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.

Canon uses both these two methods for the metering of the flash pulse. The first way is called the "Through The Lens" (TTL) system. It was first used by Canon in their T90 model during it debut in the 1980s. Later the TTL system became a standard facet of the EOS range of Cameras. In fact the T90 became the only Non EOS camera to ever use the TTL flash metering system.

The TTL system works by calculating the amount of light which is reflected off the subject into the camera after the flash light is emitted by the flash unit. It actually measures the light which is reflected of the film in real time and this done by using an OTF (Off The Film) sensor. When enough light has entered the OTF sensor, the flash will be deactivated. Due to the fact that the Canon EOS Digital cameras do not use films, the Digital range of Canon cameras does not support the TTL system.

The TTL metering system works in the following manner and order:

1. When the shutter button is depressed halfway the ambient light metering operation runs its course. The shutter speed and aperture is set by the camera or the user depending on what mode is used. For example; P mode, AV mode or M Mode.

2. Only when the shutter button is depressed all the way, the internal mirror is turn up and exposes the film. Power is then sent to the flash unit which will emit a flash light to illuminate the scene. The length of the flash pulse is determined by the OTF sensor in the camera which is metered for normal operational conditions. If the picture is taken under bright lighting conditions, the auto fill reduction will take effect and reduce 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.

You have to note that the OTF sensor, reflect the amount of light off the film. Therefore it follows that the composition of the film will also affect the amount of light which will be reflected. Normally this will not be an issue as all cameras are adjusted to work with normal films. This is only an issue if you use specially coated films which have a different level of tolerances to light exposure.