Originally posted by fikkser 
If it's reflections from the blades, it should be a problem with some other lenses too. I've never heard about it before.
I'm more than willing to substitute that theory, which is at least possible, with any other and better theory that you may wish to propose.
The fact that several of us experience this phenomena, that it only happens when the blades predominate (small apertures) and increases with higher f number and that you can experience it with sun out of influence (even in cloudy scenarios) doesn't leave much else.
But be my guest and fire away with another plausible possibility.
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