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Glossary Terms
The acronym TTL most commonly refers to,

Through The Lens: referring to in-camera focusing (reflex cameras) or exposure metering with photocells somewhere in the light path of the taking lens. This was an improvement over older designs where a CdS or selenium meter cell was attached somewhere atop the camera body, but could not take into account light loss from filters, lens extension, etc. TTL exposure metering was possible in the 1960s after the introduction of CdS cells that were small enough to fit in a camera body.

TTL flash exposure control is another common meaning. But depending on context the term might also differentiate viewfinder designs, autofocus mechanism designs, or other camera systems.


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.