Sigma 17-50 vignetting
I have Sigma 17-50 f2.8 EX HSM lens on Nikon D7100 body. I have noticed recently quite strong vignetting in the corners under certain conditions. The effect is visible at long end with the aperture fully open. Here is a picture of a white uniform surface taken with this lens at 50 mm and f2.8:
The corners seem excessively dark to me. There was nothing on the lens: no filter, no hood.
This effect disappears when stopped down to about f4-5. It also disappears at shorter focal lengths.
The lens correction in Photoshop usually controls vignetting very well, and creates flat uniform exposure. With the above image, Photoshop corrects the overall vignetting, leaving the very corners dark. This makes me think that something might be wrong here.
Can anyone comment on this?