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Flash over exposing in ETTL

I've used a yongnuno ETTL flash twice over last couple of weeks with a 5d mk3 I borrowed and both times and both sunny dates the flash was blowing out. I was using AV and gone down a stop and it still blew out. To bring the exposure back to acceptable levels I went 3 stops under in camera and 2 stops under with fec. My ISO was down at 100, it baffled me completely and went anything I've ever read about flash. I was on spoit metering initially and switched to evaluative but no difference or not significant anyway.

Any ideas?



  • TonyTTonyT Member
    I suspect that your flash sync speed in AV mode was fixed to either 1/60th or between 1/60th and the cameras max flash sync speed.
    Hence when it was sunny your cameras light meter would have been dictating a higher shutter speed but was being overridden by your flash sync in AV mode i.e. much slower than needed for proper exposure,  therefore giving you a blown out exposure.

    Change the setting to auto and you should be good to go.
  • Also, if the flash is not mounted properly in the hot-shoe, then it will tend to fire at full-power.  That is something to check for.
  • Neil vNNeil vN Administrator
    I agree with Tony that this is your most likely scenario - when you switched your flash on, the HSS wasn't enabled, and your camera brought your shutter speed down to max flash sync speed. 
  • MaryMary Member
    So in such bright conditions if I use Hss it should rectify this? Thanks so much for responses, much appreciated
  • TonyTTonyT Member
    Mary, if you use AV mode you will need to change your 'flash sync speed in Av mode' setting, which is probably set to the cameras max flash sync speed. Change this to auto then camera will operate the shutter at the required speed for correct exposure. Changing the flash to HSS alone without changing the sync speed in Av mode will leave you in the same boat.
  • MaryMary Member
    It was set at 1/60 to 1/200, so I've now changed to auto. Thanks again Tony this was really really frustrating me
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