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I own a few lenses with a maximum aperture of f/2.8. For this question, I'll focus on two specifically:

While the price difference is significant, the Rokinon is both MF only, as well as prime, meaning that design, material, and production costs would be considerably lower -- on paper, anyway.

One thing I've noticed with the Sigma is far less of a need to step down to reduce astigmatism-related blurring -- in most reasonable lighting conditions, it's barely noticeable even wide open at f/2.8.

Not so on the Rokinon, which needs to be stepped down to f/7.1 in brighter environments to get to an acceptable level of astigmatism-based abberation.

What causes such a significant difference in astigmatism blur reduction? The only factor I can think of is diaphragm blade count and shape -- the Rokinon has 6 blades, while the sigma has 9 -- but I'm not at all convinced that's the only thing at play.

  • High apertures like f/22 or low apertures like f/2.8? – Michael C Oct 4 at 4:20
  • What type of subject are you shooting? Is it a flat test target or a three dimensional scene? In the former case, it might not be astigmatism at all. It could just as easily be field curvature. CA can also come into play at wide apertures. – Michael C Oct 4 at 4:22
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Diffraction is not what causes lack of sharpness in any mainstream setups (as in, you are not operating an aerial camera in a bomber) wide open - at f/2.8, it will come from astigmatism, spherical aberration, operator error, autofocus error, and several other causes that have nothing to do with diffraction.

Diffraction is what sabotages the film-era adage "just stop down as much as you can to be sharp", it appears when you stop DOWN, usually starting noticeably at f/8 or f/11 on a mainstream high resolution (24MP APS-C, 36MP FF) sensor.

In the case of these two lenses: Both are retrofocal ultrawide lenses, which adds complexity and requires compromises to be accepted - however, A 115° wideangle is far more extreme than a 85° design, regardless of absolute focal length and/or zoom capability. Also, cynically speaking, there might have been much more money spent on the sigma design, since it is likely that far more sales of a professional style zoom vs an exotic ultrawide prime can be expected.

  • Astigmatism seems to be what I meant to describe in this case. I'll update the question. – verandaguy Oct 3 at 22:10
  • All lenses are plagued by the twin demons of diffraction and interference. – Alan Marcus Oct 4 at 3:13
  • @verandaguy close but no cigar. Always combined effects, but diffraction is the least of your worries at f/2.8. – rackandboneman Oct 4 at 10:35
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There are seven major lens aberrations. So far, they remain a plague as none have been eliminated. We can however mitigate. There are two that are color related (chromatic aberration) and five monochromatic aberrations. Astigmatism is the most complex. Astigmatism is mitigated by carful control of the figure (curve) of the various lens elements. Special attention is paid to reduce thickness of the various elements at the margins. The density of the glass elements is selected with care. Some elements are separated by an air gap. This gap is tasked to function as a lens element. The placement of the iris diaphragm is critical in the mitigation of astigmatism.

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Any optical error that is corrected by stopping down is being added by the periphery of the optical elements. The errors that are greatest at the periphery are astigmatism, field curvature, lateral CA, and Coma. And they are the greatest at the periphery because the light from those areas has to be bent more in order to form the focused image circle.

There are two things that makes correcting the light from the periphery of the elements harder for the Rokinon. First, it is collecting light from a wider field of view (FOV); which means the light from the additional FOV has to be bent more in order to create the same sized image circle. Secondly, I suspect that the Rokinon has an objective element with a greater curvature to start with. Which means there is a much greater differential in the amount of light bending occurring across its' profile, and that makes it that much harder to correct uniformly.

There are ways of correcting the different optical errors; but even if both lenses have the same level of design and correction incorporated, the UWA Rokinon is more demanding/difficult to correct.

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