Lens Test: Pentax 21mm f/3.2 DA AF Limited

This well constructed, light and compact 21mm is an excellent choice for composed or from-the-hip snapshooting, as well as landscapes and interiors.


One of Pentax's exclusive Limited series lenses, the lightweight and compact 21mm f/3.2 ($470, street) features all the traits that have made a name for the Limited series: solid all-metal materials, extremely high manufacturing standards, engraved barrel markings, and uncompromising optics. Its high-tech pluses include a hybrid aspheric element and Pentax's relatively new SP coating that the company claims will make glass surfaces unusually easy to clean. Mounted on a Pentax DSLR, the 21mm's angle of view is equal to that of a 31.5mm lens on a 35mm film SLR.


Solid-feeling though lightweight (under 5 ounces) and compact (1 inch deep!), this wide-angle owes its svelte profile, in part, to the lack of an aperture ring. Its bright green aluminum barrel ring tells you from across the room that the lens is a DA, digital-only Pentax. Its handsomely engraved markings include depth-of-field and focusing scales (feet in blue, meters in white.)

What's Hot • Extremely well constructed. • Very light and compact. What's Not • Filter system requires purchase of 43mm and 49mm filters.

Physically, the lens' most unusual trait is the hood (included), which, with its elegant black-on-black markings, has a rectangular shape that resembles a mask more than a typical lens shade. The dual 43mm/49mm filter system is also nonstandard. Threads inside the lenshood accept 43mm filters, but because the bayoneting hood prevents finger access to these filters, the design is inappropriate for circular polarizers, split filters, and other lens-mount accessories that require accurate positioning. To use such filters, you remove the lens shade and thread 49mm filters, more conventionally, onto the outer barrel rim.

The lens's AF action is fast, accurate, and quiet, while the manual focus ring turns smoothly (even in the AF mode), with a nice, well-damped, and even action.


SQF test results showed sharpness and contrast in the Excellent range - as expected for a Limited-series lens. To measure distortion, we used the DxO Analyzer 2.0 system, and found barrel distortion at the low end of the Visible range (0.35%) - somewhat better than expected. (By way of comparison, a Leica 21mm f/2.8 Elmarit-M, full-frame lens a few years back showed 0.15% barrel distortion.) Edge vignetting left the Pentax corners by f/4.5, an average performance. The maximum magnification ratio of 1:5.7 at the close-focus distance of 7.25 inches showed great magnifying power for such a wide angle lens. The ratio suggests significantly greater magnification than the 1:7 Pentax claims.


An excellent choice for composed or from-the-hip snapshooting, as well as landscapes and interiors, this lens' true forte -- due to its compact, carry-on proportions -- is probably travel photography. It will easily fit in the smallest of camera bags. Also, with that tight close-focus distance and great magnifying power, it's an unusually good choice for dramatic, wide-angle close-ups.


@ 21mm: VISIBLE barrel (.35%)

@ 21mm by: f/4.5

1:5.70 @ 21mm (7.25")