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 methods of flash metering which Canon uses for their system. The first method as described above is the Through The Lens (TTL) system. This method was first introduced by Canon during the 1980s with the introduction of the T90 model. The system was later adopted by the EOS range of Cameras as a standard feature. This makes the T90 as the only non EOS Camera which uses the TTL flash metering system.
The TTL system works by measuring the light which bounces of the subject matter and entering the camera lens after the flash light pulse has been emitted. In essence, the TTL system, measure the actual light which is reflected off from the film in real-time. This is done by using a OTF (Off The Film) sensor. When enough light has entered the sensor, to get a proper exposure level, the light from the flash will be extinguished. Because Digital Camera does not use film, the Canon Digital EOS cameras do not support TTL.
The sequence of operation of the TTL 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. 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.
As the OTF sensor measure the reflected light off the film, the chemical composition of the film will also affect the amount of light which will be reflected. In most cases, this will not be a problem as cameras nowadays are adjusted to operate with normal films. This only becomes a problem if you have to use specially coated films with different level of light tolerance. - 16732
These are the two methods of flash metering which Canon uses for their system. The first method as described above is the Through The Lens (TTL) system. This method was first introduced by Canon during the 1980s with the introduction of the T90 model. The system was later adopted by the EOS range of Cameras as a standard feature. This makes the T90 as the only non EOS Camera which uses the TTL flash metering system.
The TTL system works by measuring the light which bounces of the subject matter and entering the camera lens after the flash light pulse has been emitted. In essence, the TTL system, measure the actual light which is reflected off from the film in real-time. This is done by using a OTF (Off The Film) sensor. When enough light has entered the sensor, to get a proper exposure level, the light from the flash will be extinguished. Because Digital Camera does not use film, the Canon Digital EOS cameras do not support TTL.
The sequence of operation of the TTL 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. 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.
As the OTF sensor measure the reflected light off the film, the chemical composition of the film will also affect the amount of light which will be reflected. In most cases, this will not be a problem as cameras nowadays are adjusted to operate with normal films. This only becomes a problem if you have to use specially coated films with different level of light tolerance. - 16732
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
