Sigma C 20 mm f/2 DG DN review - Coma, astigmatism and bokeh

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Sigma C 20 mm f/2 DG DN

5 July 2022

Maciej Latałło

7. Coma, astigmatism and bokeh

The Sigma C 20 mm f/2 DG DN doesn't correct comatic deformations perfectly well. Images of a diode are slightly enlarged already in the corners of the APS-C sensor, and you can notice not only coma (that turns that diode into a comma) but also aberrations of higher degree that create these characteristic wings. Unfortunately, stopping down the aperture to f/2.8 doesn't allow to reduce this aberration in a significant way.

Center, f/2.0	Corner APS-C, f/2.0	Corner FF, f/2.0

Center, f/2.8	Corner APS-C, f/2.8	Corner FF, f/2.8

Astigmatism, understood as an average difference between vertical and horizontal MTF50 function values, amounts to 3.7% and it is a low level. The lens should be praised in this category.

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The optical construction of the Sigma 2/20 features as many as three aspherical elements and their influence is visible when it comes to light spread in the out-of-focus circles – they are hardly smooth, with slightly accented onion-ring bokeh structure. Still, for a wide angle lens, such a result seems to be quite sensible. It's also worth noticing that mechanical vignetting makes itself felt even by f/4.0.

Center, f/2.0	Corner APS-C, f/2.0	Corner FF, f/2.0

Center, f/2.8	Corner APS-C, f/2.8	Corner FF, f/2.8

Center, f/4.0	Corner APS-C, f/4.0	Corner FF, f/4.0