Sigma A 17-40 mm f/1.8 DC
7. Vignetting
The situation changes significantly at the shortest focal length. Here it is obvious the Sigma has problems not only with classic coma, that transforms a point-like source of light into comma, but also with optical aberrations of higher degree that are responsible for characteristic 'wings'.
| 17 mm, f/1.8, center | 17 mm, f/1.8, Corner APS-C/DX |
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| 28 mm, f/1.8, center | 28 mm, f/1.8, Corner APS-C/DX |
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| 40 mm, f/1.8, center | 40 mm, f/1.8, Corner APS-C/DX |
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In order to make our test more interesting we also decided to show the image of a bright star put in the frame corner. These photos confirm the conclusions from the paragraph above. Additionally the coma doesn't disappear completely even on stopping down the aperture to f/3.5 so by 2 EV. Stopping down the aperture reduces efficiently aberrations of higher degree because the wings disappear practically completely. At the same time the star test confirms good coma correction at the longer end of the focal spectrum.
| Corner APS-C/DX, 17 mm, f/1.8 | Corner APS-C/DX, 17 mm, f/2.5 | Corner APS-C/DX, 17 mm, f/3.5 |
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| Corner APS-C/DX, 40 mm, f/1.8 | Corner APS-C/DX, 40 mm, f/2.5 | Corner APS-C/DX, 40 mm, f/3.5 |
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Astigmatism, understood as an average difference between horizontal and vertical MTF50 function values, amounted to 5.4%. It is a value on the borderline between low and very low values so you don't have any serious reason to worry. We should also add that the highest astigmatism level we noticed at the longest focal length but differences aren't especially pronounced.
Defocused circles of light look well for a zoom lens. There are no distinct heterogeneous areas inside and the presence of slight onion ring bokeh shouldn't surprise anyone because in this optical construction there are as many as four aspherical elements. At the same time some limitation in the size of the lens make themselves felt in a form of truncated circles of light in the corner, visible even on stopping down the aperture by 2 EV.
| 40 mm, f/1.8, center | 40 mm, f/1.8, Corner APS-C/DX |
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| 40 mm, f/2.5, center | 40 mm, f/2.5, Corner APS-C/DX |
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| 40 mm, f/3.5, center | 40 mm, f/3.5, Corner APS-C/DX |
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