Metering for ambient light setting and metering for flash pulse occurs at two different times of operations. Ambient light metering happens the camera’s shutter is opened. For the Canon EOS cameras, this happens when you press the shutter button down halfway. Flash Pulse metering, on the other hand, is done only when you depress the shutter button fully. There are two main ways which one can meter for flash pulse. The first method is to measure the flash light as it is being emitted by the flash unit. The second method is by doing what we call a pre-flash test. What this means is that you send a low powered test pulse of established brightness before opening the shutter.

These are the two metering system used by the Canon flash metering system. The Through the Lens (TTL) metering system adopts the first method of metering for flash automatically described above. Canon first introduces the TTL metering system with their T90 model during the 1980s. This feature was later incorporated in the EOS line of camera as a standard feature. As such the Canon T90 is the only non EOS that can use the TTL flash-metering system.

TTL system works by calculating the light which is reflected of the subject and entering the lens after emission of the Flash Pulse. In short the TTL system actually measures the light which is reflected off from the film surface in real time. This is archived by the use of the “Off The Film” (OTF) Sensor. Once there is enough light entering the sensor, the flash will be turn off. As Digital Camera does not use films, the Canon Digital EOS Camera does not support the TTL flash metering system.

The progression of the TTL flash metering system is as follows:

1. Once the shutter button is depress midway, the metering for ambient lighting takes places. The speed of the shutter and size of the aperture is programmed by the camera or User depending on which program mode the camera is using. For example whether the camera is using the AV, P or M mode.

2. Once the shutter button is depressed all the way down, the internal mirror is flipped up and the film is exposed. Then Power is sent to the flash unit illuminating the area. The duration of the flash burst is determined by the OTF sensor which meter for a normal operational condition. If the picture is taken under bright lighting situation, the auto fill reduction will apply reducing the illumination of the flash.

3. When the foreground is sufficiently illuminated, the power supply to the flash unit is terminated which in turn extinguishes the flash light. The shutter is opened during the entire duration of its speed run. After that, the shutter then closes and the mirror flips back to it original position. For those cameras which have a warning light to confirm the success of the flash exposure operation, the warning light will then glow to indicate the success of the flash metering operation.

As the OTF sensor measure the reflected light off the film, the chemical composition of the film will also affect the amount of light which will be reflected. In most cases, this will not be a problem as cameras nowadays are adjusted to operate with normal films. This only becomes a problem if you have to use specially coated films with different level of light tolerance.