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 superior to the TTL and A TTL system because its algorithms is better at applying natural and subtle fill flash to daylight photos. The E- TTL is also linked to the auto focus point which results is better and finer grained exposure than that of the multiple zoned TTL flash sensor system. The E-TTL system is supported by all recent Canon EOS film camera as well as the Digital Camera after the introduction of the Canon D30.The sequence of 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.
If you are taking pictures under bright lighting conditions, the camera auto filled reduction feature will operate to reduce the flash intensity from ranges between 0.5 to 2 stops. The internal mirror then flips up to expose the film or sensor during the shutter run. After the shutter closes, the mirror will flip back down. If the camera has such a feature as a warning light for confirming the flash exposure operation, then it will start to glow.
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.
The E- TTL is superior to the TTL and A TTL system because its algorithms is better at applying natural and subtle fill flash to daylight photos. The E- TTL is also linked to the auto focus point which results is better and finer grained exposure than that of the multiple zoned TTL flash sensor system. The E-TTL system is supported by all recent Canon EOS film camera as well as the Digital Camera after the introduction of the Canon D30.The sequence of 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.
If you are taking pictures under bright lighting conditions, the camera auto filled reduction feature will operate to reduce the flash intensity from ranges between 0.5 to 2 stops. The internal mirror then flips up to expose the film or sensor during the shutter run. After the shutter closes, the mirror will flip back down. If the camera has such a feature as a warning light for confirming the flash exposure operation, then it will start to glow.
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.
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
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