Other features found on many SLR cameras include through-the-lens (TTL) metering and sophisticated flash control referred to as 'dedicated electronic flash'. In a dedicated system, once the dedicated electronic flash is inserted into the camera's hot shoe and turned on, there is then communication between camera and flash. The camera's synchronization speed is set, along with the aperture. Many camera models on the market today actually measure the light that reflects off of the film plane, controls the flash duration of the electronic flash (some flash units have a flash duration range of 1/1000 of a second to 1/50,000 of a second), and then terminate exposure when the camera has received enough light for the exposure.
Some electronic flash units can send out several short bursts of flash to measure the distance, use the camera's inboard sensor(s) to determine the amount of light that is reflected from the subject, then send out a main pulse of light of just the right amount of light energy for a perfectly exposed photograph. Sophisticated cameras can even make it easy for the photographer to balance electronic flash and available light for an evenly balanced scene. While these capabilities are hardly unique to the SLR, manufacturers included them early on in the top models, whereas the best rangefinder cameras adopted such features later.
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