E-TTL flash metering system, or otherwise know as Evaluative Through The Lens system, was introduce in 1995 by Canon with the debut of the Elan II/50 camera. Even though the metering is still done through the lens, the system works on an entirely different principle from that of the TTL system. Furthermore this system is also not backward compatible. The E-TTL system operates by firing a per-flash of known brightness from the flash unit to determine the correct level of flash exposure. It does this by measuring the Pre-flash light which is reflected off the scene. It then calibrates the correct flash pulse to achieve a mid tone level on the subject.
The E-TTL is more advanced than the TTL or A-TTL system because the algorithm of the E-TTL system is able to fill flash daylight photos better. It is also connected to the auto focus point and as such the exposure result is better and finer grained than the multiple zoned TTL flash system. Currently the system is supported by all late model Canon EOS film cameras and the digital range after the introduction of the Canon D30. The progression of the operation of the E-TTL system is as follows:
When the shutter button is depressed halfway, current ambient light level are metered in the normal way. Aperture and shutter speed are set by the camera or by the user depending on the mode of operation selected. Once the shutter button is depressed down all the way, the flash unit fires a pre-flash from the main flash unit. The reflected light is then calculated by the metering system to determine the correct power output for the actual flash burst to achieve the correct exposure level.
Auto filled reduction is applied if the photo taking operations occurs under bright lighting conditions. With the auto filled reduction process, flashlight output can be reduced by 0.5 to 2 stops. The mirror then flips up and while at the same time the shutter opens exposing the film or sensor element if it is a digital camera. The shutter then remains open during the entire time of the shutter speed. After this, the shutter will close and the mirror will down back to its original position. If the camera has a LED light confirming the flash exposure, then it will light up if the metering process is successful.
The main drawback of the E-TTL system is that the pre-flash will cause those who blink quickly to be photographed while in mid blink. The predicament gets worse when you use the second curtain sync with a slow shutter speed. To get around this problem, you can after firing the pre-flash wait a short while before commencing the main flash burst. It would be wise to forewarn the subjects that you are just firing a pre-flash so that they will not assume the actual picture has been taken.
The E-TTL is more advanced than the TTL or A-TTL system because the algorithm of the E-TTL system is able to fill flash daylight photos better. It is also connected to the auto focus point and as such the exposure result is better and finer grained than the multiple zoned TTL flash system. Currently the system is supported by all late model Canon EOS film cameras and the digital range after the introduction of the Canon D30. The progression of the operation of the E-TTL system is as follows:
When the shutter button is depressed halfway, current ambient light level are metered in the normal way. Aperture and shutter speed are set by the camera or by the user depending on the mode of operation selected. Once the shutter button is depressed down all the way, the flash unit fires a pre-flash from the main flash unit. The reflected light is then calculated by the metering system to determine the correct power output for the actual flash burst to achieve the correct exposure level.
Auto filled reduction is applied if the photo taking operations occurs under bright lighting conditions. With the auto filled reduction process, flashlight output can be reduced by 0.5 to 2 stops. The mirror then flips up and while at the same time the shutter opens exposing the film or sensor element if it is a digital camera. The shutter then remains open during the entire time of the shutter speed. After this, the shutter will close and the mirror will down back to its original position. If the camera has a LED light confirming the flash exposure, then it will light up if the metering process is successful.
The main drawback of the E-TTL system is that the pre-flash will cause those who blink quickly to be photographed while in mid blink. The predicament gets worse when you use the second curtain sync with a slow shutter speed. To get around this problem, you can after firing the pre-flash wait a short while before commencing the main flash burst. It would be wise to forewarn the subjects that you are just firing a pre-flash so that they will not assume the actual picture has been taken.
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
