Originally posted by Alex645 Nice theory, but a back reflection at f/11 on a f/1.8 modern lens?
Ummmm...yes. Why not? My comment is based on personal experience with the reflect-back anomaly. The relative size of the aperture outline is a clue, though the anomaly is fairly hard to characterize except that it is most common with lenses having a fairly exposed, wide diameter rear element. The Sigma 18-35 fits that description. As for blade reflectance...I don't have the Sigma 18-35 in hand, though if the blade finish is anything like that on my Sigma 17-70/2.8-4 (C), it would be a medium gray anodized brushed surface with albedo comparable to that of blades of most of the lenses on my shelf.
I know, it is a apples vs. beefsteak comparison, but here is a sensor reflect hot spot at f/11 from a K10D. The image has not been cropped and the taking lens was a vintage M42 Auto-Rikenon 55/1.4 (single coated) with a deep hood fitted. The Rikenon has a notoriously wide and prominent rear element and while it is not real evident in this shot, there is a halo of reduced contrast extending several degrees concentric to the aperture spot, but which does not veil over the full field.
I had some other shots taken of a scene with bright highlights oblique to the lens axis and out of frame that resulted in a hot spot almost identical to the example posted here. Unfortunately, those were bit-bucketed some years ago.
Anyway, I am not married to that explanation.
Steve