Sigma A 18-35 mm f/1.8 DC HSM review - Coma, astigmatism and bokeh

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Sigma A 18-35 mm f/1.8 DC HSM

18 June 2013

Arkadiusz Olech

7. Coma, astigmatism and bokeh

The coma is not corrected in a perfect way. Its influence can be noticed the easiest at the shortest focal length where the shape of the diode in the frame corner is noticeably distorted. Still we don’t consider this aberration’s level as high – medium would be its best description.

The coma also makes itself felt at 24 and 35 mm focal length still its level there is lower than at 18 mm so it is not a serious factor, contributing to the decrease of the image quality.

Sigma A 18-35 mm f/1.8 DC HSM - Coma, astigmatism and bokeh

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The average difference between vertical and horizontal MTF50 values amounted to just 3.5% so it allows us to describe the astigmatism as very low. It’s worth mentioning here that the 24 mm focal length and f/1.8-2.0 apertures make the biggest contribution to that value.

Defocused images of light points, provided by the Sigma 18-35 mm, are presented below.

The only thing that can make you worried are the concentric rings, perfectly visible at all apertures. Fortunately their intensity is not very high. You should praise a nice, circular aperture and, by and large, the lack of vignetting already by f/3.5.