Originally posted by Papa_Joe Wonderfull pictures Sandy and Derek!
So, as has been suggested base your decision mainly on your personal stile (for me 24 mm is wide enough) but keep in mind, that in terms of sharpness the A or FA 20 has the better perfomance on the whole frame. While the 24 mm has a excellent center sharpness you need to stop it down the usual two stops to get it decent sharp at the edges and down to f 8.0 to get it good over the whole frame.
The dancers from Sandy will be a wonderfull subject with the 24/2.0 while it might struggle with the landscapes.
I stop down to ƒ8 for landscapes because I need it for the depth of field. You seem to be suggesting ƒ4 or ƒ2.8 landscapes are a thing. ON APS_c and FF both ƒ5.6 is the sharpest for all but premium glass. SO, if you want excellent sharpness, you need to be shooting at ƒ5.6 already. But neither lens has good edges at a ny focal length.
FA 20
FA* 24
Compare those to the DA*55
My point being if edge sharpness is concern, you won' t be using either of the lenses.
If edge sharpness is a concern then the FA 20-35 might be the better option.
As Sandy's photos demonstrate, these are "lenses for the way people take pictures lenses, not for the test charts." The exhibit pleasing characteristics that go beyond test chart analysis.
If you're looking for edge to edge sharpness, you're going to be looking at heavier, more expensive, more modern glass.
The chart for a EF 24mm f/1.4 USM L II
$1500 and a lot more size and weight for a lens that would seem to be only marginally better in terms of it's curve being consistent across the chart.
Canon EF 24mm f/2.8 USM IS fo $549.
Not really a lot better than the Pentaxes, if better at all.
Talking edge sharpness in wide angle lenses, my question is who's done it and what does it cost? You might want to look at Sandy's images and decide if you like the rendering. That's what's going to distinguish these lenses, not the assumption you're going to find one that's sharp edge to edge.
As far as I know no FF lens has excellent edges in the 20-24 range, but as always, I'd be happy to be proved wrong.
However on APS-c my 18-135 has excellent edges at 24mm, as do many other APS-c lenses.
https://www.opticallimits.com/pentax/597-pentax_18135_3556?start=1
Before you start asking for something, you really should establish it's practically possible.
---------- Post added 09-12-19 at 02:02 PM ----------
Just on whim I decided to check out the Sony ZA (Zeiss) 24mm ƒ2 S SM
IN terms of the quality for consistent edge and centre sharpness it might be the best, for a mere $1400. (But still not as good as my DA* 55 1.4). Go for it.