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Lens review

Sigma C 17-70 mm f/2.8-4.0 DC Macro OS HSM

13 March 2013
Arkadiusz Olech

7. Coma, astigmatism and bokeh

The coma makes itself felt the most at 17 mm where the point-like image of a diode turns into a distinct comma. However the aberration decreases as the focal length gets longer so there are no reasons to complain. Still there is nothing to admire either, especially that the situation is a bit worse than the results of its predecessor.

Sigma C 17-70 mm f/2.8-4.0 DC Macro OS HSM - Coma, astigmatism and bokeh

In the case of the older model of the tested Sigma the astigmatism was on a level of 4.3%. When it comes to the new model it is a bit higher, reaching 5.3%. Still taking into account the margin of error those results can be considered similar and low at the same time. In this category there are no reasons to complain.


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At the end of this chapter let’s find out how the blurry areas look, with the help of photos of defocused light points, positioned in the frame centre and in the corner of the frame. At the maximum relative aperture the light spread in the circle is more or less even. After stopping down a distinct rim on the edge appears - it might spoil a bit the positive perception of the blur.

Sigma C 17-70 mm f/2.8-4.0 DC Macro OS HSM - Coma, astigmatism and bokeh