TTL Flash Metering System of the Canon Camera
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. 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.
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.
