The 55mm F1.4 is a member of Pentax's DA* family of lenses, and the construction and finish is extremely good. It's at least on a par with other lenses in this line, such as the DA* 16-50mm F2.8 and DA* 50-135mm F2.8, and arguably best in its class. The barrel is reassuringly weighty and rigid, and a rubber seal around the lens mount provides an external clue to the weather-sealed design. The front element is very deeply recessed in the barrel (at least at infinity focus), in effect providing a built-in hood for protection against flare.

Compared to Pentax smc FA 50mm F1.4

The smc DA* 55mm F1.4 SDM, despite being notionally an APS-C lens, is rather bigger and heavier than its predecessor, the full-frame smc FA 50mm F1.4. This is due to a combination of factors; the built-in autofocus motor demands a larger diameter barrel, and the optical unit is also slightly larger. The lens also uses a floating focus mechanism, with the rear element staying fixed as the main lens group moves forward on focusing closer; this results in a longer focus travel and therefore a longer barrel (in principle, it should also give superior results at close focus distances compared to the older lens).

Externally, the other clear differences are the loss of the aperture ring, and the much larger, easier to grasp manual focus ring. The narrower spacing of the marks on the depth of field scale reflect the fact that it is calibrated for the APS-C format, rather than 35mm (as you'd expect given the lens's DA* designation).

On the camera

The DA* 55mm F1.4 SDM may be large for its type of lens, but it's actually only a little bigger than the 18-55mm F3.5-5.6 kit zooms, if rather heavier. It balances well on all of Pentax's DSLRs from the large(ish) K20D to the diminutive K2000, and is a particularly good match for the K-7. The large focusing ring is especially easy to use, and the focus mode selection switch is well-placed for operation by the left thumb.

Lens body elements

The lens mount is Pentax’s KAF3 iteration, using a mixture of electronic and mechanical connections to interface with the camera. The two gold contacts for the autofocus can be seen on the left.

There's a rubber seal around the mount to protect from dust and water ingress into the body.
The filter thread is 58mm, and does not rotate on autofocus, which should be welcome for polarizer users.

The front element is very deeply recessed in the barrel - by about 20mm when focused to infinity. The main optical group moves fully 14mm forward on focusing down to 0.45m, with the 'floating' rear element staying fixed.
The ribbed rubber focus ring is 20mm wide, with a reasonably smooth (if slightly loose) action. It rotates 125 degrees anti-clockwise from infinity to 0.45m.

The angle of view noticeably decreases on focusing closer, as is often the case with primes.
A distance scale is provided with markings in both feet and meters, and includes a depth of field scale marked for F8 and F22. This is DA lens designed for use on cameras with APS-C sensors, so it's calibrated using a suitable circle of confusion for this format. This therefore gives a narrower indicated depth of field than the (full-frame calibrated) FA* 50mm F1.4.
A conventional AF/MF switch is located on the side of the lens barrel, perfectly placed for operation by the left thumb. Focus can also be adjusted manually when the lens is set to AF.
If lens hoods won prizes, Pentax would own a cabinet full. The bayonet-fit PH-RBH 58mm hood supplied with the lens is nice and deep, covering almost the entire length of the lens when reversed for storage. The inside is lined with black flocking to minimize reflection of stray light into the lens, and the crowning glory is a ring of rubber armour around the front to protect against shock. A mini-masterpiece.
There's even a slide-out section at the base of the hood to allow easy rotation of polarizers. Unfortunately it can't be removed while the hood is actually attached to the lens, and it's the kind of small part that can easily be lost (we can envisage legions of Pentax shooters searching the ground for where they dropped theirs). Overall though it does give the impression that Pentax lenses are designed by photographers for photographers.

Reported aperture vs focal length

This lens allows an aperture range from F1.4 to F22 to be selected.