Recently started astrophotography with a used Tokina 11-20mm f/2.8 lens purchased from the original owner. The lens is scratch free, clean, and well kept. All focus options work perfectly in manual and autofocus modes. I checked "backfocus" with chart, and all is in order.
I have noticed a half-formed dark "ring" in images taken at night. There are no marks, blemishes, distortion, or half ring in shots taken during the day, at any time from sunrise to sunset.
Images were shot at 11mm f/2.8 at 25 sec with ISOs up to 6400. With lower ISOs, such as ISO 800, the mark becomes darker and is not really seen. If shot at f/3.2, the mark is almost gone, but then the stars are not sharp.
Can anyone explain the mark and whether anything can be done to fix it?
Here are some sample images to illustrate the mark, located at the bottom of the unrotated image.