LensTip.com

Lens review

Sony FE 20-70 mm f/4 G

17 January 2023
Maciej Lata││o

6. Distortion and field of view

Distortion

When you attach the Sony FE 20-70 mm f/4 G to a camera, an option of distortion correction control gets disabled in its menu. It's quite obvious that optics specialists meant the software to do the job of optics and the camera forces automatic correction of this aberration.

It also means additional work for the testers. We have to check distortion levels not only on the APS-C sensor and on full frame but also for corrected JPEG files and uncorrected RAW files. We get four combinations that, additionally, have to be checked at different focal lengths.

Let's start with the easiest case – the smaller APS-C sensor plus corrected JPEG files. Below, you can find thumbnails of images we got on such a combination at 20, 24, 28, 35, 50, and 70 mm.


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Sony A7RáIII, APS-C, JPEG, 20ámm
Sony FE 20-70 mm f/4 G - Distortion and field of view
Sony A7RáIII, APS-C, JPEG, 24ámm
Sony FE 20-70 mm f/4 G - Distortion and field of view
Sony A7RáIII, APS-C, JPEG, 28ámm
Sony FE 20-70 mm f/4 G - Distortion and field of view
Sony A7RáIII, APS-C, JPEG, 35ámm
Sony FE 20-70 mm f/4 G - Distortion and field of view
Sony A7RáIII, APS-C, JPEG, 50ámm
Sony FE 20-70 mm f/4 G - Distortion and field of view
Sony A7RáIII, APS-C, JPEG, 70ámm
Sony FE 20-70 mm f/4 G - Distortion and field of view

Here the task was the easiest: angles of view aren't extremely wide and the lens gets some help from the software of the camera too. As a result you can notice some traces of barrel distortion but only at 20 mm, where we got -0.78%. At 24 mm that value decreases to -0.43%, and at 28 mm it is -0.28%. At the longer end of the focal spectrum the results are, within the margin of error, practically zero. At 35, 50, and 70 mm we got, respectively: -0.07%, -0.07%, and -0.01%.

Now let's have a glance at RAW files from the smaller APS-C sensor in order to check what happens if you switch correcting software off.

Sony A7RáIII, APS-C, RAW, 20ámm
Sony FE 20-70 mm f/4 G - Distortion and field of view
Sony A7RáIII, APS-C, RAW, 24ámm
Sony FE 20-70 mm f/4 G - Distortion and field of view
Sony A7RáIII, APS-C, RAW, 28ámm
Sony FE 20-70 mm f/4 G - Distortion and field of view
Sony A7RáIII, APS-C, RAW, 35ámm
Sony FE 20-70 mm f/4 G - Distortion and field of view
Sony A7RáIII, RAW, JPEG, 50ámm
Sony FE 20-70 mm f/4 G - Distortion and field of view
Sony A7RáIII, APS-C, RAW, 70ámm
Sony FE 20-70 mm f/4 G - Distortion and field of view

This time you won't have any problem with noticing deformations. At 20 mm you deal with a surprisingly high level of -5.79%. Additionally, at 24 mm you still deal with a quite high value of -3.76%, and at 28 mm you see -2.14%, a level which is also quite distinct. A bit better results we got for higher focal lengths: at 35 mm it's still barrel distortion of -0.53%, but at 50 mm you already see the pincushion variation which level reaches +0.66%. That pincushion only increases to +1.05% after changing the focal length to 70 mm.

Now let's progress to full frame. Thumbnails below show the performance of JPEG files after the correction.

Sony A7RáIII, FF, JPEG, 20ámm
Sony FE 20-70 mm f/4 G - Distortion and field of view
Sony A7RáIII, FF, JPEG, 24ámm
Sony FE 20-70 mm f/4 G - Distortion and field of view
Sony A7RáIII, FF, JPEG, 28ámm
Sony FE 20-70 mm f/4 G - Distortion and field of view
Sony A7RáIII, FF, JPEG, 35ámm
Sony FE 20-70 mm f/4 G - Distortion and field of view
Sony A7RáIII, FF, JPEG, 50ámm
Sony FE 20-70 mm f/4 G - Distortion and field of view
Sony A7RáIII, FF, JPEG, 70ámm
Sony FE 20-70 mm f/4 G - Distortion and field of view

You get some surprises here. First of all, at 20 mm you see barrel distortion of -1.65%, quite a lot for an already corrected image. As if it wasn't enough, there are traces of moustache distortion left behind after the correction. Strange? Wait till you reach the RAW file analysis...

