Originally posted by StefanMikes Although I much agree with these views, let us bear in mind the fame of the FA31 originated in the times when none of Pentax-brand owners ever planned making an FF digital SLR. FA31 is certainly excellent with APS-C but I believe it is not the top lens for high resolution FF sensor; it is enough to study the MTF figures. The FA31 was tested by Photozone at 10 Mpx and the center resolution at f/1.8 was 1848.5 lw/ph. Sigma A 35 as tested on an FF Nikon gave center resolution of 3262 lw/ph at f/1.4. You don't need pixel-peeking to determine that: It is clearly visible in pictures taken.
Although certainly the FA31 is a respectable lens, its high price and the old design make the lens natural to be reused by existing owners on the K-1 body but I would think thrice before I recommended the FA31 to a new user. Why to pay twice what you spend for Sigma to buy a lens that is outresolved by the K-1 sensor? And well, a 6-degree angle of view difference between both lenses? Is it worth paying the double price? I doubt and that's why I chose the Sigma for my coming K-1 body though I could afford the Pentax Limited lens.
1848 on 10MP APSC with low pass filter mechanically means 2772 on a a 24MP FF with low pass filter. Now that bring both lenses within 15% of each other. Because the systems are different, may have different low pass filter and due to sample variation, from that one sample it is quite hard to conclude the FA would perform any worse on that nikon FF is you where to manage the conversion.
But more importantly, the 16MP shows that the FA31 manage 2403 wide open on a 16MP APSC with low pass filter... So that about 3600 on a 36MP with low pass filter. On what can typically expected by removing the low pass filter, you could add 10-20% more resolution, so more than 4000. That's at f/1.8
Closed down a bit the FA31 saturate the 16MP APSC sensor at just f/2.8 setting meaning it would outresolve the K1 too...
There nothing to fear.
To be honest, in the 28-85mm range any prime lens is going to out resolve a mere 36MP sensor at least on the center. In fact there no bad lens in that focal range. Basically any 30 year old manual lens on K1 is going to mop the floor with the sigma if it is only put on a 24MP Nikon FF...
The choice shouldn't be the sharpness as all lenses perform well in that focal range but more the size/weight, the rendering, the price... The FA31 has a typical FA rendering that is quite different from many modern lenses and has also exceptional bokeh for a lens of that type.
The sigma is nice but is big/heavy, noticably its doesn't provide in body lens corrections for the jpegs and the rendering is different. But it provide f/1.4. That the main reason to buy it: f/1.4 instead of f/1.8-f/2.
As both are very expensive, as a potential buyer I would really be carefull at what rendering I prefer, potentially less dof, heavier/bulkier gear or the rendering of the FA31... Also I'll consider the framing I'll prefer. FA31 is a bit wider and the sigma a bit narrower.
To me at 31-35mm on FF, I'am not sure there that much use for the extra 0.5 stop. I dislike unecessary bulkyness/weight and finally I tend to prefer the rendering of the old FA to modern lenses so the choice would be obviously FA28, FA31 or FA35. As I already own the 31, that would be it, except if I was to sell it for an FA28 as f/1.8 and outstanding bokeh is a bit of a waste for me as such focal length on FF and I get lighter/smaller lenses and get back some cash. I'd never consider the sigma as it is simply too big to be a pleasure to use.
Other people will prioritize different aspects, maybe the max apperture. Or they particularly dislike the FA type of rendering and for them the sigma might be the best choice.
In all cases, we can't really decide for them.
Today I use the FA31 on APSC. The performance from photozone of about 2400 wide open on 16MP with low pass filter hint that on my K3, 24MP and no low pass filter I already get comparable resolution to the sigma on the Nikon FF. For me it serve more as a standard 50mm equivalent of course. A bit wider and I appreciate that. I use the lens reguarly and I am often stunned of its performance. It is extremely sharp at all settings and render beautifully. Better than most lenses I tried in the range. That lens is outstanding.