The acronym TTL can refer to:

Through The Lens
Usually referring to in-camera exposure metering or flash exposure control, but also used to differentiate viewfinder designs, autofocus mechanism designs, and other systems on lens-based cameras. Measuring light or focusing through the taking lens can be more accurate than separate metering, viewfinder or focus systems, as this measures the light that will reach the film, from the exact point-of-view of the lens - and so eliminates parallax and stray light from outside the frame.
Transistor-Transistor Logic
TTL is a type of digital electronic circuitry which uses transistors and resistor elements (usually made of transistors) to make logic gates. It is rarely, if ever used in camera circuitry as it has fairly high power consumption. Other logic families, with low power demands are used, so as to prolong battery life - for example CMOS (Complementary Metal-Oxide Semiconductor) or even IIL (Integrated Injection Logic, used in the Canon A-1).

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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Glossary Terms