold your camera. If you are only shooting in auto you are also lacking control of your camera, your settings and the outcome – so take control! If you are not ready for manual mode, then I recommend starting with aperture priority.
A high ISO comes at a price however- and that is noise. Noise or grain are little dots in your photo that can make it look quite… well…noisy. You will see more noise in your photos as you increase the ISO. To avoid this I recommend using a low ISO. I like to keep mine at 100 (the lowest setting on my camera), and I change just the aperture and shutter speed to get a good exposure. If I am in a low light situation I will have to increase the ISO though. To keep noise to a minimum I won’t change it until I have to.
To a certain extent noise can be corrected in post processing. Noise can also be added to give the final image a filmy look. You see, noise tend to look good in film photos, but not really in digital images, so it’s best to avoid it.
Now, get out your camera and try it!