The SMC Pentax-DA 40mm F2.8 XS lens is a new ultra-compact lens launched alongside the K-01, and it's part of one of the kit options for the camera (the other being a two-zoom 18-55mm & 50-200mm kit). Outside of the kit it's priced at an MSRP of $249, but if you buy it in a kit, you'll save $100 or more. Pentax says that the lens was also designed by Marc Newson, hence its unique appearance.
This lens suits the K-01 perfectly as it makes the camera handle perfectly and keeps it light. The lens itself weighs only 52g and is less than a centimeter long, making the weight of the camera just 612g when mounted. Pentax claims that this is the smallest interchangeable lens they've ever built. It has a 27mm front filter thread, and will accect UV filters of that size.
In addition, the design of this lens matches that of the K-01, and it actually goes along with the K-5 quite well also.
The DA 40mm F2.8 XS seems to be optically identical to the DA 40mm F2.8 Limited, featuring 5 elements in 4 groups and 9 aperture blades. We found it to deliver the same superb image quality and bokeh as its bigger cousin.
As far as differences go, obviously, the XS lens is smaller: Pentax managed to shrink the casing by removing the quick-shift feature as well as the distance scale. Also, the XS lens has two different contacts on its mount, and there is a strange gap on the back of the lens, as shown below. It doesn't come with a lens hood, and its uses a rubber push-in cap rather than a screw-in cap of the limited version.
Our only complaint about this lens is the lack of quick-shift and the noise it makes while focusing, but fortunately, its short throw helps counteract this. It is also somewhat difficult to mount as there isn't much to hold on to. Considering that it only costs about half as much as the limited version and that it's smaller, though, this isn't a very big deal.
The full specifications of the 40mm XS lens, as well as user reviews, can be viewed in our lens review database. We provide numerous sample photos from this lens later in the review.
It should be noted that although this particular lens will work on any Pentax DSLR, Pentax plans on using the DA "XS" lens series for lenses specifically designed for the K-01 and future mirrorless cameras by implementing protruding rear elements.