The Canon TTL Flash Metering System
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.
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 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. When you depress the shutter button fully, the internal mirror will then flip up and exposes the film. A signal is then sent to the flash unit to emit a light to illuminate the scene. The duration of the flash pulse will depend on the OTF sensor which is metered for normal operational lighting conditions. If one is working the Camera under bright lighting, the auto filled reduction will automatically take over and reduces the illumination of the scene.
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.