Sony FE 16-35 mm f/2.8 GM II
6. Distortion and field of view
Field of view
A full frame rectilinear 16-35 mm lens should provide angles of view ranging from 107 to 63.4 deg. Of course some exceptions are allowed. After all, as we are going to see in a moment, a lens doesn't correct distortion in a perfect way so is not rectilinear at all. What's more, its real focal length might slightly differ from the one officially stated. The producers always round the numbers up and 2-3% of difference is nothing strange.Fortunately we don't have to keep guessing, as we were able to measure fields of view of the new Sony model, tested here, and we did it very precisely, for rays of light falling from infinity. Such measurements were possible because we took photos of starry sky, transformed the pixel layout (X,Y) from the photo into the equatorial coordinate system (right ascension and declination), and found their position expressed in pixels.
In case of the 16 mm focal lenght we used over 200 stars positioned evenly across the frame and average mesh-fitting error amounted to a tad over 4 minutes of arc. The field, measured this way, amounted to 109.0 deg with measurement error on a level of 0.3-0.4 of a degree. A good piece of news is such that the lens provides a bit wider field than the one officially declared.
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For a change in case of the 35 mm focal length our result was in almost perfect accordance with official declarations. Tranformation based on over 150 stars allowed us to measure the field and our result amounted to 63.6 deg with an error amounting to about 0.1 of a degree.
Distortion
In case of the smaller APS-C detector distortion made itself feld only at the shortest focal lengths. At 16 mm you see slight barrel of -2.79%, that decreases to -1.52% on passing to the 20 mm focal length. At 24 mm we get almost imperceptible level of -0.53%, and at 28 mm deformations practically become zero because our result amounted to +0.05%. At the maximum focal length you deal with slight pincushion of +0.72%.
| Sony A7R III, APS-C, 16 mm | |||
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| Sony A7R III, APS-C, 20 mm | |||
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| Sony A7R III, APS-C, 24 mm | |||
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| Sony A7R III, APS-C, 28 mm | |||
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| Sony A7R III, APS-C, 35 mm | |||
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When it comes to full frame, the official results aren't especially high. Unfortunately, optics specialists had to introduce moustache distortion to curb them down. At the 16 mm focal length the result, averaged out across the frame, reaches -3.57%; however, if you limit your measurements to the 1:1 markings area, where straight lines don't bend due to moustache distortion, the result increases to a quite high level of -4.62%.
After passing to the 20 mm focal length distortion decreases to -1.97%, but moustache deformations still remain visible. What's interesting, their delicate influence is noticeable also at 24 mm where our result, averaged our across the frame, is -0.54%.
At 28 mm you deal with a very slight 'pincushion' of +0.48%. At the maximum focal length that value increases to a noticeable level of +1.95%.
| Sony A7R III, FF, 16 mm | |||
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| Sony A7R III, FF, 20 mm | |||
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| Sony A7R III, FF, 24 mm | |||
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| Sony A7R III, FF, 28 mm | |||
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| Sony A7R III, FF, 35 mm | |||
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