Kind of interesting. Leaf shutter SLR (Voigtlander Bessamtic). The camera was on on loan to a friend when the shot was taken, but I assume it was at a fast shutter speed (250 or 500) with the aperture close to wide open. The shadow of all 5 shutter blades are clearly visible. I've done some googling on this effect and all I've found is exposure correction tables for leaf shutters at small f-stops and high shutter speeds. My best guess is that the Bessamatic is a one of those few 35mm leaf shutter SLRs with the shutter behind the lens, rather than in the optimal location (between the elements), resulting in the vignetting rather than plain old over exposure.