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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.

sample image sample image

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    This looks more like a sensor/image processing issue than a lens issue. Have you tried the same exposure settings under the same lighting conditions with other lenses? I would expect using all of your other lenses that you haven't been using for astrophotography will also demonstrate this same artifact under the same shooting conditions.
    – Michael C
    Jul 9, 2018 at 14:59
  • Could you try another lens with the same camera for astrophotography? Maybe the lens coating is the culprit. Somebody could have cleaned the lens to thoroughly. Not sure if the lack of coating would show up like this though.
    – Grebu
    Aug 16, 2018 at 19:55

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Based on the consistent position of the "ring", I suspect the cause is a sensor issue (as MichaelClark comments). This would be consistent with appearance of the ring at higher ISO and longer shutter speeds. To confirm that this is a problem with the camera body and not the lens, try the lens with a different body or a different lens with the same body.

The ring is particularly visible in the Red channel, barely in the Green channel, not at all in the Blue channel. It seems that the Red sensels in that part of the sensor have reduced sensitivity.

If you want to remove the ring in post processing, blur or dodge the red channel so that the ring is no longer visible. Then use color and luminosity layer blending modes with a copy of the original to restore details. Finally, over the original image, put a copy of visible with masking to isolate the affected region of the image.

image1r image2r

image1 image2

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Artefacts in colour images may be caused by debayering, but unlikely in this case.

Sharp edges around stars are vulnerable to artefacts particularly with gradient type algorithms. Bilinear and PPG are better candidates in that case.

A flat frame could verify obstructions in the optical path, which could be corrected by proper image calibration/reduction.

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  • While demosaicing can cause color artifacts, such artifacts would not be consistently located in the same location across multiple images.
    – xiota
    Aug 23, 2018 at 20:54
  • Agree. I skipped over your images which show the artefact well. Given the light source to the right and the shape of the ring, it could be optical as previously mentioned. Blue transmission is typically lower at night. Presumably, the camera is not modified and the red response attributed to a weak IR cut filter.
    – GeorgeC
    Aug 24, 2018 at 2:18

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