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.
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.
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 sequence of operation of the TTL 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. - 16747
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.
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 sequence of operation of the TTL 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. - 16747
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
