Automatic flash unit (auto flash).
A self-contained flash unit which determines correct flash metering itself - flash metering is handled neither by the camera nor directly by the photographer.
Such flash units have small light sensors on the front which measure the amount of light reflecting back to the unit from the scene. The unit then automatically shuts off the flash tube (usually with a device called a thyristor) when it has determined that a correct exposure has taken place.
Since thyristors were a new technical innovation three decades ago, older flash units may be proudly marked as “auto thyristor” devices. “Auto” means that the flash unit has an automatic light sensor and “thyristor” means it uses a thyristor control device. However, since nearly all flash units contain thyristors or similar devices these days, you don’t see the term used as a marketing feature much anymore.
Unlike dedicated flash units which communicate data to and from the camera, the flash metering of automatic flash units does not occur through the camera lens and so can easily be fooled under certain circumstances. However, the photographer does not have to measure distances and calculate correct flash exposure settings manually like he or she would with a purely manual flash unit.
Such flash units are still sold, but dedicated flash units which rely on the TTL (through the lens) metering are more common today.
cf. dedicated flash unit, flash unit, protocol, thyristor, TTL.
Entry last updated 2002-05-20. Term 92 of 1487.
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