The Pentax HD DA 20-40mm F2.8-4 ED Limited DC WR ($999.95 direct) is the first zoom in the company’s Limited lens series, and also the first lens in that series to boast a weather-sealed design. It’s got an impressive all-metal build quality, and is quite compact for a zoom, but it’s no match for our Editors’ Choice Sigma 18-35mm F1.8 DC HSM, a larger zoom that captures an impressive amount of light throughout its zoom range. The 18-35mm hasn’t been released for Pentax cameras as of yet, but it is coming and available for preorder.
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The 20-40mm is not that far off in size from a typical 18-55mm zoom lens, but’s clear that the build quality is much higher. It measures 2.7 by 2.8 inches (HD), is compatible with 55mm threaded filters, and weighs about 10 ounces. The barrel is metal, and there’s a knurled zoom ring and a manual focus ring. It does support the Pentax Quick Shift focus system, which allows you to override autofocus lock with a quick turn of the manual focus ring—without having to change focus settings on the camera. There’s an o-ring seal around the lens mount, which provides full weather sealing when paired with a sealed body body like the K-50.
I used Imatest to check the sharpness of the lens when paired with the company’s top-end K-3 D-SLR. It’s an impressive performer, bettering the 1,800 lines per picture height that we require of a sharp image at every tested focal length and aperture. At 20mm f/2.8 it manages 1,861 lines using a center-weighted test, but there’s a bit of softness towards the edges of the frame. Stopping down to f/4 improves things all around; the average score is 1,959 lines there. It improves to 2.068 lines at f/5.6 and peaks at 2,248 lines at f/8. Distortion is an issue here; the lens shows 2.3 percent barrel distortion, which makes straight lines appear to curve outward. The K-3 has in-camera distortion control for JPG shooting, but if your camera doesn’t have that feature, or if you shoot in Raw, you’ll need to correct the distortion in an editing program such as Lightroom.
Distortion goes away quickly as you zoom; it’s gone by 30mm. There the maximum aperture narrows to f/3.5 and the lens notches an impressive 2,159 lines. Stopping down to f/5.6 only delivers marginal improvement, and again the lens peaks at f/8 with 2,315 lines. At 40mm f/4 the lens records 2,164 lines, a score that improve marginally until it hits 2,391 lines at f/8. Sigma’s 18-35mm lens shows similar distortion at its widest angle, but when we tested it on a Canon APS-C body its scores were more impressive. On the T3i it managed 2,383 lines at its widest angle and aperture, 2,470 lines at 24mm, and 2,399 lines at 35mm.
The Pentax HD DA 20-40mm F2.8-4 ED Limited DC WR is impressive from an optical quality, especially when you consider its size and weather-sealed design, but its price is a bit high when you consider its limited zoom range and unambitious aperture. Our Editors’ Choice Sigma 18-35mm F1.8 DC HSM is still the best wide-to-standard angle zoom that you can get for an APS-C camera. It’s bigger, but it’s less expensive and is sharp enough that it can replace a couple of prime lenses in your bag. The only complaint we had when we reviewed the Sigma was that it wasn’t optically stabilized. Pentax shooters won’t have to worry about that, as the company builds shake reduction into its cameras. The lens is coming soon for the Pentax system, and if weather-sealing isn’t a must, is the one to get.
Article source: http://www.techradar.com/news/photography-video-capture/cameras/nikon-d5200-10-things-you-need-to-know-1110221
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