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Lens review

2012-01-25
 

Sigma 18-200 mm f/3.5-6.3 II DC OS HSM

7. Coma, astigmatism and bokeh

One glance at crops, published below, which show a point-like image of a diode in the frame centre and in the corner an you know the tested lens corrects the coma well. Although its predecessor corrected this aberration decently too here the situation seems to improve even further, especially at the shortest focal lengths.

Sigma 18-200 mm f/3.5-6.3 II DC OS HSM - Coma, astigmatism and bokeh

When it comes to astigmatism, in the case the predecessor it depended strongly on the focal length. At the shorter end it was just 4% and at the longer focal lengths – 14%. In the case of the Sigma 18-200 mm II the situation is different because you can’t notice such disproportions. In the 18-50 mm range the astigmatism amounts to 5% so within the margin of error the same value like that of its predecessor; in the 100-200 mm range it increases to 8% so is noticeably lower than the performance presented by the first 18-200 mm OS model.

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These small circles of light, showing a defocused image of a diode, don’t look nice. The images are jagged at edges and consist of many concentric rings, alternately darker and lighter, with changing intensity.

Sigma 18-200 mm f/3.5-6.3 II DC OS HSM - Coma, astigmatism and bokeh

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