The Evaluative Through The Lens (E-TTL) flash metering system was introduced by Canon with the 1995 Canon Elan II/50 Camera. Although metering is still done through the lens, the E-TTL flash metering systems works completely on different principles to that of the TTL system. It is also not backward compatible. The system works by firing a pre-flash of calibrated brightness from the main flash unit to ascertain the correct flash exposure. The system measure the Pre- flash light reflected from the scene and then recalculates the correct flash pulse to achieve the proper mid tone on the subject matter.
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:
Once the shutter button is depressed halfway the ambient light metering process runs it normal course of operation. Aperture size and Shutter speed are programmed by the user or camera depending on which mode the camera is operating under. After the shutter button is press down all the way, the camera fires a pre-flash from the flash unit. The light which is reflected of the scene is then measured by the metering system to ascertain the right power output for the actual flash pulse to get 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 problem with the E-TTL system is that, the pre-flash will result in those who blink quickly to be snapped while in mid blink resulting in "droopy eyes" looks. The situation gets more problematic when you use a slow shutter speed and the second curtain sync together. To overcome this problem, just wait a short while before firing off the main flash burst. Remember to forewarn the subjects as they might think the actual photo has been taken after you fired the pre-flash. - 16089
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:
Once the shutter button is depressed halfway the ambient light metering process runs it normal course of operation. Aperture size and Shutter speed are programmed by the user or camera depending on which mode the camera is operating under. After the shutter button is press down all the way, the camera fires a pre-flash from the flash unit. The light which is reflected of the scene is then measured by the metering system to ascertain the right power output for the actual flash pulse to get 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 problem with the E-TTL system is that, the pre-flash will result in those who blink quickly to be snapped while in mid blink resulting in "droopy eyes" looks. The situation gets more problematic when you use a slow shutter speed and the second curtain sync together. To overcome this problem, just wait a short while before firing off the main flash burst. Remember to forewarn the subjects as they might think the actual photo has been taken after you fired the pre-flash. - 16089
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
