Through-the-lens metering, also known as TTL, is a method of measuring light for photographic exposure purposes. In TTL metering, the light is measured using an in-camera light meter, eliminating the need to use a separate, hand-held light meter. The light enters the photographic lens and is read by the meter, which then outputs a display through a gauge or dial viewable through the lens. TTL flash is a type of flash that is metered using through-the-lens metering. In TTL flash, the flash head works with the camera's internal metering system to determine flash output for proper exposure of the scene.
Prior to the invention of TTL metered flash, the photographer had to manually calculate the correct flash output based on the measurement of ambient light. This method required extra time and patience, and was prone to mathematical error. The first TTL flash was developed by Olympus in the mid 1970s with the introduction of the OM-2. This technology was quickly adopted by other camera manufacturers. It has since become the standard for on- and off-camera flash units.
Through-the-lens flash metering uses an "off-the-film" (OTF) sensor to detect the light being reflected off the exposing film in real time. This sensor determines the length and strength of the flash pulse emitted by the flash tube, eliminating the need for the photographer to calculate this measurement. Off-the-film sensors are still present even in modern digital camera equipment, and the theory of TTL flash metering remains the same whether the camera uses film or a digital image sensor for image capture.
Types of TTL
The original concept of TTL is sometimes referred to as "standard TTL." Many modern flash units also have the ability to use a setting known as "advanced TTL." In advanced TTL metering, the flash unit fires one or more "pre-flashes" of either infrared or white light before exposing the film. This is then used to determine an approximate distance to the main subject of the composition, which in turn is used by the camera to determine settings, such as aperture, when in any semi or fully programmed mode. After the distance and settings are determined, the A-TTL flash works in the same fashion as a standard TTL flash, using the OTF sensor to determine the strength and duration of the flash pulse.
The advantage of using a through-the-lens flash metering system is that the photographer is not required to perform the calculation of flash output, which eliminates a margin of error. This saves time while shooting, ensuring that less action is missed. In some situations, a TTL flash can eliminate the need for a hand-held light meter, which means less equipment needs to be carried by the photographer. In basic photography, TTL metering ensures a reasonably correct exposure while using flash.
The disadvantages of through-the-lens flash metering lie in the limitations of the technology. In using an internal light meter, the TTL system may be "fooled" by very bright or very dark subjects, resulting in an incorrect exposure. In some cases, it may be more accurate for the photographer to carry a separate light meter for accuracy. Also, there's the issue of compatibility; not all TTL flashes are able to be used with all camera bodies. These units may have limited or no functionality, making it necessary for the photographer to calculate flash output manually.