Why do stars show different aberrations on the left and right sides of my image?
Asked 1/1/2014
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2 answers
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When shooting a star field with a Samyang 14mm f/2.8 on an APS-C camera, I’m seeing different star distortions near opposite edges of the frame. The center looks fairly sharp, but on one side some stars appear split vertically, while on the other side brighter stars show a colored offset or smear.
The effect appears consistently across multiple images. The photo was taken on a tripod with stabilization off, 20 seconds, f/4, ISO 3200, with manual-focus bracketing. What could cause aberrations to differ from left to right like this?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
12y ago
2 Answers
2
What you're seeing is most likely the artifacts of slight misalignment of the lens, the sensor plane is not exactly in line with one or more lens elements which gives rise to slight variations from left to right.
All lenses have a degree of misalignment, and generally it is more visible the shorter the focal length. So for a 14mm lens even a miniscule misalignment can be visible in images. I would expect that your 14mm lens is within factory spec.
Normally such effects are masked by the depth of field but shooting a starfield at infinity with a wide angle lens is a bit of a torture test for the lens designer!
Edit: on closer inspection the artifacts look like comatic aberration (that's comatic, not chromatic) which is common in wide angle lenses, and is probably the restult of a slight misalignment of a lens element/group, you might be able to send the lens in for service, but it may be deemed to be within spec.
Originally by user1375. Source · Licensed CC BY-SA 4.0
user1375
12y ago
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Generated from our catalog & community — verify before relying on it.
The most likely cause is slight lens misalignment (decentering), where one or more lens elements are not perfectly aligned with the sensor plane. That can make aberrations look different on opposite sides of the frame.
In star photos this is especially noticeable because stars are tiny point sources at infinity, which is a very demanding test for a lens. Wide-angle lenses often show coma near the edges, and if the lens is even slightly decentered, the coma can appear uneven from left to right.
So the issue is probably not your technique, but the lens’s optical alignment combined with the fact that astrophotography reveals these flaws very clearly. Some degree of this is common, especially in very wide lenses.
If needed, some colored fringing can be reduced in raw processing, but the underlying star shape distortion is optical rather than a simple camera-setting problem.
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