Sigma 70-200 mm f/2.8 EX DG APO OS HSM
7. Coma and astigmatism
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The Sigma 70-200 mm OS corrects the coma very well. At 135-200 mm you can hardly notice any influence on the image of the diode and it is true for the APS-C/DX frame corner and full frame alike. Very slight coma can be noticed only after attaching the converter in the full frame corner at 70 mm so in a place where the resolution of a lens suffers the most.
We mentioned astigmatism in the chapter about the lens’s resolution. It has a noticeable growth tendency with the increase of the focal length and it increases sharply after attaching the converter. At 70 mm its value is still negligible amounting to just 2%. At 135 mm it reaches a not very bothersome but already visible level of 10%. At 200 mm its level is not much higher, because 11%, but here it features an unpleasant property of being visible even after a significant stopping down. However real problems start only after you put the converter on because then the astigmatism level rises to a high level of 21%.
