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You can find quite a few comparisons between the various full frame 35mm lenses if you look around.
I'll preface my comments by saying that I've only used the 31mm LTD on Pentax. I have used the Zeiss 35mm on Canon (APS-C) and the Sigma 35mm 1.4 on Nikon (FF) bodies before.
The Zeiss is the most pleasant to focus manually, for sure, although the 31mm is solid as well. The difference is more in the aesthetic experience than the actuality of focusing, as both can be focused comfortably and accurately manually. The Sigma 35mm 1.4 is my least favorite for manual focus (no hard stops, more of an 'airy' feeling). The Zeiss is, of course, not able autofocus which I think greatly limits it's critical use outside of live-view.
The Pentax 31mm and the Zeiss 35mm are both 9 elements in 7 group constructions, although they are different in layout. The Zeiss 35f/2 is a newer design, not derived from the Contax era designs as some of the other ZK lenses are. The Sigma 35mm 1.4 is a considerably more complex and modern optical construction of 13 elements in 11 groups.
For pure resolution/MTF, I have not shot the lenses side by side, but based on use and other research, I'd place rank the Sigma 1st, the Zeiss 2nd and the Pentax 3rd. The Pentax as notable purple fringing issues that the other lens do not have, and it is less sharp wide-open. It also has more vignetting. I never formally tested, but all seem to be reasonably flare resistant; the built-in hood of the Pentax might be annoying if you use a unified filter system of some kind.
The 31mm was clearly designed to maintain a balance between size and image quality, which it achieves very well compared to the other two lenses. The presence of amorphous 'magical' qualities is often cited by Pentax users, but I would generally be as skeptical of that as I would be of some kind of Zeiss 'magic'. I like the 31mm because it has an aperture ring, provide adequate autofocus and manual focus, and is about as big of a 28-35mm sized lens as I would want for travel purposes. If you are shooting on a Pentax body, and especially if you use both film and digital bodies, it is simply the most versatile, despite having some more problematic elements to it's IQ. It's also the most unique, being the only 31mm lens available from any manufacturer that I am aware of.
At F4 (or even F2.8) all of these lenses are sharp enough that I think the differences would rapidly become academic. If you aren't shooting on a tripod, I doubt the differences in output quality would even be visible. If you want speed and highest possible quality at large apertures, the Sigma is the choice, but it will cost you in weight, size and manual focus.
If the sole purpose of the lens for you is landscape on tripod, I would probably veer towards the Zeiss as it likely has better corner performance, and the lack of autofocus would be unlikely to be an issue. If that is the case, it may also be worth looking at the Pentax-K 35mm f/2, 8 elements in 7 groups, which is considerably lighter and while aged, likely equal the more modern lenses at landscape apertures (but probably considerably worse wide-open). I shoot a mixture of subjects and the lack of autofocus on these lenses took them out of the running for me.
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Post #5 by jspillane
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