K: the original K-Mount lenses, full 35 mm frame coverage, made out of metal, extremely solid and well constructed, great mechanical feel. Capable of only manual metering and aperture priority
M: like the K-series, but generally smaller and lighter
AF: like the M series but, but with an AF motor in the lens
A: like the M series, but with an "A" setting on the aperture ring, thus supporting additionally shutter priority and program operation
F: like A series, but supporting modern-age auto-focus (AF motor in the camera body) and digital communication with the camera body
FA: further development of the F-series, more extensive communication with the camera body
FAJ: like FA lenses but without an aperture ring, thus only operable on bodies that can control the aperture electronically
DA: like FAJ, but optimized for digital bodies and covering the image circle of the reduced-size digital sensor
D FA: like FA, but the optics are optimized for both film and digital
DA L: like DA, but featuring a very light construction, thus optimally paired with very compact and light bodies like the K-m
*: the star sign follows the lens series designation (e.g., A*, FA*), and denotes a high-performance lens that uses special-glass elements.
Limited: Similar to the * designation in that the lenses are of very high quality, but not meaning that the lens is being produced in limited quantities. Star-lenses are generally large and expensive zooms and Limited lenses are generally compact and expensive primes with not very wide apertures.
AL: a lens that uses one or more aspherical elements (aimed at achieving flat field and corner sharpness with wide-angle lenses).
ED: a lens that uses extra-low dispersion glass elements (aimed at reducing color aberrations in telephoto lenses).
IF: a lens that uses an internal focusing mechanism (lens groups are being moved with respect to each other instead of the entire optical assembly being moved towards or away from the film plane); IF is typically used for longer lenses with large front elements in order to reduce power requirements for the focusing motor and to make such lenses easier to use with polarizing and graduated filters. IF lenses typically change their focal length slightly during focusing. They get wider as they get focused closer.
Fixed Rear Element Extension (FREE): a lens whose last cell i remains fixed during focusing. FREE lenses achieve low field curvature and low spherical aberrations not only when focused close to infinity but also at very short distances. For normal applications the FREE system leads to small physical extensions, and thus allows the design of physically smaller and lighter lenses. For macro applications FREE lenses exhibit non-proportionately long extensions which aid precise focusing. However, to achieve these positive effects, the FREE design causes the lens focal length to decrease as the focus distance decreases. Pentax has listed FREE as a feature only for prime lenses even though there are zooms that also have a fixed rear elements (e.g., SMC-A 70-210/4, SMC-A 28-135/4).
SDM: System, assures smoother, quieter auto-focusing operation by using a
built-in supersonic motor
Weather-Resistant (WR): a lens that features special seals and is thus resistand to dust and light rain.