TTL—a term most commonly associated with single lens reflex (SLR) cameras—is used to refer to incamera exposure metering and viewfinder designs that preview the image directly through the lens.
TTL metering is provided by cameras that have a light sensor inside the camera body behind the lens. Light levels from the scene are therefore measured precisely through the lens, as opposed to a separate metering window. Cameras with light meters mounted to the body of the camera are less precise as they can be affected by stray light.
Analog TTL viewfinders employ a special optical design that enables you to see precisely the image that will be recorded through the lens: what you see is what you get. This is desirable because it eliminates parallax inherent to rangefinder cameras. This is caused by framing an image through a viewfinder window that is separated from the lens.