Why do concentric rings appear in a long-exposure night-sky photo from my phone?
Asked 10/25/2018
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I used my phone’s long-exposure mode to photograph the night sky with focus set to infinity/far. In the resulting image, faint concentric circles are visible, especially after adjusting levels to make them easier to see. What causes these rings in a phone long-exposure image?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
7y ago
2 Answers
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The circles themselves are likely caused by vignetting of the camera lens.
As for the discrete colours that you're getting: Did you save your image as jpg or raw? If it's jpg then it's highly likely that the levels are due to digitising noise. Grey level in each channel are only represented by 256 values, so in the dark sky, the pixels would only have low values like 1,2,3,4. When you adjust the curves/contrast in your image these discrete levels gets stretched out to an extend that you eye can distinguish them. Compression of jpg files may also have a role to play in this posterisation effect but are not the dominant cause in your case.
Originally by user71741. Source · Licensed CC BY-SA 4.0
user71741
7y ago
0
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The ring pattern is most likely a combination of lens vignetting and posterization becoming visible after heavy editing.
- Concentric rings: These can come from the lens/sensor shading pattern of the phone camera, often seen as vignetting or uneven illumination from center to edge.
- Visible bands after levels adjustment: In a very dark sky, pixel values are clustered into only a few low brightness steps. If the image was saved as JPEG rather than RAW, each color channel has limited tonal precision, so boosting contrast or levels can stretch those tiny steps into obvious bands or colored rings.
- JPEG compression may add to the effect, but the main issue is limited tonal depth in dark areas.
So the circles were probably faint in the original capture, and editing made them obvious. Shooting in RAW (if your phone supports it), using gentler adjustments, and avoiding excessive shadow lifting can reduce the effect.
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