HD Pentax-DA 21mm F3.2 Limited
Focusing
For a screw-drive lens, the DA 21mm Limited showed decent speed and accuracy in most settings.
In our controlled-light settings, the DA 21mm Limited achieved focus from infinity to 2.5 meters in three separate scenes. The first was a very low 2EV scene with medium contrast subject; the second a relatively dim 4EV but with a very low contrast subject; the last was in acceptable natural light with a medium contrast subject. The table below summarizes the results of our focus tests.
Scene | Average AF Time | Fastest Time | Slowest Time |
| 2EV, medium contrast | 0.55 s | 0.26 s | 0.90 s |
| 4EV, low contrast | 0.74 s | 0.39 s | 1.11 s |
| 6EV, medium contrast | 0.25 s | 0.13 s | 0.36 s |
Auto focus times for the DA 21mm Limited.
In our field testing, the DA 21mm Limited was very good, but not perfect, at acquiring focus. Unlike its smaller cousin, the DA 40mm Limited, the wide 21mm angle introduced a few errors using a K-5 IIs. On a few occasions, the lens reported in focus when the subject was slightly missed. Additionally, the lens would on a few occasions fail to find focus at all.
The lower magnification due to the wide angle of view and the smaller F3.2 aperture is likely the culprit for the focus errors. The total number of misses was in the single digits out of hundreds taken in varied lighting throughout our tests.
Quick Shift and Manual Focus
The DA 21mm Limited has Quick Shift, which allows manual fine-tuning even while the camera is in auto-focus mode. This can be exceptionally useful in difficult scenes, like those with competing foreground or background objects.
The manual focus feel is decent, with a damped ring which turns approximately 70 degrees from near to far. Unlike some modern lenses, the ring actually focuses the lens rather than the focus-by-wire which utilizes the auto-focus motor to move the lens elements.
Verdict
The DA 21mm Limited isn't the fastest overall screw-drive lens we've tested, but in decent light it can be lightning quick, with a 0.13 second fastest time in our controlled tests. The lower magnification inherent to the wide angle of view contributes to the camera's difficulty finding focus in some situations, which of course is typical for the class.
Both the Quick Shift and manual-only focusing worked well, much better than most other manufacturers' implementations.
Because it utilizes a screw-drive auto focus, the lens makes an audible noise during the process, something which videographers will need to take care to avoid.
