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 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. 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.
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.
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. 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.
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.
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
