Pentax SMC DA* 55mm F1.4 SDM
... the Pentax smc DA* 55mm F1.4 SDM is undoubtedly a lens capable of producing wonderful images, with a blend of optical characteristics near-perfectly suited to its intended purpose as a portrait lens. Shoot at F1.4 and you'll get flatteringly soft, but still finely detailed images, with characterful, almost painterly backgrounds. Stop down to F2 or thereabouts (arguably the optimum aperture for portraits) and you'll see a touch more sharpness, with slightly smoother, less distracting bokeh. If you want biting definition and more depth of field, shoot at F4 or smaller. All the while that 55mm focal length is offering the angle of view of a classic 85mm lens from the days of film, giving you a slightly longer working distance and more flattering perspective compared to a 50mm prime on APS-C.
Over the course of what has become a rather protracted regime of testing, we've looked at three samples of this lens, and each has shown a clear tilt to the focus plane; this was most pronounced in the first sample, but relatively minor in the last (indeed in this sample it was only really detectable in our studio tests). This does more than simply mess up our studio tests - it clearly results in real-world image quality and focusing problems too. In fact we've never before seen a lens so capricious when it comes to focusing - we're used to F1.4 primes being challenging for cameras' AF systems, but the 55mm was most difficult of all.
The lens ends up being recommended, but with reservations.
DPReview: Test gallery of 30 full-resolution JPEG images.