Sigma S 500 mm f/5.6 DG DN OS
6. Distortion and field of view
Field of view
A full frame rectilinear 500 mm lens on a full frame should offer you an angle of view of 4.95 deg. The official Sigma specifications state a value of full 5 degrees and it's hard to say whether it is a result of rounding up or the producers just wanted to emphasize the fact that their lens features a slightly lower focal length than the officially declared value.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.
The result we got amounts to 5.13 deg with an error not exceeding a value of 0.05 deg. It is clear that the real field of view of the tested lens is a bit wider than the one stated in official specifications and it corresponds a focal length amounting to 483 millimeters. Does that fact have any significance at all? We are going to answer this question and say more about it in the next chapters.
Distortion
Telephoto long-focus lenses usually don't have big problems with distortion and the tested Sigma is not an exception here. In case of the APS-C sensor you deal with slight pincushion variation that we measured as +0.29%. After passing to full frame that 'pincushion' increases to +0.49% and it is a level that you still can describe as very low. In short - no reservations in this area.
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| Sony A7R IIIa, 500 mm, APS-C | |||
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| Sony A7R IIIa, 500 mm, FF | |||
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