The Pentax HD DA 18-50mm F4-5.6 DC WR RE ($299.95) is the first retractable zoom lens for the K-mount camera series. It sacrifices just a little bit of light gathering capability and zoom range when compared with the company's standard DA 18-55mm F3.5-5.6 AL WR ($199.95) zoom, so you're paying a premium for the retractable design. It's a fine option as a starter lens (especially if you value its size and don't want to spend a lot more on the HD DA 20-40mm F2.8-4 Limited DC WR zoom), but it's certainly not an Editors' Choice winner. That honor goes to a big, expensive lens, the Sigma 18-35mm F1.8 DC HSM, which is pretty much the antithesis of the svelte 18-50mm in terms of design.

When retracted, the HD DA 18-50mm is very small—just 1.6 by 2.8 inches (HD). It nearly doubles in length when extended, so storing it in the collapsed position will save some room in your gear bag. The lens is also quite light at just 5.6 ounces, but it achieves this through plastic construction, and there's definitely some wobble to the barrel that you can notice while holding it. The zoom action is pretty smooth, however, and the lens retracts without issue when mounted on a camera. A rubber seal around the mount prevents water from seeping in at the connection point, complementing the weather-sealed design of most Pentax SLRs.

The HD model of the lens includes a clip-on hood that adds just about an eighth of an inch to its height. Pentax also has a version of the lens that's billed as the SMC DA 18-50mm F4-5.6 DC WR RE, which it doesn't sell on its own but instead bundles with certain cameras. (Currently it's only available in a kit with the K-S2.)  The SMC version doesn't ship with the hood, and claims to use the company's older lens coating process. Performance in our test lab was very similar for both lenses—well within the limits of sample variation—and mechanically the lenses offer the same level of build quality.

The HD DA 18-50mm uses 58mm lens filters, which are a bit larger than the 52mm filters used by the standard 18-55mm Pentax kit zoom. It can focus as close as 11.8 inches, which gives it a 1:4.3 reproduction ratio. That's not quite macro territory, but it lets you get fairly close to your subject. If you want a zoom lens that covers this range and has a decent macro capability, consider instead the Sigma 17-70mm F2.8-4 DC Macro OS HSM. It's more expensive, and certainly not as small as the 18-50mm, but it's better built and captures images at a 1:3 magnification at its longest focal length and closest focus point.

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I used Imatest to check the optical quality of the HD DA 18-50mm when paired with the 20-megapixel K-S2. At 18mm f/4 it scores 2,047 lines per picture height on a center-weighted sharpness test, which is better than the 1,800 lines we look for in an image. Edge performance is a little weak at 1,705 lines, but that's a fine result for a starter lens. Stopping down to f/5.6 offers very modest improvements in clarity. Barrel distortion is an issue at 18mm; the lens shows 1.9 percent. This gives straight lines a noticeable outward curve, but it's an effect that can be corrected with image editing software.

Zooming to the midpoint, 35mm, narrows the maximum aperture to f/4.5. The lens is fine in terms of sharpness, scoring 2,041 lines, with edges that are pretty crisp (1,830 lines). Image quality is about the same at f/5.6 and f/8, and distortion isn't an issue here. At 50mm the maximum aperture is f/5.6, with 2,025 lines and edges that approach 1,800 lines. Performance is just about the same at f/8, and there is a bit of pincushion distortion, about 1 percent. That gives images a slight inward curve. You may notice it in some shots, but you'll have to look for it.

The Pentax HD DA 18-50mm F4-5.6 DC WR RE is aimed at Pentaxians who are looking for a zoom lens that is small, light, and weather-sealed. But it's expensive for what it is, especially when you take its build quality into account. If you're buying a new Pentax SLR with the SMC version of the lens—which is of identical build and optical quality, just with an older type of lens coating—it's an easier sell. But I wouldn't recommend the HD DA as an upgrade if you currently shoot with the standard 18-55mm Pentax kit lens.

If you are looking to spend some money on a new standard zoom lens, there are better options out there. The Sigma 17-70mm is more expensive and lacks weather sealing, but it offers significant gains in terms of maximum aperture, zoom range, and build quality. Pentax also has its own SMC DA 18-135mm F3.5-5.6 ED AL (IF) DC WR, which is significantly larger and priced around $500, but offers a longer zoom range. Our Editors' Choice for standard zoom lenses is the Sigma 18-35mm f/1.8, though it isn't particularly compact. Pentax has its own premium compact zoom, the HD DA 20-40mm, but it's a very expensive alternative.