HD Pentax-DA 20-40mm F2.8-4 Limited
The 20-40mm Limited zoom from Pentax is difficult to summarize simply because it's such a unique and unusual lens. Its exceptional build quality rivals that of film-era glass, and with the addition of weather sealing, it ventures a step above the existing HD Limited primes. We found the silent DC autofocus to be another much-needed and a very welcome feature, and we're happy with it despite the fact that the lack of internal focusing slows down the overall AF speed. Even though the lens is considerably larger than any of the Limited primes, it's light, remarkably easy to hold, and only slightly bigger than the 18-55mm kit lens, which means that it's still small enough to go just about anywhere. Current users of Limited primes might need a while to get used to the 20-40mm, but it's certainly much more compact than a premium F2.8 zoom.
Even though some might consider the short 2x zoom range to be a bit restrictive, we enjoyed this lens because it offers plenty of "normal" focal lengths in addition to moderate wide-angle capabilities. This makes it a great everyday lens for a variety of artistic photographs, and we're sure that seasoned photographers will agree. On the other hand, the 20-40mm certainly doesn't offer the same degree of versatility as the 18-55mm kit lens or the DA* 16-50mm, so we don't recommend it for beginners looking for an all-in-one lens, or professionals that might need a longer zoom range or wider aperture for their work. Thus, we conclude that the 20-40mm is designed for enthusiasts who have plenty of time when shooting in the field, but who might not always want to carry around a bag of primes.
The formal testing that we performed revealed a number of optical strengths of this lens, as well as a number of weaknesses. In terms of image quality, the thing that impressed us the most was the bokeh, which was perhaps the best that we've seen in a zoom lens with these focal lengths. The optical design of this lens enables it to focus very close and deliver a maximum magnification of 1:5, which is ideal for close-up hand-held snaps. Other strengths of the lens include minimal barrel distortion, which is easily-corrected at 20mm and negligible at other focal lengths, and virtually no chromatic aberrations except at the extremes of the frame. The new HD coating also a good job of controlling ghosting, and the lens delivers accurate, warm colors and enables spot-on metering on the Pentax K-3. The out-of-camera colors and contrast rarely needed any adjustments, based on the samples that we took.
One of the areas in which this lens doesn't shine is sharpness. Even though the center of the frame is looks good at the wider zoom settings, at 40mm you need to stop down to F6.7 or F8 (1.5-2 stops) for optimal performance, as contrast suffers at wider apertures. The corners never really catch up with the center of the frame, but they do become acceptably-sharp at 20mm and 30mm when stopped-down. At 40mm, however, distortion in the corners makes them appear very soft at virtually all aperture settings. The sub-optimal sharpness won't always be a problem unless you pixel-peep, but it is a bit disheartening to see a $999 lens only perform well at such a small number of apertures, especially when you consider the fact that it is already somewhat slow to begin with. This means that we can't recommend this lens for low-light photography, nor can we recommend it as a permanent replacement for a trio or duo of Limited primes. Next up is the vignetting, which worsens as you zoom in and needs correcting either in-camera or in post. The in-camera lens corrections offered by modern Pentax cameras will fix the vignetting almost perfectly in JPEGs, but as they don't affect RAW files, the vignetting will introduce overhead in post. Finally, since the 20-40mm ships with a tiny lens hood, it is especially prone to lens flare. Sacrificing the benefit of a modern petal hood for the more elegant looks of a round hood is a mistake, in our opinion. If you do decide to get this lens, consider purchasing an extendable rubber hood to go along with it.
Overall, we greatly enjoyed using this lens in the field, not only because the zoom range gave us more flexibility when composing photographs, but also because the lens delivered beautiful bokeh, excellent colors and contrast, and versatile close-up capabilities. Many users have also captured and posted some fantastic photographs with this lens on our forum. With that said, we realize that this lens is not for everyone. Although not overpriced, this lens will likely never be an impulse buy. Some will not be able to swallow its optical shortcomings at a $999 price point, while others will prefer a wider zoom range or lenses with a faster apertures. But for advanced photographers who often shoot with primes yet want a bit more convenience in the field and who would value a weather-sealed metal build quality, this lens might make an excellent everyday companion! Lastly, let's not forget that this lens looks and feels great on your camera. Plus, since it's available in both black and silver, it can match both the standard black Pentax DSLR bodies, as well as the special-edition silver ones.
- Excellent color rendering and contrast
- High-quality metal construction
- Elegant design
- Stunning bokeh
- Great for close-ups (1:5)
- Very easy to hold
- Low, easy-to-correct distortion
- Minimal aberrations and ghosting
- Good "normal" zoom range
- Good center sharpness at 20 and 30mm
- Quick-shift full-time manual focus override
- Silent DC autofocus
- Lack of internal focusing slows down AF speed
- Poor corner sharpness, especially at 40mm
- Low contrast wide-open at 40mm
- Sub-par center sharpness at 40mm
- Moderate vignetting if left uncorrected
- Small hood makes it prone to flare
- No filter threads on hood, unusual 55mm diameter
- Expensive ($999)
- Slow aperture makes it a suboptimal replacement for primes
- Considerably larger than comparable primes
- Not as versatile as the DA* 16-50mm
- Not ideal for low-light photography
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