Please correct me if I'm wrong... but this is my understanding of TTL & flash TTL.
TTL stands for 'Through The Lens'. It means in order for the camera to have an accurate reading of the amount of light available to capture a picture at a particular setting, this is the best way to get such a reading, since this is also the same path that light will have to take to travel to reach the CCD in order to produce a picture.
Compared to readings taken from outside the camera, using remote sensors, which may be affected by other factors like different light angles, sensor sensitivity, stray shadows...., the TTL way is more accurate.
In this case, the flash unit gets info from the camera about the amount of light that is actually travelling through the lens barrel. This info is then used by the flash unit to calculate how strong the flash output should be. Less light in the lens=stronger flash output.
Rite?

TTL stands for 'Through The Lens'. It means in order for the camera to have an accurate reading of the amount of light available to capture a picture at a particular setting, this is the best way to get such a reading, since this is also the same path that light will have to take to travel to reach the CCD in order to produce a picture.
Compared to readings taken from outside the camera, using remote sensors, which may be affected by other factors like different light angles, sensor sensitivity, stray shadows...., the TTL way is more accurate.
In this case, the flash unit gets info from the camera about the amount of light that is actually travelling through the lens barrel. This info is then used by the flash unit to calculate how strong the flash output should be. Less light in the lens=stronger flash output.
Rite?