Sigma A 20 mm f/1.4 DG HSM
5. Chromatic and spherical aberration
Chromatic aberration
The longitudinal chromatic aberration is not corrected in a perfect way. The slight cast (yellow for images after the focus and blue for images before the focus) is visible at the maximum relative aperture even on stopping down to f/2.0.
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When it comes to the lateral chromatic aberration the situation looks much better. Its function, depending on the aperture values, for both types of detectors presents a graph below.
All results range from 0.04 to 0.06% which means the aberration is practically imperceptible. This is a performance very similar to that of the Nikkor AF-S 20 mm f/1.8G ED as in its case we got a level of about 0.06–0.07%.
| Canon 5D III, f/1.4 | Canon 5D III, f/8.0 |
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Spherical aberration
The tested lens didn’t show any „focus shift” effect and images you get by f/1.4 are very sharp in the centre of the frame, without the characteristic contrast decrease connected to the presence of the spherical aberration. In photos of defocused circles of light you still can notice a very delicate influence of that aberration, though. The circle we got in front of the focus has a slightly darker edge than the circle behind the focus. Still that effect remains slight and it doesn’t change our positive assessment of the lens in this category.
| Canon 5D III, f/1.4, in front of | Canon 5D III, f/1.4, behind |
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