Sigma 18-50 mm f/2.8-4.5 DC OS HSM
7. Coma and astigmatism
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A quick glance at the clippings below shows that apart from chromatic aberration, it’s coma that’s the main reason for decreasing the image quality at the edge of the frame. At the shortest focal length and in the middle of the range it’s very large and becomes acceptable only at the maximum focal length.
Astigmatism is often a good way of measuring the build quality of a lens and the amount of plastic in it. All the inequities in lens’ setting, maintained by a sloppy lens body display in a “blast” of off-axial aberrations, especially astigmatism. Sigma 18-50 OS passes this test without much of a trouble. Astigmatism here equals 5% which is a result between good and very good.
