Pentax HD DA 20-40mm ED F2.8-4 ED LIMITED DC WR (799.95 USD) is the new Limited zoom. It also features a weather sealed design. Although it has a modest zoom range of just 2x it is compact. The all-metal construction gives it an elegant look. It's a great choice for an APS–C Pentax system if you appreciate its small size and need an all-weather zoom.
If you are willing to spend more, then the Sigma 17-70mmF2.8-4 DC Macro OS MSM provides greater coverage and is cheaper. You can also get a shorter zoom for the same price, the Editor's Choice Sigma 18-35mmF1.8 DC HSM. This lens maintains bright apertures throughout the range but is heavier and larger than the 20-40mm.
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Design
Although the 20-40mm lens is similar in size to an 18-55mm zoom lens it is clearly larger. The lens measures approximately 2.7x2.8 inches (HD), can be used with 55mm threaded filter, and is about 10 ounces in weight. It is made of metal and has a knurled manual focus ring as well as a zoom ring.
It supports 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. The lens mount has an o-ring seal that provides weather sealing when used with the K-50 sealed body.
The 20-40mm is available in either black or silver from Pentax, with an accent ring of red. The compact hood and slip-on cap are included. The coverage is restricted to APS-C cameras. Full-frame photographers can consider the zoom as covering a 30--60mm field.
The change in aperture is fairly linear—at 30mm you can shoot at f/3.5. The barrel is slightly curved when zooming, and it becomes shorter at 30mm. There's no opitcal stabilization, but that's a nonissue for Pentax owners as the feature is built into camera bodies—any lens you attach benefits from stabilization.
The focal lengths are indicated at 20-25, 30, 35 and 40mm. A distance scale is located just below the manual focus ring. It measures in meters and feet. A corresponding depth-of-field scale is marked at f/4 to f/8 and f/16 to indicate the appropriate aperture. You can get images as close to 1:15 at 40mm settings with the 20-40mm zoom. Although it is not macro-friendly, the Zoom has some flexibility. Sigma 17-70mm can project images up to 1:13 at 70mm.
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Image Quality
Imatestwas used to test the sharpness and compatibility of the lens with the K-3 DSLR. The lens is a solid performer and achieves more than 1,800 lines per pic height, which we expect from a sharp image at any focal length or aperture. It manages 1,861 lines at 20mm f/2.8 using a center weighted test. However, it has some softening towards the edges.
The average score of 1,959 lines is achieved by lowering the aperture to f/4. The average score is 2,068 lines at F/5.6 and 2,248 at F/8. The lens has a 2.3 percent barrel distortion that makes straight lines look outwardly. You can adjust the distortion using Lightroom or your camera's distortion control feature for JPG shots.
Zooming quickly reduces distortion; the difference is only 30mm. The maximum aperture drops to f/3.5, and the lens measures in at 2,159 lines. The lens' peak at f/8 is 2,315 lines. A drop to f/5.6 does not provide much improvement. The lens registers 2,164 lines at 40mm f/4, which is marginally better than the 2,391 lines recorded at f/8.
Corners are a bit dim at the widest aperture—about 1.8 stops (-1.8EV) behind the center at 20mm f/2.8, -1.3EV at 30mm f/3.5, and -1.5EV at 40mm f/4. This issue can be resolved by reducing the iris to a minimum of one stop. The periphery of the frame will then become within a half stop of the middle in terms of brightness. This issue can be avoided if you shoot JPG. Raw photographers can correct the distortion with one click using a lens profile.
It is not intended to be used with full-frame cameras such as the K-1 ($1,599.00 on Amazon).The K-1's zoom range is limited by the K-1's image circle, which does a good job of covering an APSC sensor. However, it shows dark corners when paired up with the K-1. The K-1 can be set to capture only a small area of your frame.
This matches the APS-C area. When this mode is selected, a black box will be visible in the optical seefinder. This allows you to always know the boundary of your frame. We're not taking any points away from the lens due to the lack of full-frame coverage — it isn't designed for it — but there are other lenses in the DA series that pair with a full-frame sensor with very little cropping.
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Conclusions
For photographers looking for an all-weather zoom, the Pentax HD DA 20-40mm ED Limited DC WR makes a great choice. Even though the zoom range may not be very ambitious, it is compact and light. The lens remains a premium choice, even though it has seen a $200 price drop from the original launch MSRP. Although the Sigma 17-70mm is slightly more expensive, the lens weighs 6 ounces less and does not offer as much weather sealing as the 20-40mm.
However, the Sigma 17 70mm has a wider zoom range than the Sigma 20-40mm and provides stronger macro capabilities. We give the Sigma 17-70mm a slight preference. The coverage of our favorite zoom is quite limited (e.g. the 20-40mm), however, the Sigma 18-35mm DC HSM wins Editors Choice due to its fixed f/1.8 aperture as well as the crisp optics.
The 18-35mm zoom is quite large and weighs in at 1.8 pounds. This is not a good choice for anyone looking for an easy-to-use zoom. Before you make a purchase, compare the Sigma 17-70mm to the Sigma 20-40mm.