Camera Exposure metering systems

Most modern cameras have a light meter built in. This has proved to be very popular among amateurs and professionals alike because of the speed and convenience it offers.

All exposure meters work towards the same end—to give the photographer exposure information—but the ways in which they operate vary widely. There are two basic types however: coupled and uncoupled. Examples of both systems are found in 35mm reflex and non–reflex as well as in smaller and larger format cameras.

Uncoupled meters

Although the light meter is built in to the camera it is not directly linked with the aperture and shutter speed controls. The Zenith EM and Zenith E are two examples of uncoupled meter cameras. Such meter cells are usually of selenium which does not require battery power but takes up a lot of space. The cell responds to light which makes a meter needle move in direct proportion to the amount of light falling on to the cell.

The film speed must first be set before a valid reading can be taken. This moves the settings on the exposure calculator dial for aperture and shutter speed in relation to each other and to the film speed. The meter needle moves along the calculator dial to give a light reading. You can then read off the combination of shutter speed and aperture which will provide a correct exposure, and set the camera controls accordingly.

Because the cell is built in to the external casing of the uncoupled meter cameras the light readings can be unreliable. The cell does not measure the light entering the lens, and can therefore be influenced by a light or dark area near to the subject but not included in the frame. If a filter or different focal length lens is fitted to the camera the meter cannot take this into account. Care must also be taken not to cover the meter cell with a stray finger because this would influence the meter into drastically over–exposing.

Coupled meters

These are far more popular than uncoupled meters because their direct link with aperture, shutter speed and film speed settings makes them more convenient to use. You do not have to waste time reading a set of figures on a calculator dial and transposing them to the camera controls.

Exposure information is generally given directly through the viewfinder, allowing the photographer to make exposure adjustments without taking his eye away from the camera. The other major advantage of a coupled system is that it is quick to use. This is important when light on the subject is rapidly changing—when clouds suddenly obscure the sun, for example. Non-reflex cameras: some 35mm compact cameras have a single light- sensitive cell built into the camera lens housing. In many cases adjusting the film speed dial changes the diameter of a hole over the cell. This obviously adjusts the amount of light falling on to the cell which in turn provides specific exposure information for different film speeds and lighting conditions.

Although the cell is very close to the lens it can sometimes be influenced by the subject’s surroundings. It is also possible accidentally to obscure the cell window with a finger which would influence readings. When filters are fitted however, they also cover the meter cell and automatic exposure compensation (for a neutral density filter, for example) is provided.

SLR cameras: because the light– sensitive cell is built into the camera body the cell must be small—selenium is therefore unsuitable and most cameras use cadmium sulphide or silicon metering cells.

The camera may have one, two or perhaps more meter cells which can be located behind the lens or close to the focusing screen. Therefore when different focal length lenses are fitted, the cell(s) can respond instantly to the resulting variation in angles of view to give accurate exposure information. This is called through the lens (TTL) metering and is by far the most popular system. The meter reading is taken only from the area actually imaged by the lens, so that dark or light surroundings outside your picture have no influence on readings, unlike external meter cells.

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