Lens review

Tamron 20-40 mm f/2.8 Di III VXD

19 July 2023
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, the best fixed focal length lenses are able to reach a level of 75-80 lpmm; the record-breakers exceed slightly 80 lpmm. Currently our list of such record-breaking lenses features the Sigma C 65 mm f/2 DG DN (82.7 lpmm), the Sony FE 20 mm f/1.8G (83.7 lpmm), and the Sony FE 50 mm f/1.4 GM (90.9 lpmm).

Let's check how the Tamron 20-40 mm f/2.8 Di III VXD compares here – its results in the frame centre at 20, 30, and 40 mm presents a graph below.

Tamron 20-40 mm f/2.8 Di III VXD - Image resolution

An even performance up from the f/4.0 aperture sticks out at once. The differences between results reached by particular focal lengths are, by and large, within the range of measuring error. What's important, all of them are excellent. By f/4.0 and f/5.6 you land on a level of 70 lpmm or higher, a brilliant performance for a zoom lens, really worth praising.

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When it comes to the maximum relative aperture you can notice some differences between shorter focal lengths and the longest one. In the 20-30 mm range even at the maximum relative aperture the lens provides again an excellent resolution with a result near 70 lpmm. At the 40 mm MTFs are lower, reaching over 52 lpmm. The difference is significant but still, in both cases you get images that are completely useful without providing any reasons to complain.

Now let's observe how the situation changes as you pass to the edge of the APS-C sensor. An appropriate graph can be found below.

Tamron 20-40 mm f/2.8 Di III VXD - Image resolution

In this case differences between focal lengths are more pronounced and the widest angle of view fares the best. Already up from the maximum relative aperture you deal with very good values, reaching almost 60 lpmm. The 30 mm focal length is a bit weaker and the 40 mm focal lenth is the weakest but even there, at the maximum relative aperture, the lens exceeds slightly the level of full usefullness so it should be only praised.

Let's deal with the most demanding edge of full frame and glance at another graph.

Tamron 20-40 mm f/2.8 Di III VXD - Image resolution

Once again the shortest focal length is the best because even at the maximum relative aperture it ensures you a decent image quality. Unfortunately the same can't be said about the 30-40 mm where, by f/2.8, images are slightly soapy. In case of the 30 mm focal length you have to stop the lens down to near f/3.5 in order to reach decent resolution; at 40 mm the lens has to be stopped down even further, to f/4.5.

Still, the summary of this chapter can be positive only. In the frame centre the tested Tamron provides excellent image quality across the focal and aperture range as well. The edge of the APS-C sensor doesn't provide any reasons to complain; only the edge of full frame might be problematic but only at the longer end of the focal spectrum and near the maximum relative aperture.

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

A7R III, JPEG, 30 mm, f/4.0
Tamron 20-40 mm f/2.8 Di III VXD - Image resolution
A7R III, JPEG, 40 mm, f/2.8
Tamron 20-40 mm f/2.8 Di III VXD - Image resolution