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

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Tokina AT-X PRO FX SD 17-35 mm f/4 (IF)

16 September 2013

Arkadiusz Olech

7. Coma, astigmatism and bokeh

Looking at the crops below and observing how the coma is distorting the point-like image of a diode you can understand at once why the lens gets best resolution results in the middle of the focal range. The coma is corrected well there and its slight influence can be noticed only in the very corner of the full frame. At both ends of the focal range the situation is noticeably worse. The fact that the influence of the coma is quite noticeable already in the corner of the APS-C/DX sensor should be especially criticized. Comatic aberration in the corner of full frame at the shortest focal length are really high – small wonder it is so difficult for the lens to produce sharp images there.

Tokina AT-X PRO FX SD 17-35 mm f/4 (IF) - 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.