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Pentax DA 17-70mm f/4 SDM Lens Review
by David Rasnake -  9/4/2008

For several years now, Pentax's DA 16-45mm f/4 constant-aperture zoom has been one of the workhorse lenses of the manufacturer's stable. Although newer, faster glass has topped the 16-45mm in Pentax's lineup, the lens has remained a favorite choice for the budget-friendly performance it offers up.

Pentax DA 17-70mm f/4 SDM

When word of a similar lens with superior specs, the Pentax DA 17-70mm f/4 SDM, hit the streets earlier this year, the contingent of Pentax shooters in the DCR office was eager to reel in a review sample. With more range and a Supersonic Drive Motor (or SDM) lens-based focusing system, would the new 17-70mm f/4 be all that the 16-45mm f/4 is and more? There's a simple way to find out – take it out for lots of shooting – and after more than a month with the new lens, we're feeling pretty settled around here on where the new glass shines, and where it's still in need of more polish.


ERGONOMICS

Lens Mount
Notably, the 17-70mm is the first lens we've looked at to use Pentax's KAF3 lens mount, which supersedes the current KAF2 in SDM focusing applications. The KAF3 variant still uses what shooters who've been with Pentax for awhile know as the "power zoom" contacts to drive the lens's internal SDM focusing system, just like with the KAF2 mount.

Pentax DA 17-70mm f/4 SDM

The primary difference, however, involves the elimination of a back-up mechanical coupling between lens and focusing system. On Pentax's KAF2 SDM lenses, the lens made auto focus allowances for camera bodies older than the K10D/K100D series – which don't support lens-based focusing motors – to auto focus using the camera body's focusing motor and a traditional mechanical coupling instead. For the KAF3 lenses, this coupling is gone, making the latest optics the likely first step in an "SDM only" direction.

While we like the idea of transitioning to lens-based focusing with supersonic motors all around (we're certainly not going to complain about faster and quieter...), Pentax shooters who use older *ist cameras as primary or backup equipment may be disappointed that the KAF3 mount relegates them to manual focus only on new lenses. On the flipside, it's hard to complain too much about Pentax's lens mount continuity, given that they've been more accommodating than many of their competitors in this regard.

Design and Build Quality
In terms of operational smoothness and overall quality, we were impressed to see that Pentax brought the 17-70mm to market with an attention to detail in construction normally reserved heretofore for the pro-grade DA* glass. Unlike most DA* units, the lowly DA is built in an all composite shell, but tight parts fit, overall heft, and a buttery zoom motion make the 17-70mm feel like a significant step up from typical consumer grade lenses.

Pentax DA 17-70mm f/4 SDM

With a 67mm-mounting front element that's a little narrower than the industry standard 72mm threads found on many pro lenses, the 17-70mm cuts a narrower profile than many constant-aperture optics, with very little barrel flare from mount to front.

Pentax DA 17-70mm f/4 SDM

A large, rubberized zoom ring dominates the center of the lens barrel, with a small forward-mounted focusing ring providing MF adjustment. Switching from auto to manual focus is accomplished via either the dedicated switch on the camera body, or Pentax's full-time manual focus override; there is no on-body AF/MF switch.

Pentax DA 17-70mm f/4 SDM

As expected for a lens in this class, the front element is stationary during focusing, making circular polarizers and the like straightforward to use.

Pentax DA 17-70mm f/4 SDM

To this end, Pentax also provides an amply large bayonet-lock lens hood with the 17-70mm, complete with the manufacturer's usual polarizer window that allows finger access to filters with the hood mounted.

Handling
With its more svelte shape, the 17-70mm falls nicely between a lightweight kit lens and f/2.8 pro glass in terms of physical handling characteristics. Persnickety users may find the bias a bit front heavy on even the larger K bodies, but the addition of a battery grip makes the heft sit back a little more and even without it the 17-70mm is certainly not unmanageable with one hand.

Pentax DA 17-70mm f/4 SDM

Even set all the way in at 17mm, the lens protrudes fairly far (measuring 3.7 inches from mount to front). Fully extending the lens to its 70mm position nearly doubles this length, making the optic a little more difficult to manage physically than a kit zoom, and physically longer than many competitive constant-aperture lenses when working at full telephoto.

Pentax DA 17-70mm f/4 SDM

Zoom and focus rings are a mixed bag. The dominant zoom control is nicely textured and strikes a good balance between resistance and smooth motion. Zoom creep is not an issue, even with the camera and lens completely inverted, and hence the lack of a lock is no real oversight.

