Sigma A 35 mm f/1.2 DG DN
7. Coma, astigmatism and bokeh
| Center, f/1.2 | Corner APS-C, f/1.2 | Corner FF, f/1.2 |
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| Center, f/1.7 | Corner APS-C, f/1.7 | Corner FF, f/1.7 |
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Astigmatism, understood as an average difference between horizontal and vertical MTF50 values, amounted to 7.5%. It's a moderate level which shouldn't worry you. It's worth noticing that the aberration will be visible mainly by f/1.2 and f/1.4; by f/2.0 it becomes practically imperceptible.
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Defocused circles of light look good but we have some reservations concerning two things. Firstly, aspherical elements in the construction caused a slight onion ring bokeh effect. Secondly, and a far more important problem, we noticed mechanical vignetting and cropping of circles in frame corners, an effect which, by f/1.2, is really significant. How does it influence brightness loss in the frame corners? That question will be answered in the next chapter.
| Center, f/1.2 | Corner APS-C, f/1.2 | Corner FF, f/1.2 |
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| Center, f/1.7 | Corner APS-C, f/1.7 | Corner FF, f/1.7 |
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Center, f/2.5 | Corner APS-C, f/2.5 | Corner FF, f/2.5 |
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