Canon Camera Basic Guide Flash metering
Author: TimHarris Total views: 3 Word Count: 570
Metering for ambient lighting conditions and flash metering has totally different requirement. The metering for ambient lighting condition is done before the shutter of the camera is opened. In the Canon EOS camera, ambient light metering is done when you press the shutter button halfway down. Flash Pulse metering however is done when the shutter button is press fully down. There are two ways to meter for flash pulse automatically. One way is by measuring the flash pulse as it is being released. The other way is to do a pre-flash test. This is achieved by a low powered test pulse of a calculated brightness before the shutter opens.
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. When the shutter button is depressed halfway the ambient light metering operation runs its course. The shutter speed and aperture is set by the camera or the user depending on what mode is used. For example; P mode, AV mode 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. 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.
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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About the Author
Tim Harris is a canon camera enthusiast. For tips on choosing and buying cameras, do check out Canon OS Camera website. Camera 40D Digital Rebel XTI 3D Blog
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