In praise of primes - which next?

I'd suggest the DA40.  It was one of the very first lenses I got when I bought my K10D, and I've not been the least bit disappointed.  I find, at least for me, that the angle of view is quite handy and natural feeling; it's just a little narrower than a "normal" lens, kind of halfway between a normal and a mild telephoto.  It does seem to be something of a love-it-or-hate-it thing, though, for the field of view.

The DA40 has the advantage of being exceptionally well-behaved under most any somewhat reasonable usage or setting.  It works well enough wide open, stopped down, and in between.  It focuses reliably and rapidly if there's anything to focus on.  In short, it's a lens you can use in a point-and-shoot sort of circumstance and not have to keep any caveats in the back of your mind.  (I suspect the 50mm lens is generally similar in that regard, but I don't have any direct experience with it.)

As a rule of thumb for my lenses, I aim for—or at least have ended up with—about a 1.5x difference in focal length between lenses; that seems to be large enough to warrant having another lens, but small enough not to have too large of gaps in between.  I've ended up with 40mm, 58mm, 90mm, 125mm, and 180mm prime lenses.  Some other people try to have a factor of about 2x between lenses, which also works out OK but of course leaves bigger gaps.  It's not a wholly scientific or mathematic matter, of course; there's far more to a lens selection than just its focal length, and some intermediate ranges are more obviously useful than others.  But it's at least food for thought in planning one's bag.

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--DrewE