Canon® EOS Digital Photography Photo Workshop

8.1. HOW ETTL II TECHNOLOGY WORKS TO YOUR ADVANTAGE

The ability of your camera to intelligently evaluate the photo you are taking and apply flash lighting to it is the result of many years of technology advances and developments. Studio photography with multiple external strobes and lights set and controlled by the photographer, and essentially invisible/unrecognized by the camera, must be metered manually. An on-camera flash, however, has a number of evaluative measurements that the camera and flash manage and that adjust the flash's intensity and area of coverage (the flash adjusts its area of coverage to match the focal length of the camera's lens). Canon's current and most advanced technology today is called ETTL II; the acronym stands for evaluative through-the-lens, meaning the camera is reading the subject through the lens for the most accurate evaluation. Canon's higher-end flashes, the 580EX II and the 430EX, both use ETTL II technology, and are backwards-compatible if you've mounted them on an older camera that doesn't support ETTL II.

As part of the flash process, ETTL II uses a virtually invisible preflash and emits a very short flash before the actual image-taking flash takes place. This preflash evaluates and gives information to the camera and flash for a proper exposure based on the reading. And, as implied by the ETTL definition, the reading taken during the preflash is done through the lens and read by a sensor within the camera. ETTL is smart enough that if ...