Sigma A 35 mm f/1.2 DG DN review - Coma, astigmatism and bokeh

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Sigma A 35 mm f/1.2 DG DN

11 September 2019

Arkadiusz Olech

7. Coma, astigmatism and bokeh

The f/1.2 aperture and a wide angle of view are pretty demanding when it comes to coma correction. The Sigma's constructors didn't manage to control that aberration completely well because in the corners of full frame the image of a diode is a bit deformed; still that effect is slight and, taking into account the degree of difficulties, overall, I suppose, we shouldn't complain.

Center, f/1.2	Corner APS-C, f/1.2	Corner FF, f/1.2

Center, f/1.7	Corner APS-C, f/1.7	Corner FF, f/1.7

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

Center, f/1.7	Corner APS-C, f/1.7	Corner FF, f/1.7

Center, f/2.5	Corner APS-C, f/2.5	Corner FF, f/2.5