Lens review

Voigtlander Color Skopar 20 mm f/3.5 SL II Aspherical

24 November 2010
Arkadiusz Olech

7. Coma and astigmatism

The astigmatism of this lens is classic. What’s more, because of its solid mechanics, it most probably stems from a faulty correction of this aberration by the optical construction itself. It should hardly surprise us, though – the lens features less elements than other full frame 20 mm devices because of its limited dimensions.

In this case the astigmatism looks so nice that we decided to draw a special graph to depict it even better. Below, we have a resolution graph, based on one of two resolution charts, used in our test. Red dots indicate the resolution values depending on the aperture and based on MTF50 results determined for vertical borders of white and black. Blue dots show the performance for horizontal borders.

Voigtlander Color Skopar 20 mm f/3.5 SL II Aspherical - Coma and astigmatism

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You can see clearly that near the maximum relative aperture the astigmatism is huge and reaches as much as 28%. On stopping down, according to the laws of optics, its value decreases. By f/8.0 the level of this aberration is already slight ( circa 6-7%) and by f/11 and f/16 it disappears completely.

It’s worth noticing here that in many tests the influence of astigmatism is omitted; this fact might cause drastically different assessments of the same lens. One glance at the graph above shows that, if we, following the example of others, took into account only vertical borders, our assessment of the Voigtlander would be definitely higher as even at the maximum aperture the lens would reach the results of 35-36 lpmm.

Let’s check now how the coma looks.

Voigtlander Color Skopar 20 mm f/3.5 SL II Aspherical - Coma and astigmatism

You can hardly be satisfied here. At the maximum relative aperture that aberration is clearly visible in the corner of the APS-C/DX sensor and in the corner of full frame it is simply huge. On stopping down the aperture by one EV value the situation in the APS-C/DX corner improves significantly but in the full frame corner it remains bad.

When a lens corrects two basic off-axis aberrations so badly it would be foolish to expect a good image quality on the frame edge…