I was planning on doing a head to head comparison with the sigma 18-35mm f1.8 and my SMCP-FA 31mm f 1.8 limited this week, but due to de-centering in my copy of the sigma 18-35mm f/1.8 I have to return the lens. The issue is clearly visible at infinity focus at 18mm f/5.6 - the left side of the image is sharper than the right side. It wouldn't be fair to compare it to my perfectly centered FA31. I have also noticed some other troubling issues with the optical performance of this lens.
I really can't say much in regards to image quality due to the fact that my copy of this lens was optically compromised, however there is some potential. Along with the lacklustre Sigma quality control ( which, according to sigma is more stringent for "C" "A"and "S" series lenses*) I will mention these issues.
My example of lens flare from the 18-35mm f/1.8 at 35mm f/4:
And some disturbing frog egg bokeh from the 18-35mm at close focus distances:
And an example of the de- centering from my lens, 100% crops, un-sharpened from the extreme left of the frame to the extreme right, the more I zoomed out the worse it got.
* Hypothetically speaking I suppose the foveon sensors Sigma use to test their lenses are less sensitive to slight de-centering due to their 3D structure, Bayer sensors being a plane only a few microns deep would be more sensitive to it.
Last edited by Digitalis; 07-14-2014 at 08:16 AM.