In modern DSLRs they all have a light meter built-in to the camera. But sometimes you want a more accurate reading, or to measure the amount of light when using flash (your camera can’t do that), in which caseyou would turn to a handheld light meter.
Check out this video from Adorama TV (host Daniel Norton) for some light meter basics.
This second video is more advanced. Once again from Adorama TV, with host Mark Wallace, this one covers how to meter for light ratios... more