Sigma C 20 mm f/2 DG DN review - Image resolution

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Sigma C 20 mm f/2 DG DN

5 July 2022

Maciej Latałło

4. Image resolution

Our resolution test (resolution meaning the MTF50 function values) of Sony FE lenses is based on RAW files from the 42.4 Mpix sensor of the Sony A7R II or the Sony A7R III camera. In the case of this kind of detector the decency level we set about 39-41 lpmm and the best fixed focal length lenses have been able to reach 75-80 lpmm, with record holders exceeding 80 lpmm. So far, the Voigtlander Apo Lanthar 50 mm f/2 E (81.0 lpmm), the Sigma C 65 mm f/2 GD DN (82.7 lpmm), and the Sony FE 20 mm f/1.8G (83.7 lpmm) have fared the best.

Let's check how the Sigma C 20 mm f/2 DG DN compares – its results in the frame centre and on edges of the APS-C sensor and full frame presents a graph below.

Sigma C 20 mm f/2 DG DN - Image resolution

The results in the frame centre aren't perhaps close to resolution records but definitely you can't complain about them either. At the maximum relative aperture you deal with a good level of over 61 lpmm and on stopping down the aperture by 1-2 EV the lens is able to reach 71-72 lpmm.

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Where's the problem? It's the Sony FE 20 mm f/1.8G, the direct rival of the Sigma. Up from its maximum relative aperture it managed to get as high as 80 lpmm and on stopping down it broke our resolution record, reaching 83.7 lpmm. Compared to that you have to admit the Sigma's performance, even if good, seems to be a tad lacking...

The results of the Sigma both on the edge of the APS-C sensor and on the edge of full frame are decent, momentarily even good. After all at the maximum relative aperture the lens is able to reach 41-42 lpmm so safely above the decency level. In case of a lens with an angle of view exceeding 90 degrees and an aperture as fast as f/2.0 it is quite an achievement.

Still we are worried a bit by the significant difference between resolution values you observe in the frame centre and on the edges. You can of course defend Sigma by saying that the edge of the frame is not a strong point of the Sony lens either, quite the opposite, there are places where the Sigma fares a tad better, for example on the edge of the full frame and near the maximum relative aperture. On more significant stopping down, however, the Sony is in the lead again with results about 60 lpmm whereas the Sigma is only able to reach over 53 lpmm on the edge of the APS-C sensor and almost 47 lpmm on the edge of full frame.

At the end of this chapter, traditionally, we present crops taken from photos of our resolution testing chart saved in JPEG format alongside RAW files which we used for the analysis above.

A7R III, JPEG, 20 mm, f/2.0