Non-symetrical chromatic aberrations?

Question

I have a canon 40d camera for about 9months now. Ive recently noticed a problem with my favourite lens.. the sigma 10-20mm. There appears to be quite heavy chromatic aberrations on the left side of the image [particularly at 10mm], yet the right is clear. My problem is that when I try to reduce them in the lens distortion section in CS3 they appear stronger on the right side as the treatment appears to apply symetrically. Most of my older pictures taken with this lens have little or no chromatic aberrations and they do appear symetrical..

1. Is my lens damaged [anelement disloged or something?] and should I return it to the shop [its 9 months old]?

2. Is there another software that can fix this problem, taking the horizontal offset into account?

Answer 1

Chromatic aberrations are caused by a positive lens behaving like a prism, ie the light being dispersed into the different colours of the spectrum and therefore focusing at different distances.

This is corrected in lens manufacture by adding a negative lens usually of different refractive index glass.

If your aberration is now non-symmetrical where it was once symmetrical, it's certain that the negative lens has been knocked out of alignment with the other lenses and should be binned.

This is a problem when you buy a cheap lens for a good camera, instead of the canon lens which I know is much more expensive.

Answer 2

My Sigma 10-20 shows no similar signs.Has yours had a knock? Also does the fault change with the light? Try a similar shot with the light in the  opposite direction..If the fault persists with the light in the opposite direction,Have you a UV filter fitted? Take it off and try again.If all else fails send it back if it is still under guarantee.As to the quality of the lens generally I find mine has less aberration than my Nikon 18-200 and it is one of the most useful I have owned in terms of quality,and that includes Hasselblad stuff!!!!

Answer 3

It sounds like something is misaligned. it could be a tilted or malformed lens element, or it could be a mechanical misalignment in either the lens or camera. Try the lens on another body if you can to see if the problem is with the lens or with the camera.