The Pentax SMC DA Star 50-135mm F2.8 ED (IF) SDM ($1,599.95 direct) is the company's top-end telezoom lens. Pentax SLRs use APS-C image sensors, making this lens the equivalent of a 75-200mm zoom on competing full-frame systems. It's a classic zoom for event coverage and photojournalism, and is also an appealing range for portraiture. And because it's part of the DA Star line, the 50-135mm is completely weather-sealed—which makes it an all-weather solution when paired with a camera like the K-5 II or K-5 IIs.

The lens itself is fairly small and light for its class, since it doesn't have to project an image circle that covers a larger full-frame sensor. It weighs about 1.7 pounds and measures 5.4 by 3 inches (HD), but mounting the included petal hood increases the height by about 4 inches. Compare this to the Sony 70-200mm f/2.8; that lens delivers a nearly identical field of view on full-frame cameras, but it weighs nearly 3 pounds and measures 7.75 by 3.5 inches. It also sports a tripod collar; the 50-135mm doesn't include one or support one, but given its length there are times where one could come in handy, especially when pairing with a smaller camera like the K-30See it at Amazon UK.

The lens has an internal focus and zoom design, so it never extends when changing the focal length, and the 67mm front element doesn't rotate when adjusting focus. The hood can be reversed for storage. There's an internal SDM focus motor for quick and quiet autofocus, but there's also a screw-drive available if you have an older Pentax D-SLR that doesn't support SDM. It can focus on objects as close as 3.3 feet throughout its zoom range.

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I used Imatest to check the sharpness of the lens when paired with the 16-megapixel K-30. It's impressively sharp at its 50mm f/2.8; it scores 2,257 lines per picture height there, using a center-weighted metric. That's better than the 1,800 lines we require for an image to be sharp, but it should be noted that the edges are a bit soft at just 1,096 lines. Stopping down to f/4 improves the edge-to-edge performance; edges improve to 1,731 lines and the overall score is 2,927 lines. At f/5.6 you can expect 2,809 lines, and the lens manages 2,836 lines at f/8 with edges that exceed 2,000 lines. There is a little bit of barrel distortion at 50mm, about 1.3 percent, but serious shooters can use software like Lightroom to straighten out the slight curvature that introduces to images.

Zooming to 90mm doesn't change performance that much. At f/2.8 the lens notches 2,229 lines with edges scoring about 1,242 lines. Stopping down to f/4 improves the score to 2,777 lines and edges sharpen up nicely to 1,854 lines. Sharpness at f/5.6 is 2,760 lines and Imatest showed 2,616 lines at f/8; edge performance exceeds 2,100 lines at both of those apertures. Distortion swings the other way at 90mm; there's about 1 percent pincushion, which makes lines appear to curve inward. It can be corrected just as easily as barrel distortion in Lightroom.

The lens delivers its weakest performance at 135mm. At f/2.8 it records 1,713 lines, shy of our 1,800-line cutoff. Edges are very weak, just 914 lines. Stopping down to f/4 improves the overall score to 2,013 lines, but edges are still weak at 1,259 lines. You'll see improvements in overall resolution at f/5.6 (2,111 lines) and f/8 (2,233 lines), but the edges never get sharper than about 1,500 lines. You just won't be able to get an image that's super-sharp from edge to edge when zoomed all the way in. Distortion also increases here; there's about 1.5 percent of pincushion distortion at 135mm.

Despite not being the sharpest lens when zoomed all the way in, the Pentax SMC DA Star 50-135mm F2.8 ED (IF) SDM is still one that serious Pentax shooters should consider adding to their gear bag. The fixed f/2.8 aperture and its field of view make it an excellent choice for event coverage, and the weather-sealing ensures that you can keep shooting in all kinds of weather conditions. Its performance is impressive at the wide and middle zoom range, and you can capture images that are sharp from edge to edge if you narrow its aperture by one or two stops. If you don't need a lens this fast you can consider the less expensive SMC DA 18-135mm F3.5-5.6 ED AL (IF) DC WR£349.19 at Amazon UK. It doesn't capture as much light, but it's also weather sealed and sells for less than a third of the price of the 50-135mm.