TTL stands for Through-the-lens. It means that the light is measured only after it has passed through the camera lens. It’s a term used in flash photography.
All Canon Speedlite flashguns provide autoflash exposure. The exposure is determined by measuring the brightness of the flash illumination reflected from the subject. However, the sensors which measure this reflected light are actually found inside the camera, rather than in the Speedlite (the Speedlites 200M and 480EG are exceptions).
When the shutter button is pressed, the camera sets the shutter speed and aperture to suit the ambient light. Then the flash starts to fire. The light reflected back to the camera from the subject is reflected from the film or sensor during the exposure and measured by flash sensors inside the camera body. When the camera determines that sufficient light has been received for correct exposure, it terminates the flash output.
The way in which autoflash metering operates has evolved over the years. A-TTL is more advanced, taking two readings to determine the exposure, whilst E-TTL is an evaluative system which deducts the ambient light meter reading to determine a flash-only reading.
Related terms
