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Through the Lens (TTL) flash metering |
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Technology -
Technology
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TTL metering means that the amount of (flash) light
is measured behind the lens. Normal daylight can be measured "off-line"
(=before the exposure). Obviously this is not possible in flash photography
because you have just one single flash burst and no constant light. Therefore
the light from a flash is measured by one or more sensors below the light
channel (behind the mirror and curtain). A certain amount of (flash-) light
is reflected from the film/sensor towards these sensors (see picture below). The
camera measures the received light and stops the flash output if necessary.
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