Sigma C 18-200 mm f/3.5-6.3 DC Macro OS HSM
7. Coma, astigmatism and bokeh
The astigmatism, understood as an average difference between horizontal and vertical MTF50 function values, amounted to 6.5%. It is a moderate result but you should keep in mind the fact that it is an average value, worked out for all focal lengths. The peak of this aberration you get near 100 mm where it is over 11% so entering medium levels.
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An advantage of defocused light points is such that the images you get in the corner don’t differ much from those in the centre. Still there are also very visible faults like the rim on the edge and their not especially round shape. After stopping down the aperture shape looks more like an egg than a circle. The producers really could have made a better effort here. By the way such savings as this one make me wonder: how would the lens be priced if, instead of the aperture you get now, the producer put a nice, circular one, with nine diaphragm blades? Somehow I don’t think the difference would be really big…
