Originally posted by BruceBanner Could it be that if the flash in P-TTL mode is firing it's weakest version and the exposure will still be too much it then has to bump something up for the shot so raises the shutter speed?
Originally posted by pschlute No, because the shutter speed will not affect the exposure provided by the flash. The flash duration is too fast for any change in shutter speed to have an effect.
Peter is right with this; its important to keep emphasising that exposure time does not influence the flash exposure level in normal operations .... only in HSS mode will exposure time have an effect on flash exposure.
If the meter detects an overexposure situation even though the flash would output its lowest possible level, then it would need to adjust another variable, ie aperture or ISO, in order to allow a good flash exposure. Which of these varies would depend on the mode of course, and whether ISO is available to vary (auto-ISO parameter settings).
If the need is to reduce flash exposure, and the flash is already at its lowest possible output, then in Av mode the camera only has one thing to vary .... ISO, which would have to be lowered. The only alternative is for the photographer to step back more and increase the distance, then try again with a new pre-flash and test shot.
Regarding shutter speed auto control, then this is determined mainly by the camera flash mode setting and the lens focal length. The system aims to provide a steady handheld shot as the first requirement, and in dim conditions this will result in a poor ambient/flash balance, unless the photographer overrides this with Slow Speed Sync mode.