Camera Exposure metering systems continued

A few sophisticated automatic cameras have a dual metering system, one to provide guidance to the photographer in the form of a viewfinder exposure display, while the other system takes a reading of light reflected from the film plane during exposure. The Olympus OM2n is an example where CdS cells provide viewfinder information while silicon cells control the exposure when the camera is set to automatic.

Stop down metering: many cheaper TTL metering cameras determine exposures at the working aperture. The lens must be stopped down until the viewfinder display indicates that the correct settings have been made. Quite ogically, this darkens the viewfinder image according to the aperture selected. Although it also acts as a depth of field preview it can make accurate focusing difficult. The best alternative is to focus the image first and then take a light reading. This is a fairly slow system and may mean that the subject takes flight before you are ready to take the photograph.

Full aperture metering: with this system the viewfinder image remains bright during focusing and exposure determination whatever aperture you may select to provide a correct exposure. A special linking mechanism on the back of the lens relays information to the meter to show what aperture has been selected. If a depth of field preview feature is important to you and you prefer full aperture metering, choose a model with a depth of field preview to gain the best of both worlds.

* Many TTL metering auto cameras have a light sensitive cell in the pentaprism housing which reads the amount of light entering through the lens. The meter provides an exposure display in the viewfinder which exactly corresponds to the shutter speed and aperture chosen for exposure.

* The Olympus OM2n, when in the automatic mode, is unusual in that the light is measured at the film plane during exposure. CdS cells in the pentaprism give a viewfinder display, while silicon cells in the mirror chamber control the exposure. When sufficient light has reached the film the shutter is closed.

* Like many SLR cameras the Leica R3 has a bottom centre–weighted meter to emphasize light from central and lower subject areas. The red areas show the sensitivity pattern and percentage reading from three CdS cells.

* Spot readings can also be taken by moving a switch on the camera. The entire light reading is taken from the central area, shown in blue. Few other cameras can do this.

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