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Spotmeter

An exposure meter for a small area of a scene


An exposure meter that measures a very small area of a scene, with an acceptance angle of 1 degree or less, is referred to as a spotmeter.

WHEN IS A SPOTMETER USED?

A spotmeter is used for precise measurement of the light reflected from small areas of a large scene. A photographer using a long-focus lens on a camera that is not equipped with a built-in exposure meter would use a handheld spotmeter to determine exposure for the small area of the scene at which the lens will be aimed.

The spotmeter has a viewing system similar to that in a camera so the photographer can see the precise area that is being metered.

A spotmeter would also be employed when the subject is at a distance and inaccessible, or if the subject to be photographed is itself small, or for highly selective measurement of different areas of a scene. If, for example, a relatively-small subject, like a performer in a spotlight on an otherwise darkened stage, is brightly-lit and surrounded by deep shadow, the camera's normal metering system may be inacapable of providing an accurate exposure for the subject alone. A spotmeter, however, will provide a precise reading of the light reflected from the subject without influence from the surrounding darkness.

SPOTMETERS ARE BUILT INTO SOME CAMERAS

Certain high-end 35mm cameras with through-the-lens (TTL) metering systems have a built-in spotmeter in addition to their normal metering system. In the Nikon F5 in spotmeter mode, for example, nearly 100% of the exposure meter's sensitivity is concentrated on a 4mm diameter area of the viewframe (approximately 1.5% of the entire frame). The photographer must select spotmeter mode to use the system.

The acceptance angle of the spot field varies with the focal length of the lens in use. With long lenses, this can be very small.

NORMAL EXPOSURE METERS CAN ALSO READ NARROW ANGLES

Some normal multipupose exposure meters can be fitted with a narrow-angle attachment that permits them to be used in a manner similar to that of a spotmeter, however aiming the meter is less accurate because there is no viewing system.

MAKESHIFT SPOTMETER

If you need a spotmeter but your camera's built-in system is an averaging or center-weighted exposure system, you can still use your system as a makeshift spotmeter by switching to a very long lens. Attach the telephoto lens, meter the desired area of the scene, then switch back to a shorter lens to take the picture using the exposure settings derived by metering through the long lens.

TURN YOURSELF INTO A SPOTMETER

If you must have an exact exposure of a small area of a scene and you do not have a spotmeter or a long lens, you can still use your camera's normal center-weighted or averaging meter to obtain the exposure you need. Simply approach close to the small area until it fills your viewfinder (something you won't always be able to do), take a meter reading and either lock it into your camera (if it has an exposure lock feature) or make a note of the exposure settings. Go back to your shooting location, and take the picture using the exposure data you got from metering close-up. If you can't approach close enough, can you meter an object that is in identical illumination (that has the same luminance as the subject you wish to photograph)? If so, use that exposure reading for your picture.