Pentax DA 17-70mm f/4 SDM

What I'm less thrilled about, though, is the focusing ring: our test unit's focus control was plenty smooth, but with poorly damped, hard stops at both ends. Even less appealing to many will be the fact that the ring requires so little resistance to adjust that it barely holds focus if the camera is shaken, and that total travel from minimum focus to infinity is a paltry 45 degrees. A touch more connectedness here, and a more gradual motion from one end of the range to the other, would both be minor but worthwhile improvements for a Mark II version.

In terms of range, the big revolution compared to the 16-45mm f/4 that the new DA optic supersedes is its additional telephoto working distance. While the practical difference between 45mm and 70mm is negligible, when combined with the crop factor of Pentax's current DSLR bodies, you're effectively working with a 105mm at the long end – an ideal portraiture length. Likewise, the ability to reach out and grab even a step or two of more telephoto length at the end of the range can make all the difference in the world for capturing some shots.

Pentax DA 17-70mm f/4 SDM

At just under a foot, minimum focusing distance with the 17-70mm isn't "macro close," but the lens provides an appropriate measure of flexibility in this regard for most walk-around shooting.


PERFORMANCE

Auto Focus
As noted, the 17-70mm utilizes Pentax's Supersonic Drive Motor auto focus system, which replaces focusing by a body-based focus motor with a smooth, near-silent lens mounted unit. I've spent lots of time with Pentax SDM-equipped lenses in previous reviews, and there's very little new to report here: the system remains fast, smooth, and basically silent in normal use.

We did find that the 17-70mm's AF system wanted to "bounce" at times once it got close to lock, but this seems more a function of our K10D test body's AF system (which behaves similarly on other lenses at times) than any fault of the lens itself. I ended up using the 17-70mm f/4 to do some close-in captures for a last-minute theater performance shooting job, and even in this demandingly dark low light situation, the 17-70mm's performance was generally up to the standards of the faster glass I'm used to for such work.

Manual Focus
Although overall construction, look, and feel of the 17-70mm share a lot in common with Pentax's DA* glass, focusing ring size and placement are squarely DA-series spec: the ring is narrow (about a quarter of an inch), lightly textured, and positioned right at the front of the barrel. While placement is good, as noted previously, I found the ring's eighth-of-a-turn of travel from one stop to the other simply too little for delicate manual focusing work. The ring's overall feel, which is light and bit jittery, furthers the idea that this lens is optimized for auto focus – probably a non-issue, unless you're considering it for use on an older Pentax body where MF will be your only option.

Through-the-viewfinder brightness is, as expected, noticeably darker than an f/2.8 constant aperture lens, but the 17-70mm's sharp optics give a nice "pop" when bringing a subject into focus manually. Still, if low light shooting is a big part of your photographic routine, the combination of slow-ish maximum aperture and the resulting TTL dimness would probably be enough to make me consider the DA* 16-50mm f/2.8 or one of the third-party f/2.8 constant-aperture zooms instead.

Image Quality
Good news first: Pentax has produced yet another first-rate lens where both contrast and color reproduction are concerned. I really couldn't be much happier with the amount of detail separation this lens is capable of producing; to get at its full potential when stopped down, you'll almost certainly have to shoot raw – a good sign.

Pentax DA 17-70mm f/4 SDM

In addition to silky highlight and shadow transitions, the 17-70mm has a warm-of-neutral color reproduction that brings life and vibrancy into outdoor shooting, especially.

Pentax DA 17-70mm f/4 SDM

Likewise, although the lens doesn't allow for extreme up-close shooting, I found the 17-70mm's bokeh particularly well suited to providing a smooth background for close-in subjects.

Pentax DA 17-70mm f/4 SDM

All of this makes the Pentax 17-70mm a joy to use in making images. Whatever shooting or lighting situation you throw at it, the lens responses with plenty of vividness and a crisp, defined look – so long as you don't open the lens up too much or look to close, that is.

Pentax DA 17-70mm f/4

Which brings us to the not so good with this lens. Wide-aperture shooting is often a cause for concern with zooms, especially, and the 17-70mm is unquestionably compromised in this regard. Center and edge crops of our standard "page of text" test target show a slightly underwhelming, but not entirely unacceptable, performance at 17mm.

