The Pentax HD DA 40mm F2.8 Limited ($399.95) is an update to one of the standout lenses for the Pentax camera system. It offers an improved lens coating and rounded aperture blades, both missing from the older SMC version, but retains its incredibly compact form factor and top-notch image quality. It's seen a significant price cut since it was first released, which certainly makes it an appealing choice for any Pentax shooter in want of a compact prime lens for an APS-C SLR. If you're a bargain shopper and don't mind a funky design, the Mark Newson Pentax DA 40mm F2.8 XS ($249.95) bundled with the K-01 is no longer in production, but still available at retail. Our Editors' Choice for this focal length is the SMC FA 43mm f/1.9 Limited, which is a full f-stop faster, sharp in its own right, and just a bit larger.

Design

The 40mm($346.95 at Amazon) is really, really small. It measures just 0.6 by 2.5 inches (HD) and weighs a mere 3.1 ounces. The included metal lens hood doesn't add much depth. It screws into the lens's 49mm filter thread, and the hood itself features a 30.5mm diameter thread for filters or the included (tiny) metal lens cap. I didn't see much use in using the cap—the hood does a more than adequate job of protecting the front element. Your best bet is to leave the cap in a safe place, because it's bound to get lost if you don't. The less expensive 40mm F2.8 XS lens doesn't include a hood, and its front element supports even smaller 27mm filters.

Pentax sells the lens in your choice of black or silver, each with a red accent ring. Because the lens is so slim, the manual focus ring is also quite narrow. But it's still comfortable to grip and turn, and it offers enough resistance to make manual focus a pleasant tactile experience. All that's needed to move from infinity to the closest focus distance is a 90-degree turn, and you can adjust focus manually at any time, even when your SLR's autofocus system is engaged. The focus scale is shown in feet and meters, and there are hard stops at each end.

Autofocus is driven by your camera's screw drive, so it's on the noisy side. But it's also quite quick due to the light design and short focus throw. The 40mm locks onto subjects as close as 1.3 feet (0.4-meter), capturing objects at up to 1:7.7 life-size. That's about the same as the 43mm ($409.00 at Amazon) , which focuses to 0.45-meter to captures subjects at 1:8.

Unlike most Pentax SLRs, the 40mm Limited does not offer an sort of weather sealing. The only lens in this range to do so is the larger 20-40mm ($496.95 at Amazon) zoom.

Image Quality
I used Imatest to check the image quality the lens delivers when paired with the K-3($419.00 at Amazon). At f/2.8 the HD DA 40mm betters the 1,800 lines per picture height we require for a lens to be called sharp, scoring 2,064 lines on our center-weighted test. Stopping down to f/4 improves that mark to 2,108 lines, and the score increases as the aperture is further narrowed, peaking at 2,352 lines at f/8.

These are strong scores, but not as strong as when the 43mm is paired with the K-3; it crosses the 3,000-line mark at f/4. Of course, it's a bit larger, and has the advantage of being designed for full-frame cameras, so its edge performance doesn't come into play when paired with an APS-C image sensor.

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Distortion isn't an issue with the lens, and it's quite even in terms of illumination. There's some dimming of corners at f/2.8 (-1.3EV) if you shoot in Raw format, and it disappears at narrower apertures. JPG shooters can enable vignette correction in camera and eliminate the mild dimming effect, and it's very easy to compensate for it in Lightroom ($9.99/Month at Adobe) if you shoot in Raw.

The 40mm isn't a full-frame lens like the 43mm, but it can be mounted to a full-frame camera like the K-1 ($1,335.00 at Amazon) . You have the option of cropping the active frame to an APS-C format if you do, but you don't have to. Despite its slim design, the 40mm shows only mild dimness at the corners when paired with the K-1. However, at f/2.8 the edges of the frame are excessively soft (1,058 lines), giving them a blurred look. Stopping down to f/4 offers significant improvement (1,808 lines), and at f/5.6 the 40mm is a strong performer, notching 2,900 lines across the frame with edges that are crisp at 2,275 lines. It's even better at f/8 and f/11, topping 3,100 lines at both stops, with a periphery that hits 2,700 line and 2,900 lines, respectively. You'll certainly get better performance (and a brighter aperture) from the FA 43mm, but if you're moving up to the K-1 from an APS-C system and already own the 40mm, it's good to know that you can use it without much worry with a larger image sensor. The image above was shot on the K-1 at f/8 and is uncropped.

Conclusions

The Pentax HD DA 40mm F2.8 Limited is a lens that has a heck of a lot going for it. It's very small, not that expensive, and delivers excellent image quality when paired with an APS-C camera. And, despite not being designed for use with a full-frame body, it does a good job with one as long as you stop down a bit. You can still buy a less expensive lens with identical optics in the form of the 40mm XS, although its design can be offputting, it doesn't include the latest HD coating this version offers, and its aperture blades aren't rounded, which can change the look of the blurred background of an image. We give preference to the FA 43mm Limited, which is our Editors' Choice, due to its wider maximum aperture and crisper optics, but at $600 it's a lot more expensive. If you think you're going to stick with an APS-C camera, the HD DA 40mm F2.8 Limited is a solid buy in its own right.

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