Pentax 20-40mm f/2.8-4 ED Limited DC WR HD DA

Lens Reviews / Pentax Lenses i Lab tested
20-40mm $748
average price
image of Pentax 20-40mm f/2.8-4 ED Limited DC WR HD DA

SLRgear Review
May 8th, 2014
by William Brawley

The Pentax 20-40mm ƒ/2.8-4 ED Limited DC WR HD DA is another entry in Pentax's growing "Limited" series of lenses, and while some of these are merely minor upgrades with their new "HD" coatings, the 20-40mm is an altogether new model -- the first "Limited" series zoom lens.

Despite the relatively retro-esque, all-metal exterior design, there are a few more modern touches to this lens than with the other Limited series glass. The Pentax 20-40 is weather resistant with seals throughout the barrel and a gasket on the mount. It also has a much quieter DC autofocusing motor, and while the aperture is variable throughout the zoom, it's fairly bright at ƒ/2.8-4 rather than the more typical ƒ/3.5-5.6 variety.

Like other HD Limited lenses, the Pentax 20-40mm ƒ/2.8-4 ED Limited DC WR HD DA is available in either black or silver. The lens ships with a push-on metal lens hood and front and rear caps for around $996.95.

Overall, the Pentax 20-40mm is a sharp lens, particularly when stopped down a bit. Wide open at 20mm, there's noticeable corner softness, but a nice, sharp center. Stopping down a bit between ƒ/5.6-ƒ/11 shows excellent sharpness with very flat blur characteristics.

Zoomed out to 24mm seems to be the "weak spot" for this lens in terms of sharpness, as it shows more softness at ƒ/3.5 -- the widest aperture at this focal length -- and the greatest diffraction limiting softness at ƒ/29. This is a bit odd because once you're at 30mm, the lens displays much more impressive sharpness, which is similar to, if not slightly better than, 20mm. There's still some corner softness when shooting wide-open, but that's easily remedied by stopping down to ƒ/5.6. At 30mm, the Pentax lens also does well at handling diffraction limiting softness; allowing for sharp images all the way down to ƒ/16.

At the longer focal lengths of 34mm and 40mm, the lens displays similarly sharp images with ever lessening corner softness at their respective wide-open apertures. Like the other focal lengths, stopping down a bit shows a marked increase in sharpness, while diffraction limiting softness does appear at the very narrow apertures, with 40mm at ƒ/32 showing similar diffraction limiting softness characteristics as we see at 24mm ƒ/29.

Chromatic Aberration
The Pentax 20-40mm lens does display some chromatic aberration on average throughout the frame, although it mostly appears as purple and green fringing in the corners when shooting at the wider focal lengths and wide-open. The amount of CA remains fairly constant throughout the aperture range with only minor improvements when stopping down at the wider focal lengths. The level of CA is not overly severe, however, and is something that minor adjustment photo editing software could easily remove.

Shading (''Vignetting'')
As typical with wide-angle focal lengths and wide apertures, the Pentax 20-40mm lens displays noticeable vignetting at 20mm at ƒ/2.8, however, it's fairly minor at just shy of 0.5EVs of corner shading. Vignetting at 20mm and 24mm quickly falls to around 0.25EVs of light falloff at ƒ/4 and decreases further to about half that by ƒ/5.6 and remains around that level throughout the rest of the aperture range.

The longer focal lengths show vignetting as well with 30-40mm all showing between 0.25-0.5EVs of corner shading at ƒ/4. In fact, 40mm shows practically the same amount of vignetting at ƒ/4 as 20mm ƒ/2.8: 0.5EV. These three longer focal lengths, particularly 34mm and 40mm, display a much more gradual reduction in vignetting as you stop down compared to the 20mm and 24mm. For example, 40mm at ƒ/16 shows stronger corner shading than 20mm did at ƒ/5.6, though it's not a severe amount of vignetting.

Like many wide-angle zoom lenses, the Pentax 20-40mm shows some amount of distortion that's more prominent on the wide end, however, like vignetting, barrel distortion is quite minor. At 20mm, the average distortion is just shy of +0.5% (with maximum distortion levels out in the corners a little higher, but under +1%). The barrel distortion smoothly tapers out and reduces as you zoom in to the longer focal lengths.

