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 measuring the light which bounces of the subject matter and entering the camera lens after the flash light pulse has been emitted. In essence, the TTL system, measure the actual light which is reflected off from the film in real-time. This is done by using a OTF (Off The Film) sensor. When enough light has entered the sensor, to get a proper exposure level, the light from the flash will be extinguished. Because Digital Camera does not use film, the Canon Digital EOS cameras do not support TTL.
The TTL metering system works in the following manner and order:
1. By pressing the shutter button halfway down, the ambient metering process takes place. Shutter speed and aperture size is calculated and programmed by the camera or user depending on the mode which the camera is running on. These modes can be the AV, P 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. Once the foreground has been adequately illuminated, power to the Flash unit is cut off which extinguishes the flash light. The shutter remains open for the entire duration of the shutter’s speed time. The shutter then closes and the mirrors flips back down. Some camera has a Flash exposure Confirmation Led light and if the flash metering operation is successful, this light will glow.
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.