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Circular Pattern in Night Sky Image

Photography Asked by pH13 - Yet another Philipp on November 16, 2020

A few days ago I took a night sky image of the milky way with my XPERIA 1 II and wanted to take a closer look at the RAW file in Lightroom Mobile. When doing so, I realised a strange circular pattern.
Though "auto" correction already revealed this pattern, I additionally increased the exposure to make it even more visible.

night sky image with circular pattern
ƒ/2,2 30 Sek. 2,67 mm ISO 50

For me, it looks like something from the lens, but I am very new to photography and especially in post-processing RAW files, so I am wondering what it might be.

Is this something that can be corrected by whatever additional images?

2 Answers

I do not think this artifact is caused by reflections in the optical path. I think this is due to software correction of light falloff as a function of distance from image center (i.e., vignetting compensation). Normally such correction is not very noticeable because the signal-to-noise ratio (SNR) is high enough. But in this case, the image is so dark, that lifting the the image by a couple stops towards the corners greatly magnifies the noise in those regions as well.

Notice that the noise at the top, above the outer ring, and in the bottom center and lower right, is substantially more pronounced ("grainier") than in the center. There is nothing optically that would cause radially-biased digital noise. That has to be a result of software increasing the signal level (i.e., digital multiplication). The upper-right and upper-left corners outside of the ring especially show a lot more noise. Optically, these regions should be darkest due to vignetting, so their SNR is lowest. After digitally boosting the corners for light falloff, the only signal there is just boosted noise.

Correct answer by scottbb on November 16, 2020

That very much looks like a reflection of the lens on the protective glass to me... along with another reflection from something outside. It does not look like banding due to editing.

This is the kind of thing one might see with an SLR lens that has an uncoated filter on it.

IDT there is anything you can really do about it. In more normal conditions it is not likely to be problematic as there is much more light; which makes the dark reflection on the glass inconsequential in the total exposure.

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It matches up a lot like this enter image description here

Answered by Steven Kersting on November 16, 2020

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