Sony FE 28-70 mm f/2 GM
6. Distortion and field of view
Field of view
A rectilinear lens with focal lengths ranging from 28 to 70 mm on a full frame sensor should provide you an angles of view changing from 75.4 to 34.3 deg but the producers in their official specifications provide values of 75-34 deg. Of course we decided to check how big this field really is. In order to do so we took photos of starry sky and saved them as uncorrected JPEG files.Then we transformed the pixel layout (X,Y) from the photo into the equatorial coordinate system (right ascension and declination), which locates a star on a celestial sphere. That way we could determine the field of view of the lens with utter precision and in the right way, so for rays of light coming from infinity.For the 28 mm focal length the transformation was based on the location of 119 stars spread evenly across the frame and the average mesh-fitting error amounted to 28 seconds of arc. We got a result of 75.73 deg with an error that doesn't exceed 0.05 of a degree. As you see our result is slightly higher than official declarations but it's worth noticing that the spare area left for a possible distortion correction is hardly significant.
In case of the 70 mm focal length we uses the location of 110 stars and an average mesh-fitting error amounted to 49 seconds of arc. The angle of view, measured by us, amounts to 34.36 deg also with an error that doesn't exceed 0.05 deg. Here the field is in perfect accordance with official declarations and the theoretical value.
Distortion
At the smaller APS-C sensor distortion won't disturb you a lot. At the 28 mm focal length you deal with slight barrel distortion of -1.24%. With the increase of the focal length distortion decreases; then it becomes zero and it changes the sign because at 35 mm we got just traces of pincushion distortion and our official result amounted to +0.25%. Then it increases to +1.23% at 50 mm and to +1.36% at 70 mm.
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| Sony A7R IIIa, APS-C, JPEG, 28 mm | |||
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| Sony A7R IIIa, APS-C, JPEG, 35 mm | |||
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| Sony A7R IIIa, APS-C, JPEG, 50 mm | |||
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| Sony A7R IIIa, APS-C, JPEG, 70 mm | |||
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The real test of how the Sony FE 28-70 mm f/2 GM deals with distortion is the performance on full frame. Let's glance at thumbnails below.
| Sony A7R IIIa, FF, JPEG, 28 mm | |||
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| Sony A7R IIIa, FF, JPEG, 35 mm | |||
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| Sony A7R IIIa, FF, JPEG, 50 mm | |||
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| Sony A7R IIIa, FF, JPEG, 70 mm | |||
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You can say that the situation is not that bad, taking into account the scale of the problem and challenges the Sony constructors had to face. At 28 mm you deal with barrel distortions of -3.39%. It''s a level that can be noticed on real life photos without any problems and, at the same time, if you want to correct it the resulting field of view most certainly will be narrower than the declared 75 deg. It's clear Sony could have given us a bit more reserve.
At 35 mm focal length you deal with an almost zero level of -0.34% and at 50 and 70 mm distortion changes the sign and you get respectively +1.51% and +1.53%. What's interesting, these levels are not especially higher than the values we observed on the smaller sensor at longer focal lengths.
The explanation of this riddle is quite simple. Sony decided to adopt a very interesting approach. Currently, in case of zoom lenses with a wide angle of view, the producers put moustache distortion in order to limit the visible 'barrel'. Sony optics specialists decided to solve that problem in a different way. At the wide angle they left a typical 'barrel' but at the longer end of the focal spectrum they decided to limit the 'pincushion' deformations bending the lines near the edge the other way. As a result at 70mm distortion reaches the already mentioned value of +1.53% across the frame that, within the margin of measuring errors, brushes against the values we observed on the APS-C detector. If you limit our measuring field to the area inside 1:1 markings pincushion distortion will increase to almost +2.6%.
