monito

I suspect that this is a mirror problem. The mirror may not be rising all the way and sealing the image chamber.

You could test for this by blocking the viewfinder window to stop light leaks coming in there. Another test might be to put it on manual with the same setting and circumstances as a picture known to exhibit the problem, but take a picture with the lens cap on and leave the viewfinder exposed. If you get a streak in that one, I would suspect the mirror.

A poster in another thread elsewhere suggested there might be a leak between the lens and camera, but I find this unlikely. You could test for this by wrapping a black cloth around the lens mount in a situation known to produce the problem.

http://photonotes.org/articles/eos-flash/index3.html#manualflash

http://photonotes.org/articles/eos-flash/

This has useful information, particularly the master page (second URL), but may not directly answer your question.

At, say 1/8 power, the flash is putting out one eighth or 12.5 percent of the full power of the flash. Exposure values are in steps of one half each (or two times, going the other way), so this is three steps.

At 1/8, you could open your lens up three stops (for shallower depth of field and to receive more ambient light), or you could increase your ISO setting down three levels (from, say, ISO 100 to ISO 800). You will get three stops more exposure from the ambient light this way too.