Improper sync gives partial frame exposure.
I hope you don't mind me asking you a question, but you have some of the same equipment that I do and I was wondering if you ran into this problem before.
I have a 70d body and 580exii flash. I shoot with either a 400 5.6 or 180mm macro 3.5 most of the time. I shoot primarily nature.
Here is the issue. My flash is not being recognized all the time by the camera.
When I put the flash into hss on the flash, it shows up on the flash as hss, but when I take any shots with a shutter speed faster than 1/340th or so, I get progressively darker image with a light section at the top and a clean line across and dark on the bottom. So, I don't think I am actually getting hss. In other words at 1/400th or so I get 1/2 of the image ok and the bottom half dark, at 1/600th or so 1/4 of the image at top ok and the rest dark.
It was working fine before I went to Costa Rica with it.
Here is the kicker. My friend's 580ex i works fine on my camera and my flash works fine on his (7d).
I have cleaned all the contacts and it seems to sit nice and snug.
Thanks for your question about the EOS 70D and your Speedlite 580EX II. Sorry to hear your sync issues. Believe it or not I still use my original 580EX when shooting professional jobs, as apposed to when I am in "snapshot" mode when I just carry along my 270EX II. The 270EX II can also be used with high speed sync, but it must be set within the camera's menu.
I have had no issues with high speed sync and have used it recently where I think I was at 1/000 second on my 5D Mark III for some outdoor portraits in the sun.
I just now attached my 580EX to my 70D and had no issue at 1/4000 second flash sync. Of course if you take the 580 off the hss setting, the flash "tells" the camera its fastest shutter speed allowed is 1/250th second.
The fact that you and your friend can switch flashes and have success is quite puzzling.
I would have guessed that available battery power is a possibility. I always use a Quantum Turbo battery with my Speedlite. Maybe if you have weak batteries, your flash is coming up short on the super-quick multiple flashes that are used for high speed sync shooting?
The partially dark image results from only a portion of the sensor being exposed from the "slit" of the focal plane shutter as it passes in front of the sensor when the single flash fires.
I would call Canon USA directly. I've found them to be quite helpful when issues arise. Hope that gives you some ideas.
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