It's not all about sharpness and resolution. No sharpness can beat the sense of depth, space and 3-D effect you can achieve for example with sigma 30/1.4. Even if the resolution at the edges is crap - that's not the part of the image your'e looking at. The subject gets obviously separated, you can have much more composition flexibility etc. And the resolution is bad only at the very edges, at F1.4 subject will be plenty sharp even if not dead center (e.g placed at golden mean).
Or give coating/distortion/optic control. If they are really good, you get what you see in real life. E.g. no flares, hazing colour casts or fringing even in strong contrast situations. Correct geometry, no light falloff and other optical characteristics unrelated to DOF and resolution can give great sense of presence, if they are good. The image feels like "in your face" instead of just "picture". Even instantly invisible visual defects detach you from the actual scene.
E.g. the ugly fringing in highlights/bokeh of FA35, instantly make you perceive it as a photograph - optically manipulated representation of the scene.
Btw..
Originally posted by falconeye The irony is that this Nikkor 35mm is an APSC lens and that (AFAWK) this Pentax 35mm is a full frame lens
So, the Pentax lens has its own merits.
Even greater irony is that Pentax has no official plans for FF camera. So you like to pay for something you won't be using anyway. You may say film, but if the performance is "as good" as some of the DA LTD's that get blurry and distorted at edges on film, thats not a real "coverage".