Lens review

Sony FE 20-70 mm f/4 G

17 January 2023
Maciej Lata³³o

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 lenses have been able to reach a level of 75-80 lpmm. Record-breaking lenses are able to exceed 80 lpmm and so far, our list of these record-breakers features the Voigtlander Apo Lanthar 50 mm f/2 E (81.0 lpmm), the Sigma C 65 mm f/2 DG DN (82.7 lpmm), and the Sony FE 20 mm f/1.8G (83.7 lpmm).

Let's check how the Sony FE 20-70 mm f/4G compares – first we are going to deal with its results in the frame centre at 20, 35, 50 and 70 mm focal lengths which are presented by a graph below.

Sony FE 20-70 mm f/4 G - Image resolution

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You can say at once that the Sony fares exceedingly well. First you notice very small dispersion because all focal lengths have similarly good results. Secondly, the best values are recorded near the maximum relative aperture and f/5.6, where the lens is able to exceed 70 lpmm without any problems. For a difficult zoom lens with such parameters, these results are really splendid.

The progress of Sony optics specialists is clearly visible if you compare these values to the results of the Sony FE 24-105 mm f/4 G OSS, tested by us a bit eariler, a lens with a bit higher zoom factor but also, in my humble opinion, easier to construct because it doesn't feature ultra wide angle. Despite that at any combination of focal lengths and apertures it didn't even approach 70 lpmm, faring notably weaker than the newer Sony FE 20-70 mm f/4 G.

The new Sony model compares very favourably also with some classic 24-70 mm f/2.8 instruments. For example the Sigma A 24-70 mm f/2.8 DG DN is able to compete with the Sony 20-70 mm only in the middle of its focal range and it distinctly loses the duel on both ends.

What about the edge of the APS-C sensor? Is the resolution level similarly high? Let's find out.

Sony FE 20-70 mm f/4 G - Image resolution

You can say at once that the situation is deteriorating, with noticeably lower results. You can also see distinct differences between particular focal lengths. The maximum focal length fares the best, with really impressive values; the 50 mm focal length is a bit weaker, and the weakest results you can observe in the 20-35 mm range. Still 'weak' in this case doesn't necessarily means bad, quite the opposite. Already at the maximum relative aperture you reach a level of about 50 lpmm – it means good image quality and sharp photos.

Finally we should examine the most demanding edge of full frame – let's glance at a graph below and check how the Sony FE 20-70 mm fares there.

Sony FE 20-70 mm f/4 G - Image resolution

Significant dispersion is also visible here and once again shorter focal lengths fare weaker than the longer ones. The results at 70 mm are good, the performance at 50 mm can't be criticized either. The maximum relative aperture is a problem, though, especially in the range from 24 -35 mm where the results are lower than 38 lpmm so a tad below the decency level. In case of the 20 mm focal length stopping down is quite efficient and image quality improves quickly. You can't say that much about 35 mm because at that focal length the lens reaches 40 lpmm only near f/8.0, clearly the weakest spot in this category. Still, it's difficult to call it a serious flaw.

The summary can be only positive – taking into account the difficulty level, the new Sony lens fares very well. Its results in the frame centre are excellent and very even, its sharpness on the edge of the APS-C sensor is good, and even on the edge of full frame it offers you sensible image quality in most of cases. Looking at physical dimensions of the tested lens it would be foolish to expect more.

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

Sony A7R III, JPEG, 35 mm, f/4.0
Sony FE 20-70 mm f/4 G - Image resolution
Sony A7R III, JPEG, 50 mm, f/4.0
Sony FE 20-70 mm f/4 G - Image resolution