Barrel distortion level is also noticeable at 24 mm where it reaches -1.07%. At longer focal lengths the problems end: at 28 mm you see a level of -0.51%, at 35 mm the result is -0.21%, and at 50 and 70 mm you just deal with slight pincushion deformations of +0.11% and 0.02%.

Now we are arriving at the clou of our chapter – distortion performance on full frame, examined on uncorrected RAW files. Let's look at the photos, presented below.

Sony A7RáIII, FF, RAW, 20ámm
Sony FE 20-70 mm f/4 G - Distortion and field of view
Sony A7RáIII, FF, RAW, 24ámm
Sony FE 20-70 mm f/4 G - Distortion and field of view
Sony A7RáIII, FF, RAW, 28ámm
Sony FE 20-70 mm f/4 G - Distortion and field of view
Sony A7RáIII, FF, RAW, 35ámm
Sony FE 20-70 mm f/4 G - Distortion and field of view
Sony A7RáIII, FF, RAW, 50ámm
Sony FE 20-70 mm f/4 G - Distortion and field of view
Sony A7RáIII, FF, RAW, 70ámm
Sony FE 20-70 mm f/4 G - Distortion and field of view

I admit I haven't encountered such a situation in our tests so far. Perhaps extreme, untypical parameters of this lens needed original solutions. Anyway the result is such that distortion at 20 mm can be called only monstrous because it reaches -10.23% in a lens that tries to pretend it's a rectilinear instrument. What's even worse, such a value is a result of high level moustache deformations. They are so pronounced that the 'straight line moustaches' get significantly upward on the edges of the frame, a curious thing of itself. Now you know why barrel distortion with traces of moustache distortion could be spotted even in corrected JPEG files. It's obvious the software of the camera failed to fix such huge deformations.

At the 24 mm focal length the situation is horrible as well – we got the result of -6.08% and, what's even worse, moustache distortion remains quite noticeable. Also at 28 mm focal length you won't get any problems with spotting deformations as they reach -3.35%. The problems end only at 35 mm, where we got -0.39%

At longer focal lengths deformations become zero, then they change sign. At 50 mm you deal with barrel distortion of +1.81% which increases to +2.36% when you pass to 70mm.

Field of view

I got this lens for testing in December and in Poland it is a month plagued with notoriously bad weather. Still I managed to catch some clearer monents and took several photos of night sky that enabled to measure the field of view of the new Sony model. I am glad I took that effort because the results are very interesting indeed.

Let's start with the 70 mm focal length. In case of JPEG files we used 107 stars for our transformation that allowed us to create an equatorial coordinate system with the relative error of just 6 seconds of arc. The result we got amounted to 34.27 degrees and, what's important, all these numbers matter because the margin of measuring error we estimate at just 0.01-0.02 deg.

For uncorrected RAW files we used 80 stars in the transformation process and the relative mesh error amounted to 8 seconds of arc. The field proved to be slightly wider, reaching 34.31 deg with a similar margin of error.

As a rectilinear 70 mm lens should provide a field of 34.3 deg our results are in perfect accordance with declarations of the producer.

Very interesting situation can be observed at the 20 mm focal length. In its case a rectilinear lens should provide an angle of view of 94.5 deg. As at this focal length you also see significant correction of huge barrel distortion with an addition of moustache deformations and image cropping, we expected the field of RAW files should be distinctly wider than the declared value. We were right.

In case of this file format we used a postion of 140 stars and the mesh-fittind error amounted to 1 minute and 34 seconds of arc. The field of view, computed by us, reached a level of 104.3 deg with the margin of error not exceeding 0.1 of a degree. As you see, compared to the official value, there are almost 10 degrees to spare.

Cropped JPEG files should have even narrower field of view. In order to measure it we used positions of 156 stars and the mesh fitting error amounted to 1 minute 53 seconds of arc. The result we got amounted to 97.7 deg with the margin of error of 0.1-0.2 deg. It is a very pleasant surprise because it's exactly the field we usually got with a 19 mm rectilinear lens. Perhaps the producers assumed that, as you deal with slight barrel distortion even for corrected JPEG files, you might want to correct it further with the software of a camera and then crop the image. Because of a healthy margin of over 3 degrees you can do that much and still enjoy a field of view which remains in perfect accordance with official declarations – a piece of very good news!