Voigtlander Ultron 27 mm f/2
6. Distortion and field of view
Field of view
A 27 mm rectilnear lens on the APS-C/DX sensor should provide an agle of view amounting to 55.4 deg. The Fujifilm X system sensor is slightly smaller than a classic APS-C/DX sensor so you should expect a slightly lower result here but still, I was surprised that the official specification of the producers features values ranging from 53.5-53.7 deg. It means that the effective focal length of the lens amounts to 28 mm, not 27 mm.We decided to check the size of the field in reality. We took photos of starry sky, saving them as distortion-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. The transformation was based on over 100 stars spread evenly across the frame, with the relative mesh-fitting error of just 9 seconds of arc.
The result we got amounts to 53.5 deg with the measuring error of 0.1 of a degree and it is in perfect accordance with the official declarations of the producer. It also confirms the fact that you deal here with the 28 mm, not 27 mm focal length.
Distortion
Optics constructors often give up on distortion correction and let camera software deal with that problem so we were curious how the constructors of the small Ultron approached this issue.
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Once again we were very pleasantly surprised. The result we got for uncorrected RAW files amounts to just +0.18% and, within the margin of measurement error, it brushes against zero. Mind you , the tested Voigtlander features a wider field than typical 50 mm standards and many of them, especially these simpler double gauss constructions, could have distortion which absolute value reaches even 2%. You have to admit compared to that the result of the Ultron looks splendidly well.
There is one interesting fact: such a small distortion on RAW files shouldn't force the software of the camera to do anything. Still, in case of the Ultron the software tries to correct this negligible distortion anyway and goes over the top. As a result corrected JPEG files feature barrel distortion of -0.33%, a level once again close to zero but also, when it comes to absolute values, higher than the performance on RAW files.
Of course it doesn't change the fact that the Ultron should be praised for its distortion correction.
| Fujifilm X-T2, 27 mm, JPEG | |||
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| Fujifilm X-T2, 27 mm, RAW | |||
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