The situation that occurs during metering for ambient lighting and metering for flash is two entirely different situations. The time to meter for ambient lighting happens before the camera shutter is open. For the Canon EOS camera, ambient lighting metering occurs when you press the shutter button halfway down. Flash pulse metering only occurs when you fully depress the shutter button. There are two automatic ways to meter for flash pulse. The first way is to measure the flash pulse when it is being released. The second way is to do a pre-flash test. To do this, you need to use a low powered test pulse of known brightness before the camera 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.
TTL system works by calculating the light which is reflected of the subject and entering the lens after emission of the Flash Pulse. In short the TTL system actually measures the light which is reflected off from the film surface in real time. This is archived by the use of the "Off The Film" (OTF) Sensor. Once there is enough light entering the sensor, the flash will be turn off. As Digital Camera does not use films, the Canon Digital EOS Camera does not support the TTL flash metering system.
The progression of the TTL flash metering 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. Once the foreground has been adequately illuminated, power to the Flash unit is cut off which extinguishes the flash light. The shutter remains open for the entire duration of the shutter's speed time. The shutter then closes and the mirrors flips back down. Some camera has a Flash exposure Confirmation Led light and if the flash metering operation is successful, this light will glow.
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. - 16651
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.
TTL system works by calculating the light which is reflected of the subject and entering the lens after emission of the Flash Pulse. In short the TTL system actually measures the light which is reflected off from the film surface in real time. This is archived by the use of the "Off The Film" (OTF) Sensor. Once there is enough light entering the sensor, the flash will be turn off. As Digital Camera does not use films, the Canon Digital EOS Camera does not support the TTL flash metering system.
The progression of the TTL flash metering 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. Once the foreground has been adequately illuminated, power to the Flash unit is cut off which extinguishes the flash light. The shutter remains open for the entire duration of the shutter's speed time. The shutter then closes and the mirrors flips back down. Some camera has a Flash exposure Confirmation Led light and if the flash metering operation is successful, this light will glow.
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. - 16651
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
