In Av mode, your camera is going to meter for ambient light and set the shutter speed accordingly (thus, the longer shutter speed).
In Tv mode, your camera will try to do the same thing ... but when it reaches its limit on aperture, it'll seem as if it is acting differently (i.e. if you pick 1/60th of a second, and the aperture can't go any wider than f/2.8, then that's what it'll use, and e-TTL will make up the difference).
E-TTL simple tells the flash to fire a pre-flash, measure the light coming back, and adjust the power on the flash accordingly. Then when the shutter opens it fires the appropriate amount of power and you get a good flash exposure.
So in Av mode, e-TTL is always trying to play "fill flash" regardless of how much light there is. In Tv mode, it tries to do that, but runs up against your aperture limit.
Personally, once flash becomes the primary source of light, I simply switch to manual mode and set the shutter and aperture to what I want, and let e-TTL fill in the rest.
|
