Tokina AT-X PRO DX 11-20 mm f/2.8
7. Coma, astigmatism and bokeh
| 50D, 11 mm, f/2.8, center | 50D, 11 mm, f/2.8, corner |
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| 50D, 16 mm, f/2.8, center | 50D, 16 mm, f/2.8, corner |
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| 50D, 20 mm, f/2.8, center | 50D, 20 mm, f/2.8, corner |
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The astigmatism, understood as an average difference between vertical and horizontal MTF50 function values, amounted here to 4.5% which we consider to be a low value. It’s worth mentioning here that its level was the highest at the 11 mm focal length and the lowest at 20 mm but the differences weren’t that pronounced.
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When it comes to the appearance of defocused images there is nothing impressive there. The circles of light in the centre are noticeably darker (and it is connected to spherical aberration, mentioned earlier) and in the corners of the frame they are deformed, with lighter rims. Still it should be mentioned that their shape is oval but not truncated which means that the vignetting is caused mainly by the optics, not mechanical obstacles.
As you see there are no serious reasons to complain especially that in this type of instruments it is difficult to get wide areas of nicely emphasized blur.
| 50D, 20 mm, f/2.8, center | 50D, 20 mm, f/2.8, corner |
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| 50D, 20 mm, f/4.0, center | 50D, 20 mm, f/4.0, corner |
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| 50D, 20 mm, f/5.6, center | 50D, 20 mm, f/5.6, corner |
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