Lens review

Sony FE 20-70 mm f/4 G

17 January 2023
Maciej Latałło

10. Autofocus

During our test we had an opportunity to attach the Sony FE 20-70 mm f/4 G to the Sony A7R III and the Sony A7R V. In both cases its autofocus was noiseless and very quick. Running through the whole distance range and confirming the focus took no longer than 0.3-0.4 of a second and such a value remained the same in case of both bodies. We have to add it's a splendid result, and in this category the lens certainly deserves to be praised. The linear XD motors perform here exceedingly well.

We also didn't have any problems with the accuracy of the focusing mechanism – it was beyond reproach, no matter whether we tested the lens in the studio or in the open air.

As you see in shots below the Sony also didn't have any front or back focus related problems.

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A7R III, 50 mm, f/4.0
Sony FE 20-70 mm f/4 G - Autofocus
A7R III, 70 mm, f/4.0
Sony FE 20-70 mm f/4 G - Autofocus
A7R V, 50 mm, f/4.0
Sony FE 20-70 mm f/4 G - Autofocus
A7R V, 70 mm, f/4.0
Sony FE 20-70 mm f/4 G - Autofocus

The Filmmaker's Eye

As the tested lens features a very useful focal range from a filmmaker's perspective and it offers a fixed aperture, we decided to check also two film parameters connected to its optical construction, and keeping the focus. We think here about 'breathing' so geometric changes in field-of-view while overfocusing and parfocality so keeping the same focal point while changing the focal length.

Let's start with focus breathing:

It's obvious that breathing is the most conspicuous at the shortest focal length but even there its level should be described as low. The 20 mm focal length, used for recording wide frames, will be also least bothersome in practice.

In the middle of the focal range you can observe how should the perfect, model focus breathing in a lens look. At 70 mm for a change there is some breathing but not much. As more and more Sony cameras offer you breathing compensation, the situation should remain under control as soon as the appropriate lens profile appears in the menu.

Taking into account small physical dimensions of the lens and the fact that you deal here with a zoom instrument with an untypical focal range, difficult to construct, we think its breathing, without being perfect, is corrected decently well.

Now let's check the parfocality of the tested lens:

As you can see, in the manual mode the focus is kept perfectly well and tracking autofocus works as long as you don't change the focal length too fast. In other words – it would be difficult to want more. We think the new Sony zoom should be considered a parfocal instrument.

Strange changes of image brightness during quick changes of focal lengths are the only thing that makes us a bit worried. They shouldn't have happened while video recording with a complete manual exposure. Still it seems that it's a software-related problem of the camera, not of the lens itself, so it won't be assessed during this test.