A used DA 40mm XS from a Pentax Forum member and a 27-52 step up ring.
My wife and I are making two international flights; one late this summer and the other next winter (separate continents) I plan on have each of us having just the two allowed carry-on items and no checked bags. I can take a dedicated camera bag that folds relatively flat and put that in my roller bag. Unfortunately, I also know that on certain connecting flights, "carry-on" isn't quite true. The smaller regional planes make you 'plane-side' check roller bags as the overhead bins are too small. What that means is ALL delicate gear and spare Lithium batteries have to fit in my day pack along with other items I want while we are in transit.
My experience is photo back packs leave little room for general day items, but multi-purpose back packs are not particularly friendly to SLR gear. A SLR with a mounted lens is an awkward shape. So during flights, I have neoprene sleeves for my K-3 body and the couple lenses I want to have. Hmmm. The
DA 40mm XS isn't much bigger than a body cap, and uses the same lens design as the
40mm limited but a lot less expense.The mounted lens fits in the neoprene body case. I get a 3rd lens and a camera ready for immediate use.
The only things I really didn't like was the 27mm filter threads and the press-in 27mm front lens cap. The lens cap relies exclusively on friction (easily lost), and would require buying 27mm filters and finding a lens hood that didn't vignette. Problem solved with an super-inexpensive 27-52mm step-up ring. I already have 52mm filters, pinch-clip lens cap and a collapsible rubber hood.