Why do I see a dark half-ring in night photos, but not daytime shots?
Asked 7/9/2018
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2 answers
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I’m seeing a dark, half-formed ring near the bottom of my images when shooting astrophotography. The setup is a Tokina 11-20mm f/2.8 at 11mm, typically around 25 seconds and high ISO (up to 6400). The lens appears clean and undamaged, and focus checks out. The ring does not appear in daytime photos. At lower ISO it becomes much less visible, and stopping down to about f/3.2 almost removes it. What could cause this, and is it more likely to be the lens, camera sensor, or processing?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
8y ago
2 Answers
4
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.
Originally by user75526. Source · Licensed CC BY-SA 4.0
user75526
8y ago
0
Generated from our catalog & community — verify before relying on it.
The consistent position of the ring points more to the camera/sensor side than the lens. Because it shows up mainly in long, high-ISO night exposures and not in normal daytime shots, it’s likely a sensor response issue or related image-processing artifact rather than visible lens damage. One answer noted it appears strongest in the red channel, suggesting reduced sensitivity or uneven response in that area of the sensor.
Best way to test it:
- Try this lens on another camera body.
- Try a different lens on the same body.
- Shoot a flat frame to check for optical-path issues.
If the mark stays in the same place with different lenses, the body is the likely cause. If it follows the lens, investigate the optics or calibration.
For processing, check whether raw conversion/debayering is exaggerating the artifact. If needed, local correction in post—especially targeting the red channel—can reduce it.
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AI8y ago
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