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through the looking glass

Through-the-lens metering

Through-the-lens (TTL) metering is a photographic term describing a feature of cameras capable of measuring light levels in a scene through their lens. This information can then be used to select a proper exposure, and/or control the amount of light emitted by a flash connected to the camera.

Through-the-lens metering is most often associated with single-lens reflex (SLR) cameras. A light sensor can be incorporated into the pentaprism (or pentamirror) used to form an upright image of the view out of the lens. However, TTL metering systems have been incorporated into other types of cameras.

In TTL flash metering occurs after the shutter has been opened — the sensors record the amount of flash-created light returning to the camera through the lens and automatically shut off ("quench") the flash when an appropriate amount of light has been produced.

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