Sigma 17-50 mm f/2.8 EX DC OS HSM
7. Coma and astigmatism
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Coma shows a distinct decreasing tendency with the increase of the focal length. At 17 mm it is very noticeable, at 30 mm – medium and at 50 mm a bit lower than medium. This performance can partially explain why exactly at 50 mm focal length you reach the best results when it comes to the sharpness of the image at the frame edge.
There will be no problem with astigmatism because the Sigma 17-50 mm f/2.8 corrects it perfectly. The average difference between vertical and horizontal MTF50 function values amounted to just 2.4%.
