Regarding your problem, it seems that there could have been some decentering issue with the lens.
Aperture f3.2 is a pretty dim for a prime. This maximum aperture would have adequate depth of field covered within the frame. A gross out of focus blurr is really unlikely unless a close focus approach is attempted.
I suggest to send this limited to pentax for an evaluation instead.
Getting back to this particular limited, it has strengths and weaknesses just like any other lens. However, Some weaknesses could work as advantages for certain photographers.
The lens is known for its unsharp corners with vignetting at f3.2 but it does not matter matter if aperture f8 is used for landscape. For night scenery, f4 is far more than sufficient to get the depth of field. The rest could use a little USM sharpening via Benjikan technique. This is relatively simple.
What makes pentax da limited special is the strong colour and excellent contrast. This type of characteristic is hard to produce from photo editing. A signature look for pentax users. Da limited does cast that rich glow to certain colour embedded with strong gradient to the image. The micro contrast from da limited also pop the images interestingly.
Da 21mm ltd in particular does not seem to have that strong micro contrast like any other da limited. Hence appear a little soft everywhere. I have found a way to get around to it by isolating the edges out with selection placing the dodgeing along these selected edges. It seems to bring back a bit more micro contrast instead.
This thin pancake prime is a great lens just for being so small. Consider a 25 km hike in the mountains. A light wide prime like this is very valuable. There is a lot a lot of envy from canikon users who hike along with me feel extremely jealous. After all, a canon EF 20mm f2.8 is quite an extremely dodgy lens. Canon EF 24mm f1.4 is 550 g or so and Nikon 12-24mm f2.8 is 1 kg. By comparison, this type of heavy lens does add a lot of stress to a long trip. Considering the tough nature of the hike, heavy lenses are nuisances.
Speaking to a casual street shoot, a light small prime is so insignificant to pedestrians or museum shooting. No one really cares about the photographer who can feel free to get the "moments" when they want to.
I have used sigma 24mm f1.8 in the street. Besides all the bravo about the fast autofocusing of this prime as well as its incredible resolution in its images, the colour and contrast are seriously inferior to Da 21mm ltd. What is more is the bulky nature of the prime that can frighten a lot of people.
I would not quickly dismiss this lens as I have previously done so. A great lens to use in deed and I had been using this for daily walk around so far. Not much touch up is needed. However, I would not say that this is the best landscape prime I have ever owned.
Some images from Da 21mm ltd
Pentax Da 21mm ltd gallery
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