Sigma C 20 mm f/2 DG DN
5. Chromatic and spherical aberration
Chromatic aberration
Although the optical construction of the tested Sigma features two elements made of low-dispersion glass (one FLD and one SLD) longitudinal chromatic aberration hasn't been corrected perfectly well. You can notice its traces at the maximum relative aperture and also on stopping down the aperture by 1 EV. Fortunately, in both cases, the problem is not especially distinct.Still the Sigma performs here weaker than the Sony 1.8/20G and the Sony's task was harder as its aperture is a bit faster.
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Now let's check how the tested Sigma fares when it comes to lateral chromatic aberration – an appropriate graph you can find below.
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It is clear this aberration doesn't depend either on the detector or aperture values. Practically all our measurements show results close to 0.04% so on the borderline of negligible and low level – the Sigma should definitely be praised here. After all, proper correcting of lateral chromatic aberration is nothing easy with such lens parameters.
| A7R III, RAW, FF, f/2.0 | A7R III, RAW, FF, f/11.0 |
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Spherical aberration
First photos of this chapter don't show any distinct 'focus shift' effect but the appearance of out-of-focus circles we got before and behind the focal point indicates that spherical aberration hasn't been corrected in a perfect way. There are distinct differences - the circle before the focus shows a brighter rim.
| A7R III, f/2.0, before | A7R III, f/2.0, after |
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