Sigma A 24-105 mm f/4 DG OS HSM review - Coma, astigmatism and bokeh

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Sigma A 24-105 mm f/4 DG OS HSM

14 November 2013

Arkadiusz Olech

7. Coma, astigmatism and bokeh

On the APS-C/DX sensor the coma is not problematic at all and even on full frame it can be hardy called bothersome. For the bigger focal lengths that aberration is low and only at a difficult combination of the wide angle and the maximum relative aperture the image of a diode in the frame corners is distorted . It is a result a tad better than that of the Nikkor – in its case the coma made itself felt also at the longer focal lengths.

Sigma A 24-105 mm f/4 DG OS HSM - Coma, astigmatism and bokeh

The astigmatism is low. An average difference between horizontal and vertical MTF50 function values amounted to 3.6%. We also noticed a slight growth tendency with the increase of the focal length: at 24 mm that aberration is just 1.0%, at 105 mm it already reaches 5.5%.

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Once again in this category the Sigma fares slightly better than the Canon 24-105L and the Nikkor 24-120 VR which astigmatism was 4.3-4.4%. Still the difference is merely cosmetic, its volume comparable to the measurement errors.

Defocused light points provide images which are far from perfect. First of all you get a lot of concentric rings (the so-called onion bokeh) – the effect is visible although not as strong as in the case of e.g. the Sigma 1.4/35. Additionally the intensity of the rim on the edge increases on stopping down and the images of the circle in the frame corners are distinctly truncated. Here the vignetting makes itself felt - we are going to write about it more in the next chapter.