Tokina AT-X PRO FX SD 17-35 mm f/4 (IF)
7. Coma, astigmatism and bokeh

When it comes to another off-axis aberration, namely the astigmatism, an average difference between horizontal and vertical MTF50 function values amounted to a moderate level of 8.1%. Still the astigmatism levels aren’t similar for all focal lengths –at the maximum focal length that aberration reaches 13%, making the most considerable contribution to the overall result. It is also one of the reasons of the weak performance of the lens on the edge of the frame at 35 mm.
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We have no reservations concerning the off-focus images. It’s true that you can notice onion rings structure but that effect is not very pronounced. What’s important, the images you get even in the very corners of full frame are oval, without any strong truncations; it suggests no parts of the casing disturb the optical paths and the sizes of the elements fit well the parameters of the lens and the size of the detector.
