TTL

The acronym TTL can refer to:

Through The Lens

Usually referring to in-camera exposure metering, but also used to differentiate viewfinder designs, autofocus mechanism designs, and other systems on lens-based cameras.

Transistor-Transistor Logic

Electronic camera circuitry, digital electronics, the ability of a device or circuit to be connected directly to the input or output of digital equipment, eliminates the need for interfacing circuitry, TTL signals are usually limited to two states, low and high, and are thus much more limited than analog signals.

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See definition research from Google define:ttl