Originally posted by kaarde 1- When I have my flash on my camera (directly), in Av mode, the shutter speed is automatically setup depending on my zoom (ranges between 1/60 to 1/125), which I presumes is the P-TTL function.
Actually, in Av mode the camera is setting the shutter speed so that the flash will balance with the ambient light. It's probably just that as you're zooming the overall reflectance value of the scene is changing so the camera is raising or lowering the shutter speed so that half the light comes from the scene, and half will come from the flash when it is fired. Try focusing on an evenly lit, solid colored wall. Stand reasonably close so that when you zoom in and out you still have nothing but wall in the frame. Does the shutter speed change?
I don't really advocate using Av mode with flash though because it relies on reflectance values, so if you have a particularly light or dark or shiny subject, the camera, and therefore the flash can be easily fooled. Try this:
1) With the flash on your camera and turned on, set your camera to Manual mode, set it to matrix metering, and compose your shot.
2) Now, without moving the camera, select the AF point in your viewfinder that lines up with your subject, the camera will give more "weight" to the exposure value of that point. Hit the green button.
3) Now check, is the aperture/shutter speed something you can live with? If not, hit the AE Lock button and adjust either the shutter speed, or the aperture and the other variable will move in the reciprocal direction to maintain correct exposure. Now take the shot.
That should give pretty good results for an average scene/subject, but if the background looks to bright or too dark, just adjust the shutter speed. It will not have moved because you are in Manual mode. If the subject looks too bright or too dark but the background looks ok, adjust the flash power either on the flash itself, or in the Fn menu on the camera. If both subject AND background look too bright or dark, adjust the aperture or ISO.