Sigma C 20-200 mm f/3.5-6.3 DG
7. Coma, astigmatism and bokeh
| Center, 20 mm, f/3.5 | Corner APS-C, 20 mm, f/3.5 | Corner FF, 20 mm, f/3.5 |
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| Center, 35 mm, f/4.5 | Corner APS-C, 35 mm, f/4.5 | Corner FF, 35 mm, f/4.5 |
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| Center, 50 mm, f/5.0 | Corner APS-C, 50 mm, f/5.0 | Corner FF, 50 mm, f/5.0 |
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| Center, 100 mm, f/6.3 | Corner APS-C, 100 mm, f/6.3 | Corner FF, 100 mm, f/6.3 |
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| Center, 200 mm, f/6.3 | Corner APS-C, 200 mm, f/6.3 | Corner FF, 200 mm, f/6.3 |
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In order to make our test more interesting we decided to show images of a bright star put in a frame corner. This time we used only extreme focal lengths but also we showed how stopping down the aperture influences this aberration.
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Elongation of point-like images of stars is accented in a different way because coma is much weaker corrected for shorter wavelengths and it leads to creation additional blue 'ghosting'. It's worth adding that, in order to see them, you have to put a really bright star in the corner and additionally overexpose it.
| Corner FF, 20 mm, f/3.5 | Corner FF, 20 mm, f/5.0 |
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| Corner FF, 200 mm, f/6.3 | Corner FF, 200 mm, f/9.0 |
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Astigmatism, understood as an average difference between horizontal and vertical MTF50 function values, amounted to 9.1%, a value on the borderline between low and medium level so in this category there is no serious reason to complain. It's worth noticing the fact that the highest astigmatism value we measured at the longest focal length, where it reaches as much as 18.8%, a level that can be called only high.
Superzoom lenses aren't known for nice out-of-focus areas but the Sigma 20-200 mm fares quite well here. Circles of light we got at the longest focal lengths are pleasing to the eye, without any local extreme points, onion-ring bokeh barely perceptible. This fact is really worth our praise because, after all in the optical construction are as many as four aspherical elements. Mechanical vignetting is a distinct problem, as it remains visible even on stopping down the aperture by 2 EV.
| Center, 200 mm, f/6.3 | Corner APS-C, 200 mm, f/6.3 | Corner FF, 200 mm, f/6.3 |
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| Center, 200 mm, f/9.0 | Corner APS-C, 200 mm, f/9.0 | Corner FF, 200 mm, f/9.0 |
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Center, 200 mm, f/13 | Corner APS-C, 200 mm, f/13 | Corner FF, 200 mm, f/13 |
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