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 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.
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.
