Canon Camera Basic Guide Flash metering
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The situation that occurs during metering for ambient lighting and metering for flash is two entirely different situations. The time to meter for ambient lighting happens before the camera shutter is open. For the Canon EOS camera, ambient lighting metering occurs when you press the shutter button halfway down. Flash pulse metering only occurs when you fully depress the shutter button. There are two automatic ways to meter for flash pulse. The first way is to measure the flash pulse when it is being released. The second way is to do a pre-flash test. To do this, you need to use a low powered test pulse of known brightness before the camera shutter opens.
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.
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. 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. 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.
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