Light meters have been in the cinematographer’s arsenal since the early days of filmmaking. While the size and ease of the device have dramatically improved — see the picture below of cinematographer Georges Périnal on the set of Life and Death of Colonel Blimp (1943) — the function of the device has stayed the same; it measures the amount of light in a scene to help determine the exposure.
Light meters were much more of necessity when every motion picture was shot with celluloid. As you wouldn’t have 100% confirmation of knowing if you have successfully set the exposure correctly until the negative has been sent off to the lab and developed.
Do you still need a light meter if you have a digital camera? You would think not as almost every low budget camera on the market is equipped with an LCD screen where you can see the image in real time and judge the light for yourself. A lot of cameras also have built-in light metering, which for the most part, can give you a pretty reliable reading.
However, the cameras that have built-in light metering only give you a reading from the current frame, not the entire set. If you have a moving shot where the camera goes in and out of different light sources, the camera will not be able to obtain a reading for the entire shot. With a light meter, you can measure the light all around the set as well as any incoming light from different directions.
A light meter is one of those tools to have your equipment bag even if you don’t use it 90% of the time. It should be noted that when you are filming against a green screen and lighting and levels have to spot on to get a perfect key, a light meter would be recommended.
To understand how to use a light meter, watch the video below provided by Ryan E. Walters