Otherwise known as the Evaluative Through the Lens System, the E-TTL system was introduced in 1995 with the debut of the Canon Elan II/50 Camera. Even if the metering process is still done through the lens of the Camera, the E-TTL system is a more advanced system and operates on dissimilar system to that of the TTL system. In addition, the E-TTL system is not backward compatible. The E-TTL system functions by firing a pre-flash, whose brightness is known, from the flash unit to ascertain the exact level of flash exposure. This is achieved by measuring the pre-flash light which is reflected off from the scene. The camera then calculate the correct flash pulse to get 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:
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.
However the main disadvantage of the E- TTL system is the fact that the pre-flash can cause those who blink quickly to be photograph while in mid blink. The problem is compounded when one uses the second curtain sync with a slow shutter speed. One way to overcome this problem is to fire the pre-flash first, then wait momentarily before actually firing the main flash pulse. Of course you must not forget to forewarn the subjects incase they think that the actual photograph has been taken when you fire the pre-flash. - 16651
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.
However the main disadvantage of the E- TTL system is the fact that the pre-flash can cause those who blink quickly to be photograph while in mid blink. The problem is compounded when one uses the second curtain sync with a slow shutter speed. One way to overcome this problem is to fire the pre-flash first, then wait momentarily before actually firing the main flash pulse. Of course you must not forget to forewarn the subjects incase they think that the actual photograph has been taken when you fire the pre-flash. - 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
