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. 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. Once the shutter button is depressed all the way down, the internal mirror is flipped up and the film is exposed. Then Power is sent to the flash unit illuminating the area. The duration of the flash burst is determined by the OTF sensor which meter for a normal operational condition. If the picture is taken under bright lighting situation, the auto fill reduction will apply reducing 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. - 15437
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. 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. Once the shutter button is depressed all the way down, the internal mirror is flipped up and the film is exposed. Then Power is sent to the flash unit illuminating the area. The duration of the flash burst is determined by the OTF sensor which meter for a normal operational condition. If the picture is taken under bright lighting situation, the auto fill reduction will apply reducing 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. - 15437
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
