Sigma C 50 mm f/2 DG DN
6. Distortion and field of view
Field of view
A classic rectilinear 50 mm lens on full frame should provide an angle of view of 46.8 deg and the producers state exactly such a value in the official specifications. We decided to check that claim in reality. In order to do so we we took some photos of starry sky and save 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, for rays of light coming from infinity. Our transformation was based on over 105 stars positioned evenly across the frame and average mesh-fitting error amounted to 28 seconds of arc.Finally we got a result of 46.7 deg with measurement error not exceeding 0.1 of a degree. As you see our value is in full accordance with the declarations of the producer.
Distortion
When Sigma started to launch their new lens series they were ensuring that the Art line would consist of uncompromising models, and their image quality would be the highest priority. In case of Contemporary series, the priorities were supposed to be different – usefulness, moderate weight and dimensions, and good image quality but without correcting these aberrations that are easily controlled by software of a camera.
We were a tad surprised when the Sigma A 50 mm f/1.4 DG DN, tested by us not so long ago, showed on full frame distortion of +2.58%, higher than in case of many simple standard double gauss lenses.
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Our surprise was even greater, but definitely positive this time, when we saw the results of the Sigma C 50 mm f/2 DG DN in this category. On the smaller APS-C sensor its result was practically zero, officially -0.06%, and on full frame it showed negligible barrel distortion of -0.48%.
As you see, the new model fares really well here, a very nice surprise; still, it makes us wonder who has decided to make a faster Art lens correct distortion in a quite different manner and why.
| Sony A7R III, 50 mm, APS-C | |||
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| Sony A7R III, 50 mm, FF | |||
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