I have both lenses. I acquired the prime several years ago and the zoom in April 2019.
When the zoom appeared in the market, it was selling in Canada for CAD $1,000, which was too expensive for me. I was looking for a zoom in the 18-50mm range, and ended up getting a DA 18-135mm instead (treating its 'softer' 60-135mm range as a bonus). Recently, a new 20-40 is selling for around CAD $725, which represents very good value for money from my perspective.
Although I haven't done an extensive shoot-off to compare the 21 and 20-40 rigorously, I have used both lenses on my K-3 II in similar settings, such as classic car shows and 'urban-scapes'. I think that I've used them enough to conclude that I'm very pleased with both. Here are some observations:
- I'm tending to use my 20-40 and DA* 50-135 as a versatile travel and walk-about kit, favouring this pair over the single 18-135, albeit with a bulk and weight penalty. On general photo outings when I might have carried the 21 and 35mm Limiteds, now I'd probably take the 20-40 instead.
- The WR feature is a big plus for the zoom.
- I'm happy with the optical aspects of both lenses. From my detailed research before I bought it, I gathered that the 20-40 could be slightly soft in the corners at certain settings, but I haven't really noticed any issues, and have been happy with its edge and corner performance.
- To my eyes, I think that my HD 20-40 yields nicer images than the 18-135 across the common focal length range.
- I haven't observed any significant difference in image quality between the 21 and the 20-40. I'd say that images are comparable; I've made good pictures with both.
- On a few shots with the 20-40, I would have appreciated a wider aperture at its long end. While f/4 is ok, f/2.8 would have worked a bit better, suggesting that the trio of 21/35/40 Limited primes does have its place. This is the only (minor) weakness I can think of, and points to the complementarity of the primes and zoom.
- On a summer vacation trip in 2018, I used the HD 21 a lot, and was really happy with the results -- it's a sweet little lens. Based on this experience, I think I'll keep the prime, even though I'll likely use the zoom more often.
- While the HD 21 is significantly smaller and lighter than the 20-40, I find that the zoom is not cumbersome at all, and can be carried with ease, either in the bag or on the camera.
- I needed to buy a new 55mm CPL filter for the zoom (Hoya Fusion CPL, CAD $125), so this was an extra cost. Otherwise, my 49mm filter fits a number of my lenses, including the DA 21. Commonality is a good thing!
Edit. At the risk of making this post even longer, I thought I'd add a couple of things. The designers of the HD Pentax-DA 20-40mm Limited
spoke to my heart. To me, the lens is a thing of beauty. Reflecting some design notes of the venerable Takumars, the 20-40 has a certain flair and style that complements the Limited primes, yet it's in a class by itself. It's easy on the eyes, and it handles easily. The zoom ring has just the right resistance without being annoyingly stiff, while the manual focus action is one of the best I've experienced amongst my modern AF lenses. If the DA 21 Limited could be called a little jewel, the 20-40 is a cool, smooth, polished gem of a lens. I enjoy using it.
I see that the HD 21 is available new in the United States for USD $425 and the 20-40 is $500. If budget is limited, I'd recommend the HD 20-40 over the 21.
Hope this helps.
- Craig
Last edited by c.a.m; 08-15-2019 at 05:08 PM.