5D mk iii external flash problem...
I recently had a similar experience. I was using a Sigma 610 flash and exposure was way off (E-TTL is not properly working). It was so much inconsistent. Then I got a Canon 430 EX II and everything is perfect. 5D Mark III is a newer camera and some of these non-Canon flashes needs software updates to work with 5D Mark III.
Also these non-canon flashes are reverse engineering the ETTL and it may not be reliable always. I understand that Canon flashes are expensive, but you may be able to get similar quality ones from Metz or Nissin (no personal experience, but my friends are rating them really high).
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