The Pentax HD DA 21mm F3.2 AL Limited ($699.95 direct) is an impressively small wide-angle lens for Pentax cameras. Its all-metal build quality, compact lens hood, and push-on metal lens cap give it the feel of a top-grade lens, but you’ll need to narrow its aperture just a bit to make it sing. If you like the idea of a compact wide-angle and are a fan of the field of view the lens delivers, it’s a good option for any Pentax SLR. But if you don’t mind a larger lens, you may prefer the flexibility that the HD DA 20-40mm F2.8-4 ED Limited DC WR lens delivers—it’s a little more expensive, but is also a bit sharper at a comparable aperture and is weather sealed.
The DA 21mm measures just 1 by 2.5 inches (HD) and weighs 4.2 ounces. If you’re using a rotating filter like a circular polarizer, you’ll want to remove the hood and use the 49mm filter threads built into the lens to do so, but the unique design of the hood allows you to screw 43mm filters into its rear. The push-on metal cap requires the hood to be mounted for proper security—it can mount to the lens directly, but it has just a tiny amount of the barrel to grip onto if the focus is set to infinity, and can be knocked off easily. The 21mm lens can focus on objects as close a 7.9 inches from the camera’s image sensor, which is impressive for a wide-angle deisgn, but it’s by no means a macro lens. Magnification is a mere 0.17x (1:5.8) at its closest focus distance.
I used Imatest to check the sharpness of the 21mm lens when paired with the 24-megpaixel Pentax K-3. At f/3.2 the performance is disappointing, falling short of the 1,800 lines per picture height that we use to mark a sharp image by about 200 lines. Stopping down to f/4 improves the center-weighted score to 1,970 lines, but there’s still some softness towards the edges of the frame. If you stop down to f/5.6 the images are impressive from edge to edge, netting a 2,354-line score. At f/8 there’s only a marginal improvement, the 21mm scores 2,373 lines there. There is some barrel distortion, about 1.7 percent, but that’s a modest amount for a wide-angle lens. JPG shooters can enable in-camera correction in the K-3 to eliminate it, and Raw shooters can adjust it with ease in a workflow application like Lightroom.
The Pentax Limited series puts an emphasis on build quality and size, and for the most part, controls the limits the maximum aperture of lenses in order to capture a tack sharp photo at every setting. The Pentax HD DA 21mm F3.2 AL Limited is the exception, as its performance at f/3.2 is a little disappointing. But when stopped down to f/4, or better yet f/5.6, it’s an excellent performer, and its compact design is impressive for a wide angle SLR lens. If you don’t need such a small lens though, there’s a better option to cover this focal length—the Pentax HD DA 20-40mm F2.8-4 ED Limited DC WR zoom, which is larger, but offers a similar build quality and weather sealing. And, if you don’t mind a (much) bigger and heavier lens, the Sigma 18-35mm F1.8 DC HSM, which will be available for Pentax cameras in the near future, is a stellar optic with a wide aperture and impeccable sharpness throughout its zoom range.
|Dimensions||1 x 2.5 inches|
|35-mm Equivalent (Wide)||31.5 mm|
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