Metering with different popup flash powers (T3i)

[daviddoria]

When I use the popup flash in Av, it fires a preflash to use for computing the shutter speed. I can then use the flash exposure compensation to change the flash brightness. I'm assuming the metering is always done with 0 EV even when the flash exposure compensation is set to a non-zero value, because the shutter speeds it computes are the same no matter what I have flash exposure compensation set to. Is there a way to get it to tell me the appropriate shutter speed for a selected flash power?

When in M, I don't understand why "flash exposure compensation" is still an option. The idea is to compensate for a misinterpreting of the settings it chooses for what you want to do with the shot. But in M, there is nothing that it chooses. Shouldn't this option just be called "flash power" at this point. (Actually, it's the same for "exposure compenstation" too - why would you ever use this in M?). Again, is there a way to get the meter to tell me what the exposure will be like with my selected flash power?

Thanks,

David

[ebiggs1]

The pre-flashes have nothing to do with exposure. It is simply for AF assist. The flash has a "guide number" that the camera is aware of. It uses this number to set the flash. Along with the ISO, aperture and shutter, of course.

Off camera flashes like the 580 EX II (E-TTL) do have exposure assist beams to help them. They do a fantastic job.