Through-the-lens metering
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In photography, through-the-lens metering refers to a feature of cameras whereby the intensity of light reflected from the scene is measured through the lens; as opposed to using a separate metering window or external hand-held light meter. In some cameras various TTL metering modes can be selected. This information can then be used to set the optimal film or image sensor exposure, it can also be used to control the amount of light emitted by a flash unit connected to the camera.
Transparent window in the mirror Topcon RE-Super
Above viewfinder in the pentaprism box Pentax Spotmatic, Nikon FM, Canon EOS, Zenit-12
Side of the condenser lens Canon F-1
Below the mirror, reflecting through a transparent window and a secondary mirror Leicaflex, Nikon F3, Olympus OM-3
Olympus OM-2
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The Olympus OM-2 is a professional single-lens reflex film system camera produced by Olympus of Japan from 1975 to 1988.
Olympus OM-2
Olympus OM-2N SLR 35mm film camera
OM-2SP Finger Grip
Olympus OM-2 with Zuiko 50mm f1.8 lens