ISO controls how sensitive your camera's sensor is to light. Low ISO (100) needs bright conditions, high ISO (6400) works in darkness but adds grainy noise. Use the lowest ISO that lets you hand-hold the shot without blur.
ISO. The ISO setting of your camera controls how sensitive its sensor is to light. A low number, like 100, denotes a limited sensitivity and necessitates the use of additional light. When a sensor's sensitivity is high, as in the case of a value of 6400, it can detect very little light. Increased noise in a photograph is a result of the ISO level. Given that noise degrades image quality, as much noise as possible would be good, unless you plan to stay up late taking night and astrophotos. If you're photographing handheld or moving objects indoors, your ISO must be increased.
