Lighting with a constant light source as you have plenty of time to adjust to the conditions, but lighting with flash, which lasts only a few seconds, is more difficult. Getting the right exposure with flash is very difficult without a light meter. Our usual ambient light meters cannot measure flash lights with precision, hence it is necessary to use a handheld flash light meter whenever one is shooting indoors, or in low light conditions.

A flash light meter is indispensable for both digital and manual cameras, because digital cameras are only equipped with reflected light meters; while you need an incident light meter to measure flash light. A flash light meter has sensors that detect each time your flash gun flashes and synchronise themselves accordingly. So when your flash gun flashes, the light meter also activates its sensors to detect the amount of light incident. Just like an incident light meter, a flash light meter also has a dome that is to be held on the subject, facing the camera. When the flash gun flashes the light, the meter reads it and gives you the reading in terms of shutter speed, as well as the aperture so you can make the settings in your camera get the right exposure. Unfortunately, even advanced flash units like the Canon Speedlite 430EX II do not include their own integrated flash units. Some flash light meters also give ISO or film speed readings for the correct exposure, and this helps you know if you are using the right film stock or not.