Pentax-FA 31mm F1.8 Limited (APS-C)
As shown in the multi shot image above the specification, the focusing extension is quite small. Focusing can be done up to 30cm (12 in) which allows for a good versatility of the lens. The front element, as expected for a quality lens, does not rotate, so the built-in hood remains in place.
Auto-focus, on the K20D, is quite good, as it has a relatively short extension, from near to far focus (or the way-around), the k20D will lock very rapidly, in less than a second in good lighting condition. Of course, tungsten and low light will be harder (although most probably very improved on the K-5) but trying to focus on a good contrasted item will help the focus in these conditions. Focusing, is necessarily noisy, as it is a screw mount lens. But the lens itself has a very smooth focus.
Of course, this is not a sports-lens, but it remains very good and if you are familiar enough with your camera body's auto focusing system, you cannot really miss any shots. However, shooting at large aperture implies small depth of field. This DOF will be even smaller as you focus on a close subject. So taking portraits with this lens can be tricky if you miss the focus on the eye and get the nose for example. This is where auto focusing has its limits, and this is mostly due to the electronics in the camera, not the lens itself (at least with the K20D large AF sensor areas).
Manual focusing is a pleasure. And you might want to actually use that in many situation where the AF sensor is not targeting the exact position you want or in low light. The focusing ring is small, compared to old full-manual Takumar lenses, but it is just right to get your thumb and index on it while keeping the rest of your hand under the lens for stability (see Lens and Handling pictures), hence a very comfortable handling.
The focusing ring is so smooth you can adjust very precisely with just one or two fingers. The ring rotates just a bit less than 90 degrees, which makes it very easy to move from one close subject to a further one. Perhaps the only regret on focusing is that the lens does not incorporate the famous Pentax mechanism allowing to quickly shift from auto to manual focusing. But that would certainly make the lens bigger for a feature you might not use that often.