Focusing Operation
Unlike some of the other Pentax HD Limited lenses, the 20-40mm lens uses a DC (direct current) autofocus motor for smooth and quiet focusing, as opposed to the noisier screw-drive system on the other Limited lenses. Autofocus is indeed smoother and quieter than these other lenses, but it's not an internally focusing lens, so the focus ring rotates and the front end extends a bit during operation. It's also not particularly fast to focus, taking a little over one second to go from closest focus to infinity. It's not shockingly or frustratingly slow, but certainly not as fast as ultrasonic AF systems.

Like the other Limited series lenses, the 20-40mm has the Pentax's Quick-Shift Focus System for instant manual focus override of the AF system. Full-time manual focusing is controlled via the camera body, and the focus ring itself is very smooth to rotate with built-in focus distance markings and a depth-of-field scale.

The Pentax 20-40mm has a close focusing distance of 0.9 ft (28cm) with a maximum magnification of 0.2x (1:5 ratio), which doesn't make it a great lens for traditional macro photography, though it can focus relatively close to the subject.

Build Quality and Handling
Like the other HD Limited lenses, the build quality on the Pentax 20-40 is excellent with an all-metal construction -- complete with engraved lettering and markings for the zoom, focus distance, depth of field scale and other branding. The styling of the zoom ring is a bit different from those on more modern-styled lenses with their rubberized bands and grips. Instead, the Pentax 20-40mm lens has gear-like knurled grooves for a nice mechanical feel while still being very easy to grip. The zooming action is very smooth but with enough resistant to avoid any creeping. Similarly, the focus ring has a nice, mechanical feel with a smooth rotation and hard stops in either end of the focus range.

The optical layout is comprised of 9 elements in 8 groups, including 1 Extra-low Dispersion and 1 Super Low Dispersion element, and a 9-bladed rounded aperture for nice, out-of-focus backgrounds. As mentioned earlier in the review, the Pentax 20-40mm is more in-line with many other "modern" Pentax lenses, in that it is weather-sealed (unlike the other HD Limited lenses) -- a hallmark feature for many Pentax cameras and lenses. Not only is there a rubbery gasket around the lens mount, there's also a total of seven seals throughout the lens barrel to help keep out the dust, dirt and moisture. It also has the Super Protect (SP) coating on the front element to repel dust, moisture and grease.

While not as small and lightweight as the Pentax 70mm ƒ/2.4 HD Limited was, the 20-40mm lens is still fairly compact for a DSLR lens, and especially a zoom that can open as wide as ƒ/2.8. In terms of a size comparison, it's similar to the Canon 85mm ƒ/1.8 prime or Panasonic 12-35mm ƒ/2.8. Mounted on our Pentax K-5 test camera, the lens felt great and properly balanced. Despite being all metal, it's very lightweight while still feeling very well-built and with a nice, solid "heft" to it. The lens does extend a bit when zooming and focusing, however its extremely minor -- about a quarter of an inch -- and has no effect on balance or the overall size in a practical sense. The front element doesn't rotate, however, so filters like circular polarizers will work with no problem.

In terms of coatings, the big change to Pentax's HD Limited Series lenses are the replacement of the Super Multi Coating for Pentax's new High Definition (HD) coating, which provides better control over ghosting and flares as well as improved color balance. The Pentax 20-40mm lens is a new model for the Limited Series, but as the name indicates, it features the new High Definition coatings. Our news editor and reviewer Mike Tomkins -- an avid Pentax fan -- had a chance to take this lens and the other HD Limited lenses out for a comparison. And while he only had the 21mm and 40mm non-HD versions to compare side-by-side, the results show a noticeable reduction in lens flare as well as increased contrast and saturation from the new HD Limited lenses -- the new High Definition coating definitely does its job.

As this is a new model of lens for the Pentax K-mount, there aren't that many direct alternatives. Perhaps the most similar current lens is the Pentax 18-55mm ƒ/3.5-5.6 AL SMC DA WR. Although it's a similar size and also has weather sealing, the 18-55mm lens provides a more versatile zoom range -- both wider and longer -- but with a dimmer ƒ/3.5-5.6 variable aperture range yet a much more affordable price point of around $200.

