Sirui Aurora 35 mm f/1.4
7. 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 Sirui Aurora 35 mm f/1.4 in their official specifications state a bit higher value, that of 65.7 deg. Of course we decided to check how big this field really is. 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 the positions of 163 stars spread evenly across the frame. An average mesh-fitting error amounted to 1 minute and 58 seconds of arc.Our result amounts to 65.50 deg with a measurement error 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 33.65 mm focal length.
Distortion
Working on the smaller APS-C sensor, you don't have to think about distortion at all as its level remains low. Our measurements showed some slight barrel deformations, and their level we would assess as -0.72%.
What's interesting, after passing to full frame, the result decreases even more, reaching a negligible value of +0.08%. It's a result of introducing slight moustache distortion that bends straight lines in the opposite direction from the lines from the frame centre. This effect is not especially pronounced but allows changing the sign of the result anyway.
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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 the optical system, not the software of a camera. Compared to its rivals, the Aurora performs really well – only more expensive Sony and Sigma models are able to offer you similar results.
| Sony A7R IIIa, 35 mm, APS-C | |||
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| Sony A7R IIIa, 35 mm, FF | |||
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