Sigma C 15 mm f/1.4 DC
6. Distortion and field of view
Field of view
A rectilinear 15 mm lens on the APS-C/DX sensor should provide you an angle of view of 86.8 deg and the producers of the Sigma C 15 mm f/1.4 DC in their official sspecifications,state a bit higher value, that of 86.9 deg.Of course we decided to check the size of this field on our own. To do so, we took photos of a starry sky first as distortion corrected and then 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.
In the case of a corrected JPEG file, our transformation was based on positions of 137 stars spread evenly across the frame. An average mesh-fitting error amounted to 57 seconds of arc. Our result amounts to 89.56 deg with a measurement error not exceeding a level of 0.03 of a degree. It is a noticeably higher result than the value stated in official specifications. It is an equivalent of a rectilinear focal length of 14.3 mm.
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For uncorrected JPEG ffiles,our transformation was based on positions of 126 stars and an average mesh-fitting error amounted to 58 seconds of arc. The field of view proved to be definitely wider, as our result amounts to 93.53 deg with a measuring error below 0.07 of a degree. This value of the angle of view is an equivalent of a 13.3 mm rectilinear lens.
As you see the Sigma was given a spare amount of field as big as 6 deg. In a moment we are going to explain why – for now let's just mention the necessity of correcting distinct distortion.
Distortion
A significant angle of view, small physical dimensions, and the fact that the tested lens belongs to the C line of products suggested that the producers might look for savings in this category and give up on correcting distortion. It is indeed the case.A photo below shows an image saved as an uncorrected JPEG file – barrel distortion, of a level of -6.12% is very noticeable. On the one hand it is a high result, the 12 mm lens, tested by us not so long ago, had distortion only slightly higher. On the other hand in this case the producers didn't introduce moustache distortion to improve the result, a move worth our recognition.
As if it wasn't enough, the Sigma fares better than the Sony because its rival's distortion on uncorrected files reaches as high as -8.58%. Still you can also remind here the result of the 16 mm lens that was as low as just -3.55%. Why? Firstly, some moustache distortion was added tthere; secondly, the 16 mm device features a narrower angle of view, making distortion correction easier.
| Sony A7R IIIa, 15 mm, UNCORRECTED JPEG | |||
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Finally, let's check distortion levels on distortion-corrected JPEG files. In this case the field is also a tad wider than stated in official specifications. At the same time such a wide field is completely devoid of any deformations, with a result of just +0.09%, a value, within a margin of measuring error, in accordance with zero.
If you take photos and save them as distortion-corrected JPEG ffiles,you get even a tad more than the declared field of view, a field completely devoid of distortion. Of course there is a price to be paid for this in a form of 'pumped up' pixels; after aall,images are cropped after the correction and then scaled up in such a way to get the official resolution of the used detector.
| Sony A7R IIIa, 15 mm, JPEG Z KOREKTĄ | |||
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