Canon Camera Basic Guide Flash metering
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. 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. After the foreground is adequately illuminated, the power to the flash unit is cut off which in turn kills the flash light. During this time the shutter is opened until it has runs it course. Once that sequence has been completed, the mirror of the camera returns to its original position. Cameras which have a warning light to confirm the success of the flash exposure operations will glow to indicate its success of the metering operations.
It is also important to note that as the OTF sensor reflects the amount of light of the film, the composition of the film itself will also affect the amount that is reflected off. But as all camera are calibrated to work with normal film this will not become an issues except with special films like slide films which has different level of tolerances.
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