Leica APO-Summicron-SL 35 mm f/2 ASPH.
6. Distortion and field of view
Field of view
A rectilinear 35 mm lens on full frame should provide an angle of view amounting to 63.4 deg and such a value exactly is stated in official specifications.We decided to check the real values of field of view with the help of photos of a starry sky; we saved them first as corrected JPEG files and then as uncorrected RAW 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 of JPEG format files was based on positions of 120 stars spread evenly across the frame. An average mesh-fitting error amounted to 20 seconds of arc. Our final result amounted to 63.73 degrees with a measurement error on a level of 0.03 of a degree. It's in accordance with official specifications.
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In case of uncorrected RAW files the transformation was based on positions of 136 stars and an average mesh-fitting error amounted to 17 seconds of arc. In this case the developed file always has a tad higher physical resolution so here the angle of view amounted to a bit higher value, that of 63.76 degrees with a measurement error on a level of 0.03 of a degree. Still the differences between both formats are slight, as they might be included in the margin of measurement error.
Distortion
Negligible differences in fields of view of JPEG and RAW files suggest that the tested lens shouldn't experience any problems with distortion correction, especially that from such a focal length you shouldn't expect any meaningful problems. Indeed, there are none of them. In case of the JPEG files we got a result of -0.56% and when it comes to RAW files the result amounts to -0.53%. Both these values are, within the margin of error, in accordance with each other and completely harmless.Such an approach should be appreciated because currently, even at 35 mm focal length, many optics producers cut corners and give up on distortion correction, leaving this task to the software of a camera or a computer.
| Panasonic S1R II, 35 mm, JPEG, FF | |||
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| Panasonic S1R II, 35 mm, RAW, FF | |||
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