Originally posted by csa I should mention I do have the Tamron 10-24. Was mainly interested in a prime lens.
Well, the DA 21mm LTD is a fine choice in a prime lens, compact, premium build quality, and a versatile FL. It would outperform your Tamron zoom lens at that FL, and has a faster aperture capability. So if you are looking for a compact, high-quality prime, this one is a joy to carry and use.
That said, if you are looking for edge-to-edge performance in your landscape shots, this lens has been tested as excellent even wide open in the central image area, while rated as good in the edge areas until f/5.6 where it becomes very good, and can deliver fine results throughout.
However, also according to tests, the DA 12-24mm easily outperforms your Tamron lens, except where it is a closer situation at 24mm between the two. This Pentax lens delivers near-excellent to excellent results edge-to-edge from wide open on. It is also constant-aperture f/4 capable throughout. Distortion is unusually low, especially for a WA zoom lens, and especially from 15mm on, which would rank it highly for architectural use, including city-landscapes.