A light meter (or exposure meter, short form meter) is a device that measures light to determine the proper exposure settings for a scene. Analog measuring instruments with photoelectric selenium photo cell as voltage source were the most common models of light meters for the average photographer. Later more light sensitive meters were made, with Cadmium Sulfide (CdS) photo resistors or photo semiconductors as sensors. Nowadays digital instruments dominate the market, with memory to store exposure values of flash situations by actually firing a flash just for measuring. But selenium meters are still popular since they need no battery.
Early light meters
Actinometers were the first light meters. They typically had the shape of a pocket watch and used light sensitive photo print paper as means of measuring. The time to darken a piece of such paper until it matches a standard tint is the input value for the scales on which an appropriate shutter-speed/aperture combination for the light situation can be found.
|Logaphot extinction meter of 1955|
with slide-rule back to determine
exposure settings for 100 ASA
Another type of early meter, the extinction meter, depended on eyesight: the user looks through the meter at a row of numbers, each behind a celluloid window of different opacity, the highest or lowest visible number determining which light situation is given. Other extinction meters have a pattern visible through the eyepiece, and a control varies the amount of light allowed into the device until the pattern can only just be seen; the position on the control then indicates the exposure.
|Mimosa extinction meter of 1947|
Light meter makers
|Bertram Chrostar Exposure Meter|
- Ernst & Wilhelm Bertram (Bewi)
- General Electric
- Kobayashi Seiki (Kopil)
- Zeiss Ikon
and many others; see Category: Meter makers.
Light meters in cameras
Automation of exposure control in cameras began with built-in selenium meters. Shutter speed and aperture had to be selected manually according to the meter's absolute or relative measured values. With the advent of electrically controlled diaphragms and shutters other light sensors like photo resistors, photo diodes etc. became common parts in cameras. Both sorts of devices needed batteries for operation. Some additional electronic circuits combining meter with shutter and diaphragm units were just needed to get exposure control automated. This sort of camera emerged in the 1960's.
A camera's built-in light meter can easily be tricked into giving an incorrect exposure. The most common problem is a bright background behind the subject such as a bright sky, or bright light reflecting off snow, sand or water. Built-in meters often darken the photo excessively.
A hand-held meter is more accurate since it measures the light falling on the subject. It is not fooled by a bright background. Nor is it fooled by objects which reflect a great deal or very little light.
Keep in mind that the camera's light meter is measuring the light reflected back off the subject. In simple terms, a green shirt appears green because it absorbs all the colors except green -- which it reflects back. A white shirt reflects all the light, while a flat black surface would reflect none.
A dark-skinned black woman in a bright white wedding gown is a difficult/impossible subject that's sure to fool a camera's meter. (The classic wedding photographer's nightmare, either the gown or the face can be exposed properly unless the photo is touched up after the fact using burning or dodging techniques.)
These problems can be corrected in a few ways. One solution is to zoom in on the desired subject so that less of the bright background/foreground is seen.
Advanced cameras offer some kind of Auto Exposure Lock (aka AE Lock, Exposure Memory Lock or EV Lock). Using AEL, it's possible to zoom in or approach the subject so that an exposure value can be read from just the desired area of the subject (for instance the bride's face). With that value now locked in, the complete subject area can now be photographed at that exposure.
The correct exposure value can also be read off a neutral test card or 18 percent gray card (or even the palm of the human hand) held in front of the subject. Like using a hand-held meter, this ensures that the light falling on the subject is measured, rather than what is reflected back by different surfaces like a bright white bridal gown.
As well, the technique of exposure bracketing can be used. Bracketing (Exposure & Exposure Bracketing) is the technique of taking multiple photos at different f-stops to ensure at least one of them will be correctly exposed.