Sigma C 90 mm f/2.8 DG DN review - Chromatic and spherical aberration

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Sigma C 90 mm f/2.8 DG DN

8 November 2021

Maciej Latałło

5. Chromatic and spherical aberration

Chromatic aberration

Inside the Sigma you get a lot of low-dispersion elements for a fixed-focal f/2.8 lens, five to be precise. Despite that fact, the constructors weren't able to reduce longitudinal chromatic aberration to zero. At the maximum relative aperture the area before the focus has slightly red colouring and behind the focus the colouring changes into green. That effect is reduced significantly on stopping down the aperture to f/4.0 but even then you cannot eliminate it completely.

Sigma C 90 mm f/2.8 DG DN - Chromatic and spherical aberration

Now let's check the correction of lateral chromatic aberration – an appropriate graph you can find below.

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The aberration increases quite quickly with stopping down the aperture; fortunately, in case of this lens, f/2.8 and f/4.0 will be used most often; in that aperture range aberration is practically zero, from 0.02 to 0.03% to be precise. What's more, such a low level is kept by the lens even if you stop down the aperture to f/8.0. Medium values appear only from f/11 onwards and only on the edge of the APS-C sensor.

A7R III, RAW, FF, f/2.8	A7R III, RAW, FF, f/22.0

Spherical aberration

First photos of this chapter have a slight 'focus shift' effect – you can notice that, on stopping down the aperture, depth of field is moved toward greater distances. Problems with correction of spherical aberration can also be noticed in defocused light circles – the one before the focus has a darker edge and the one behind the focus features a brighter rim.

Both effects, described in this chapter, show unanimously that the Sigma C 90 mm f/2.8 DG DN cannot correct spherical aberration in a perfect way.