Sigma S 70-200 mm f/2.8 DG DN OS
6. Distortion and field of view
Field of view
A full frame rectilinear 70-200 mm lens should offer you a range of angles of view from 34.3 to 12.3 deg. Of course some exceptions are always possible – after all, as we are going to show in a moment, the lens doesn't correct distortion in a perfect way so is not completely rectilinear. What's more, its real focal length might be a tad different than the one officially declared. The producers always round such values to whole numbers so several percent of difference is nothing strange.Fortunately, when it comes to fields of view of the tested Sigma, we don't have to guess. We measured them precisely as it should be done, for rays of light coming from infinity. Our measurements were based on photos of starry sky; 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.
In case of the 70 mm focal length we used 81 stars spread evenly across the frame, with the mesh fitting error amounting to 6 minutes of arc. The field, measured by us, amounted to 32.7 deg with measuring error not exceeding 0.05 of a degree. This field is noticeably narrower than official declarations of the producer, fitting rather 74 mm, not 70 mm focal length.
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In case of the 200 mm focal length we used 58 stars for the transformation with the mesh fitting error of 6 minutes of arc. The field, measured by us, amounted to 6 seconds of arc as well and the field, measured by us, was 12.05 deg, with measurement error of 0.05 of a degree. The field is also narrower than the declared but this time the difference is not big, quite negligible with such a focal length value.
Distortion
In case of the smaller APS-C sensor distortion doesn't give you any serious reasons to complain. At the widest angle of view you deal with very low pincushion deformations of +0.36% which then increase with the increase of the focal length. At 100 mm you see a level of +0.98%, at 135 mm it is already +1.12%, at 170 mm it amounts to +1.22% and at 200 mm it reaches a value of +1.30%.Here the Sigma fares better than the Tamron but weaker than the more expensive Sony model.
| Sony A7R IIIa, APS-C, JPEG, 70 mm | |||
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| Sony A7R IIIa, APS-C, JPEG, 100 mm | |||
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| Sony A7R IIIa, APS-C, JPEG, 135 mm | |||
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| Sony A7R IIIa, APS-C, JPEG, 170 mm | |||
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| Sony A7R IIIa, APS-C, JPEG, 200 mm | |||
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In case of full frame the problems are noticeably more serious. At 70 mm they are probably not so acute, with a slight 'pincushion' of +0.82%, but after passing to 100 mm the level of distortion increases quite significantly, amounting to +2.06%. After employing 135 mm you have to take into account +2.52% of distortion and at 170 and 200 mm you deal with results, respectively, of +2.77% and +2.91%. Once again the Sigma wins the duel with the Tamron – despite a wider focal range it has practically the same results in the middle and at the longer end but at the 70 mm focal length it fares better.
Still, it's worth reminding yourself old good times when optics constructors tried to actually fight off distortion. For example, in case of the Sigma S 70-200 mm f/2.8 DG OS HSM the weakest value reached just +1.62%
| Sony A7R IIIa, FF, JPEG, 70 mm | |||
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| Sony A7R IIIa, FF, JPEG, 100 mm | |||
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| Sony A7R IIIa, FF, JPEG, 135 mm | |||
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| Sony A7R IIIa, FF, JPEG, 170 mm | |||
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| Sony A7R IIIa, FF, JPEG, 200 mm | |||
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