Pentax DA 17-70mm f/4 SDM
17mm f/4, 100% crop (edge)
Pentax DA 17-70mm f/4 SDM
17mm f/4, 100% crop (center)
Pentax DA 17-70mm f/4 SDM
17mm f/8, 100% crop (edge)
Pentax DA 17-70mm f/4 SDM
17mm f/8, 100% crop (center)
Pentax DA 17-70mm f/4 SDM
17mm f/16, 100% crop (edge)
Pentax DA 17-70mm f/4 SDM
17mm f/16, 100% crop (center)

What's more troubling, however, is not the degree of softness that we experienced across the board in our lab tests at f/4, but rather, the extent to which this softness persists at narrower apertures – especially at full telephoto.

Pentax DA 17-70mm f/4 SDM
70mm f/4, 100% crop (edge)
Pentax DA 17-70mm f/4 SDM
70mm f/4, 100% crop (center)
Pentax DA 17-70mm f/4 SDM
70mm f/8, 100% crop (edge)
Pentax DA 17-70mm f/4 SDM
70mm f/8, 100% crop (center)
Pentax DA 17-70mm f/4 SDM
70mm f/16, 100% crop (edge)
Pentax DA 17-70mm f/4 SDM
70mm f/16, 100% crop (center)

Standing by themselves, these results suggest a concerning sharpness drop-off at full telephoto below f/11. Following our usual procedure, we took several groups of these shots, checking every conceivable setting and visually confirming auto focus hits throughout the range only to yield the same result over an over again.

While the lab tests may overstate the impact for real-world shooting, the fact is that the 17-70mm f/4 – or at least, the one we reviewed – was soft across the board at f/4. The issue is relatively well masked (or even so slight as to be covered by aggressive sharpening) in compositions of objects at varying distances where the foreground and background are pulled intentionally out of focus by the narrow depth of field of a wide aperture. Against a uniform texture at f/4, however, it's hard to ignore the softness.

Pentax DA 17-70mm f/4

The 16-45mm f/4 was also plagued by some chromatic aberrations (in the form of a bright blue fringe in this case) when shooting wide open. While Pentax has made what looked to the trained Pentax shooting eyes around here to be some improvements in controlling CA at wide angle and wide apertures, the right combination of conditions and settings can cause a flare up that's hard to correct, even with a raw file to work from.

Pentax DA 17-70mm f/4 SDM

Even in this case, the issue isn't shot ruining, and as noted some definite improvements have been made (we rarely experienced the issue with any severity when stopped down at all on this lens, for instance). That said, there's still some disappointment over the fact that performance wasn't cleaner across the board here.

Finally, we can't let Pentax off the hook without a bit of chiding for the amount of corner darkening at wide angle (it's a solid 1.5 EV at 17mm and f/4), as well as the pronounced barrel distortion and corner softness at the wide end of the lens.

Pentax DA 17-70mm f/4 SDM
Wide-Angle

Pentax DA 17-70mm f/4 SDM
Telephoto

Though it dissipates quickly, full wide-angle shots have an odd, barely rectilinear (corner sharpness and brightness fall-off don't help here) about them that can be difficult to deal with depending on the subject.


CONCLUSIONS
Testing a lens always comes with some variability: optics are precise instruments, and even among copies from a top manufacturer like Pentax there will be better and worse performing samples. Beyond the technical, even, how much a particular lens's look appeals to you – and how much its flaws limit you – will depend heavily on what your needs and preferences are.

With excellent contrast, nice color handling, and improved CA control, the 17-70mm is capable of superior shots, especially in the context of landscape shooting. Several days of intense outdoor shooting with the 17-70mm, and the very pleasing shots that resulted, are a testament to just how good this optic can be. Compromised sharpness throughout at wider apertures will be a sticking point for some, which is a shame really: for certain kinds of shooting, the 17-70mm works quite well.

Given just how good Pentax's current-generation 18-55mm kit lens is, however, a bigger concern may be whether the 17-70mm's $600 price tag makes it a worthwhile upgrade. The more expensive lens buys you a little bit of speed at the telephoto end, and a little bit of range; when sharpness is firing on all cylinders at narrower apertures, there's simply no comparison between the two. The concern, though, is that for lots of general, walk-around shooting (which implies, at least, the lack of a tripod, and thus makes narrower apertures a challenge depending on light), it may be hard to realize the benefits of the more expensive glass.

The 17-70mm should appeal to architectural and nature photographers without question, and might even serve well as an urban shooting or general purpose lens, but even with all of the progress that Pentax has made in this series to improve build quality and correct wide-open aberrations, the adherence to a slow-at-the-wide-end f/4 constant aperture and nagging questions about sharpness leave us feeling a bit tossed up about the 17-70mm at the end of the day.

Pros:

Cons: