HD Pentax-D FA 21mm F2.4 ED Limited DC WR

Focusing

The D FA 21mm uses a modern autofocus system with Pentax’s DC motor. The lens lacks the mechanical screw-drive coupling required for use with early digital bodies, but suports autofocus on most bodies since 2007.

Autofocus

Surprisingly, the AF system is easy to hear in a silent environment. The video below illustrates this, with the lens raking from infinity to close distances.

This amount of noise is disappointing for a 2021 lens. Other recent Pentax lenses, in particular the D FA*, are almost completely silent.

Focusing is internal and the length of the lens remains constant. The focus ring does not rotate as the lens focuses, and neither does the front element.

The autofocus is accurate and generally responsive. The lens does not feel like a speed demon, however. Again, some other recent Pentax lenses, namely the PLM motors, have impresed us more.

The lens allows quick-shift for manual focus override at any moment.

The lens includes no AF switches or controls.

Live View

Live view focusing feels slightly less responsive than using the viewfinder. This subjective difference is small from a user's perspective.

Manual Focus

Manual focus with the 21mm is on par with other Limited lenses. The ring move clearly and with a finely-tuned amount of friction, especially with the QuickShift function. It comes to hard stops at both ends and offers a clear distance scale, complete with DOF markings for F4, F8, F16, and F22.

The lens exceeds our expectations regarding manual focus.

AF (Autofocus) Speed

We tested the AF speed with a Pentax K-1 II, using both live view (contrast-detect) and the standard PDAF (Phase Detection AF, through the viewfinder). The subject was a black cross on a white background, about 1.5 meters in front of the camera. We used the central focus point. We set the lens at infinity before each test, and three measurements were averaged for each data point. Measurements were performed by recording the AF noise, at various levels of ambient light.

For reference, the Pentax K-1 II's autofocus sensor is rated for ambient light levels as low as -3 EV.

Note that 5 EV corresponds roughly to a small room lit with a 60 W bulb, and a sunny day corresponds to 16 EV, and a moonlit night to -2 EV.

The graph supports our comment above that the lens is not particularly fast at autofocusing. Even in good light, it never does much better than 0.6 seconds. In very low light, it requires over a second to lock focus.

This is not bad, per se, but certainly not the best that Pentax delivers these years. The D FA 21mm is actually a bit slower than its screw-drive siblings in the FA Limited line.

One positive aspect is that there is no practical difference between the viewfinder and live view. Another is that the lens' focus accuracy is high.

Remember that actual speeds will vary greatly with the subject and lighting conditions.

Hunting

We experienced absolutely no occurrences of hunting with the D FA 21mm during our tests. The lens always locked focus exactly where it should, regardless of the focus mode used.

Verdict

The Pentax D FA 21mm's AF speed is adequate but not class-leading. Its accuracy is high.