TTL or Through The Lens is a camera setting whereby the camera determines values by metering through the lens of the camera.
The camera uses TTL in one of two places:
- To determine exposure when one or more elements of the photograph (e.g. shutter speed or aperture) are being controlled d by the camera.
- To set the level of light emitted by the flash or flashes connected to the camera to ensure the subject of the photograph is exposed correctly.
TTL is useful in situations where light or exposure can vary quickly and where the photographer doesn't have time to adjust exposure or lighting manually - e.g. when a toddler is being photographed outside in an area with areas of light and shadow.