Sigma A 35 mm f/1.4 DG II
6. Distortion and field of view
Field of view
A rectilinear 35 mm lens on a full frame sensor should provide you an angle of view of 63.4 deg and the producers of the Sigma A 35 mm f/1.4 DG II in their official specifications state exactly the same value. Of course we decided to check how big this field really is anyway. To do so, we took photos of a 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 were able to measure the field very precisely and exactly as it should be done for rays of light coming from infinity. Our transformation was based on positions of 173 stars spread evenly across the frame. An average mesh-fitting error amounted to just 32 seconds of arc. Our result amounts to 62.95 deg with a measurement error on a level of 0.04 of a degree. It's a tad lower than stated in official specifications and suggested by the focal length value but the differences are slight. It seems the resulting angle of view is the equivalent of the distortion-free 35.3 mm focal length.Distortion
Working on the smaller APS-C ssensor, you don't have to think about distortion at all as its level remains very low. Our measurements showed some slight barrel deformations, and their level we would assess as -0.65%.After passing to full frame the result is a tad higher, reaching -1.10%. Considering the parameters of the tested lens, especially its angle of view, you can say that it corrects distortion really well. What's important, it uses successfully the optical system, not the camera's software, and it doesn't make this task easier by introducing moustache distortion.
The Sigma fares here better than the Nikkor (-5.31%) and the Samyang (-1.47%), but a tad weaker than the Sony (+0.73%).
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| Sony A7R IIIa, 35 mm, APS-C | |||
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| Sony A7R IIIa, 35 mm, FF | |||
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