I have both lenses - the HD DA21 and the HD DA20-40...
I feel like a scratched record for saying this yet again, but... try as I have, I just can't fall in love with the DA20-40
Of course, it has a
lot going for it... the focal lengths of three DA Limited primes (and everything in between) with good close-focusing capability, near-silent DC motor AF and a WR body that's beautifully built and presented. At the wide end, it's a tad faster than the DA21 (f/2.8 vs f/3.2) and the distortion - whilst
slightly worse - isn't bad and is easily corrected. Out of focus highlights aren't as silky smooth and uniform as with the DA21, but decent nonetheless, and out-of-focus rendering is generally very nice. Centre sharpness is really good throughout the focal length range, with borders becoming decent around f/5.6.
BUT... at the longer end, it has runway style field curvature that is most noticeable when focusing at distance and with something in the borders at closer range, and it's there even when stopped down. I don't believe I've encountered this behaviour with any other lens. Focus with the centre AF point at something in the distance, and anything close to you at the sides of the frame will be disconcertingly in focus. For a lot of shooting, it's not an issue... But it can matter for certain landscape, city / architectural and other types of photography where a flatter field would be preferable. And once I noticed it the first couple of times, I just couldn't "un-notice" it.
I'll say again, though - the DA20-40 has a lot going for it. And I've made a real effort in recent weeks and months to use it more, to the point where I'm at least enjoying and getting some value from it. I've no intention of selling it, because it's certainly versatile, and the WR is very useful. But it's far from being my favourite lens.
The DA21, on the other hand, is - or, rather, it's one of my favourites. And I can't quite put my finger on why, because it's not a stunning performer on paper. It's not especially fast. Centre performance is very good, but the borders are just OK unless it's stopped down (although they're better than the DA20-40). But I
love the rendering. It has a character that is subtly but noticeably different from any other modern lens I own that covers this focal length, almost like a modern incarnation of an old film-era lens (maybe that's why I like it, as there are so many vintage lenses I really like). Plus, it's compact and cute, and nice to use