As a step-up alternative from the 18-55mm "kit" lens, the Pentax 16-45mm ƒ/4 ED AL SMC P-DA again offers a more versatile zoom range compared to the Pentax 20-40mm lens, as well as a constant ƒ/4 aperture. And while this lens is also more affordable at around $600, the constant ƒ/4 aperture makes the lens larger and heavier than the 20-40mm. It also lacks weather sealing.

The last alternative is the Pentax 16-50mm ƒ/2.8 ED AL IF SDM SMC DA*, which is one of Pentax's high-end lenses. Again, with a more versatile zoom range than the 20-40mm lens, the Pentax 16-50mm ƒ/2.8 provides a bright, constant ƒ/2.8 aperture, weather sealing and faster, quieter SMC focusing. This lens, however, is much larger, heavier and a good bit more expensive at around $1,300.

All in all, the Pentax 20-40mm ƒ/2.8-4 ED Limited DC WR HD DA is a solid lens and a proud addition to the HD Limited lineup. The lens displays great sharpness, especially in the center of the frame or, better yet, when you stop down a bit. Chromatic aberration, distortion and vignetting are all present to varying degrees depending on focal length and aperture, but it's all very minor and something your average photo editing software should be able to clear up.

The mechanics all work well and feel great on this lens -- solid build quality with smooth zoom and focus rings. The AF system is much quieter than the other screw-drive based Limited-series lenses, but the DC AF motor on the 20-40mm lens did feel a little slow.

The biggest downside to the Pentax 20-40mm lens is the limited focal length range (and price). At 20mm, it's not very wide, and at 40mm, it's not very telephoto, especially considering there are less expensive, weather resistant lenses like the 18-5mm WR lens. Even more so, at around $1000 for the 20-40mm lens, Pentax shooters could buy the 18-55MM ƒ/3.5-5.6 AL WR, 50-200MM ƒ/4-5.6 ED WR and 35MM ƒ/ 2.4AL for a much more versatile setup and still have money left over.

On the other hand, the 20-40mm lens has a brighter aperture range than the typical kit zoom, all-metal construction, stylish retro-ish design, weather sealing, and Pentax's new HD coatings that reduce glare and increase contrast all in a compact and lightweight package.

Product Photos

Sample Photos

Check out some sample photos shot by our senior lens technician Rob Murray. You can view more sample photos, plus download the full-resolution files, over at our Flickr page.

The VFA target should give you a good idea of sharpness in the center and corners, as well as some idea of the extent of barrel or pincushion distortion and chromatic aberration, while the Still Life subject may help in judging contrast and color. We shoot both images using the default JPEG settings and manual white balance of our test bodies, so the images should be quite consistent from lens to lens.

As appropriate, we shoot these with both full-frame and sub-frame bodies, at a range of focal lengths, and at both maximum aperture and ƒ/8. For the ''VFA'' target (the viewfinder accuracy target from Imaging Resource), we also provide sample crops from the center and upper-left corner of each shot, so you can quickly get a sense of relative sharpness, without having to download and inspect the full-res images. To avoid space limitations with the layout of our review pages, indexes to the test shots launch in separate windows.

Pentax 20-40mm f/2.8-4 ED Limited DC WR HD DA User Reviews

9.0/10 average of 1 reviews Build Quality 10.0/10 Image Quality 9.0/10
  • 9 out of 10 points and recommended by (7 reviews)
    sharp, compact, weather-sealed, excellent build quality
    first copy was decentered, required exchange

    My first copy of this lens was very decentered but BH made a quick, no-fuss RMA exchange. The second copy was quite good, including at the 24-30mm range that others, including SLRGear, found wanting. I suspect that this is a quality control and assembly problem rather than a design deficiency. My first copy was terrible at focal lengths SLRGear's review showed to be good and good at the 24mm setting that was deficient here. My sense is that this design is capable of producing sharp images at all focal lengths, when stopped down to f/5.6 to f/7.1 but that misassembly can cause problems at a variety of focal lengths. Now that I have a good copy, I do like the lens and appreciate its compact, light design compared to the Sigma 18-35mm, my other possible option. The weather-sealing is appropriate for a K-3 or K-5.

    reviewed May 10th, 2014 (purchased for $997)