ISO

The ISO refers to the film's sensitivity to light. The higher the number, the more sensitive to light the film would be. Higher sensitivity film produces a more grainy effect. Low sensitivity means that the film has to be exposed to light for a longer period of time, which might result in a better quality image. For example, on a sunny day, you would probably use a film rated 100 or 200, but if you were taking pictures indoors, you would use a higher sensitivity like 400 or 800. ISO is useful when indoors and not able to use a flash.  In film photography, you have to remain the same ISO throughout the roll.

JAM