Pentax SMC DA Star 16-50mm F2.8 ED AL (IF) SDM

By Jim Fisher

The Pentax SMC DA Star 16-50mm F2.8 ED AL (IF) SDM ($1,499.95 direct) ?is a lens that is designed to replace the standard 18-55mm kit lens that is bundled with Pentax D-SLR cameras. It's expensive, but delivers a fast f/2.8 aperture throughout its zoom range, weather sealing, and an internal focus motor. Unfortunately, it isn't all that sharp at its maximum aperture and displays significant distortion at the wide end of its zoom range.

Pentax currently only offers APS-C D-SLRs, giving it a 24-75mm field of view in terms of 35mm full-frame photography. The lens is a bit longer than the Sony's similar 16-50mm f/2.8 Standard Zoom Lens , which likewise can only be used with APS-C cameras. It's 3.3 by 3.9 inches (HD) and weighs 1.3 pounds, and has a 77mm front filter thread.

There are two control rings on the lens?the smaller one closer to its base controls the zoom, while the larger ring allows you to adjust focus. It supports the Pentax Quick Shift focus system, so you can adjust the focus manually after the camera locks on to an object via autofocus, without having to toggle the AF/MF switch on the camera body or lens. The lens would have been better served to reverse the size of the control rings, as the zoom ring is likely to be used more often than the manual focus ring, and gripping such a narrow ring can be tricky as it is located very close to the base of the lens.

I used Imatest to check the performance of the lens. In terms of sharpness, it does quite well at the 16mm focal length, notching 1,915 lines per picture height at f/2.8 and an outstanding 2,851 lines at f/4. We use 1,800 lines as the cut off for a sharp image. Distortion is a big factor at 16mm, as the lens exhibits a very noticeable 3 percent barrel distortion, which makes straight lines appear to be slightly curved.?The Sigma 17-50mm F2.8 EX DC OS HSM , available for Pentax cameras, does a slightly better job with distortion?it shows 2.7 percent at its widest setting. That lens is also a bit soft at f/2.8, but quite sharp when set to f/4.

As you zoom in with this lens, sharpness starts to decrease. At 35mm f/2.8 it only manages 1,418 lines and it also suffers at 50mm, notching only 1,660 lines there. Stopping down to f/4 improves image quality?at that aperture it records 2,285 and 2,259 lines respectively. The Sony 16-50mm lens does better, as it stays above 1,800 lines at f/2.8 throughout its zoom range.

If you're a Pentax shooter in need of a zoom lens that delivers better image quality than the standard kit lens, you'll undoubtedly be drawn to the Pentax SMC DA Star 16-50mm F2.8 ED AL (IF) SDM. It's a bit soft at f/2.8, but it does offer weather sealing. If you don't need as much speed, consider the SMC DA 18-135mm F/3.5-5.6 ED AL (IF) DC WR ?lens. It's significantly less expensive, has a longer zoom factor, is sharp throughout its focal range, and also offers weather sealing. If shooting in inclement weather isn't a requirement, the SMC Pentax DA 17-70mm f/4 AL (IF) SDM lens is an option?it's only a stop slower and is reasonably priced at $600.?If you're willing to go with a third-party lens, the Sigma 17-50mm F2.8 EX DC OS HSM shouldn't be discounted?it's less expensive and performs slightly better in terms of sharpness at f/2.8.

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