Autofocus with the DA 21mm mounted on the K20D is both fast and reliable. At all times, the system feels responsive, and missed focus only occurs with targets almost devoid of contrast, in low light. The AF drive generates a low amount of noise, especially for a screw-drive AF system. The small weight of the optics certainly contributes to that low level of noise. The focus ring rotates when AF is engaged.
The following graph displays the measured AF speed in relation with the brightness of the subject. The camera was set on a tripod, SR was turned off, and the lens was focused at infinity before each test. A remote was used to trigger the shutter. The focus chart used as the focus target was positioned 35 cm in front of the lens. Time to achieve focus was measured by recording the sound of the AF drive. An average of three measurements was used for each reported value.
At very low EV values, the time to lock AF is significantly higher, but still perfectly acceptable. The AF time decreases very fast and stabilizes once the scene brightness reaches around 4 EV. It must be stressed that this test was performed in a controlled environment; field tests with lower-contrast scenes are likely to yield longer AF times.
The lens features the quick-shift focus found on almost all DA lenses. This feature allows the user to manually focus the lens at any time (except when the AF drive is acquiring a lock). Although the lens focuses very accurately, having the option to correct the focus point as needed is a handy feature. The focus throw is around 80 degrees, which means fine-tuning the focus point can be tricky with narrow depths of field. The focus ring is surprisingly well damped for an AF lens.