Canon Speedlite 580EX II

Question

What's the difference between ETTL and TTL?

Answer 1

ETTL is Electronic-Through-the-Lens metering. The strobe fire a preflash (Or multiple preflashes)  to determine how powerful the immediately-following actual flash needs to be. With Through The Lens TTL when the shutter is tripped, the light from the flash fires. This light hits the subject then bounces back to the camera. The light travels through the lens and strikes the digital CMOS or CCD sensor. The processor in the camera reads the light as it builds up exposure. Once the sensor and the camera software determine enough light has entered  for the correct exposure, the camera's computer turns off the flash.

I have Nikon not Canon and they call it iTTL instead of ETTL but its the same thing. Its supposed to give a more accurate flash exposure and can be set to do things like balanced fill light flash.

Oh and for film cameras instead of hitting a CCD or CMOS sensor it bounces off the film to a light sensor.