I got a photo of starry sky where I find different kind of aberrations on the left side and the right side of photo. It seems consistent in various photos so far, but this lens is quite new to me and I've had only so much time to get familiar with it.

The lens is a Samyang 14 mm f/2.8 ED AS IF UMC but do not take my sample photo as a proof of low quality. The sample is taken in high humidity weather with an entry level cheap system camera by an unexperienced star photographer - end of disclaimer.

This photo was taken with 20 sec exposure, f/4.0 and ISO 3200. Focus should be "quite good" for the photo is the sharpest one from a set of five that I took by manual focus bracketing. Image stabilizing is Off because I have the camera on a tripod and using a wireless remote shutter release. The Samyang 14mm lens is made for full frame camera, but mine has an APS-C size sensor, which means I'm not even getting the worst of the aberrations on my photos :)

Center of photo at 100% pixels:
enter image description here

Left and right side near edges:
enter image description here enter image description here

On the left side some of the stars appear like vertically split in half.
On the right side the brightest stars have a bright blue shadow cast on the right side of the star and less bright stars have a lesser shadow.

What can cause the aberrations to be different on either side of the photo?

Full size fine quality jpeg image here (12 MB).

  • APS-C or FF? How long was the exposure? What aperture? How are you stabilizing the camera during exposure? Are you using magnified Live View to focus? I see the Croma on the right and the ghosting on the left.
    – Michael C
    Jan 2 '14 at 2:34
  • @MichaelClark - Thanks for reminder, exposure info is there now. For the focus I used LiveView focus magnifying feature, with my strongest reading-glasses on my nose. Best focus I could find was far from the end of focus ring movement, but the lens is a prime lens... so to be on the safe side I did a five shot focus bracketing. Jan 2 '14 at 5:40

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.

  • It is probably the lens in this case, but slight variations in flange focus distance will be most noticeable at wider angles of view and apertures. Just a few microns can make a difference at 14mm.
    – Michael C
    Jan 3 '14 at 6:45

Lens element alignment is slightly different from one side of that lens to another. The causes of this can vary from simply lens assembly quality, or it could be more serious like uneven grinding. For the price though it's a good lens. You can make a preset in Camera raw to remove the CA.

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