In Digital Photography ISO measures the sensitivity of the image sensor. The same principles apply as in film photography – the lower the number the less sensitive your camera is to light and the finer the grain.
What ISO setting do I use?
The ISO setting you use depends on the amount of light in the scene you are photographing. The more light you have to work with the lower you can set your ISO. As settings go, ISO is one of the key camera menus you need to learn. ISO numbers typically range from 100 to 6400 in most newer cameras.
* Want to know more? Click the link to see an article about what ISO means.