Lens Reviews / Pentax Lenses i Lab tested

17-70mm	$497 average price

SLRgear Review
August 17, 2009
by Andrew Alexander

The Pentax 17-70mm ƒ/4 SDM was introduced in the summer of 2008, one of the first new Pentax lenses that uses the KAF3 lens mount. This mount does not support a mechanical focusing screw of previous designs, meaning the camera must support the SDM focusing standard to focus the lens.

The lens features a constant ƒ/4 aperture, and on a Pentax digital camera body, will produce an equivalent field of view of 26-105mm. The lens takes 67mm filters and ships with a petal-shaped lens hood for an approximate price of $600.

Sharpness
The Pentax 17-70mm produces very sharp results, even when used wide open at ƒ/4. The best results for sharpness were obtained at 50mm and ƒ/8, where image sharpness hovers just over one blur unit, or what we like to call ''tack-sharp.''

Like most lenses built to a lower price point, optimal performance is obtained in the middle of the zoom range, but the lens is no slouch at wide angle (17mm) or full telephoto (70mm). When used wide open (ƒ/4) at wide angle (17mm) the lens shows very sharp results in the center, just under 1.5 blur units, and corners that are just slightly soft at just under 2 blur units. The results are the ''worst'' you'll see with this lens, as stopping down or zooming out offers marginal improvement in image sharpness. Wide-open performance is at its best at the 35mm mark, with almost edge-to-edge sharpness results of just under 1.5 blur units (technically, the corners are slightly softer than that). Our copy of the lens exhibited some de-centering at 70mm, with the major area of sharpness in the upper left hand corner; however, image sharpness still didn't go above 2 blur units in this case.

Stopped down, the lens offers improved performance, but at this point the improvements are marginal and in real-world usage, for sharpness, your results at ƒ/4 will look a lot like your results at ƒ/5.6 or ƒ/8. Diffraction limiting sets in above ƒ/11, but even at ƒ/16, image sharpness is still below 2 blur units, corner to corner. Fully stopped-down at ƒ/22, image quality declines, which for this lens, means it actually surpasses the 2 blur unit mark. At 70mm and ƒ/22, we note some uneven sharpness and around 3 blur units.

In short, very good performance for sharpness, to the point that any degradation in image quality will most likely be a result of the presence of chromatic aberration or corner shading.

Chromatic Aberration

Upper left corner, 17mm, ƒ/4.

The Pentax 17-70mm ƒ/4 has higher-than-we'd-like results on our chromatic aberration testing, particularly in the wide angle setting (anything less than 35mm). As with image sharpness, we note CA in areas outside of the middle of the zoom range: at 35-50mm, CA is actually quite low. But at the widest angle our sample photos tell the whole story, with purple fringing evident in areas of high contrast; as is typical for wide angle lenses, it's mostly in the corners, but average CA performance is fairly high as well. Stopping down doesn't really alleviate the problem. At the telephoto end (70mm) CA is quite low when used wide open, but stopping down increases CA dramatically in the corners. Anything above ƒ/8 at 70mm will become noticeable.

Shading (''Vignetting'')
Corner shading isn't much of an issue with this lens, with the only noteworthy results being at 17mm, with the aperture set wider than ƒ/8. Wide open at ƒ/4, the corners are just over a half-stop darker than the center; at ƒ/5.6, this light falloff decreases to just 1/3 of a stop. At any other setting, light falloff is at the quarter-stop mark or lower, or effectively negligible.

Distortion
Results for distortion testing were typical for this type of lens: strong barrel distortion at the wider angles (less than 25mm), approaching +0.1% barrel distortion in the corners. There is no distortion-free focal length; you will always have some level of barrel or pincushion distortion in images produced by this lens, but happily, the numbers aren't overly high. Above 25mm, distortion quickly devolves into a mix of pincushion distortion in the corners, and barrel distortion throughout the rest of the image. This level of distortion reaches its most objectionable state by 33mm, where we note -0.4% pincushion distortion in the corners, but just +0.1% barrel distortion on average through the rest of the image.

Autofocus Operation
The Pentax 17-70mm ƒ/4 uses Pentax's SDM (supersonic drive motor) focusing technology, with autofocus being driven by a motor within the lens, rather than a mechanical focusing screw which has been (until recently) the standard method for Pentax. Pentax has been moving in this direction for the last couple of years, bridging the gap with the KAF2 standard, which offered both lens-driven and body-driven autofocus techniques to support older and new camera bodies. With the KAF3 standard, your camera must support SDM lenses, or you'll be limited to manual focus only.

In terms of performance, the 17-70mm is positively snappy, with fast and near-silent performance. Autofocus is achieved in just under a second, no doubt aided by a very short focus throw. With the Pentax quick-shift system, autofocus results can be overridden at any time by just turning the focus ring.

Macro
With a minimum close-focusing distance of 28cm (just under a foot) and a magnification rating of 0.31x, the Pentax 17-70mm ƒ/4 offers respectable results for macro.

Build Quality and Handling
The Pentax 17-70mm ƒ/4 is a meaty lens, weighing in at just over a pound (17.1 oz, 485 grams) but balancing nicely on both K10D and K-7 test bodies. Being a DA lens, it isn't outfitted with refined touches such as weather sealing, but the quality level is quite high (for example, it uses 7 rounded blades to make up the aperture). The lens uses a fair amount of plastic construction to keep the weight down, but key areas such as the lens mount are metal. The optical formula is quite complicated, 17 elements in 12 groups, but I can't help thinking that the addition of a couple of ED elements might help to reduce chromatic aberration at wide angle and telephoto.

The lens has no control surfaces other than the zoom and focus rings. In the KAF3 standard, auto / manual focus operation is controlled by the camera, and autofocus results can be overridden via the quick-shift system. The lens has a distance scale on the focus ring, with measurements in feet and meters, but there is no depth-of-field scale. The lens takes 67mm filters, and polarizer users will be happy to note the front element does not rotate. However, the instruction manual indicates that using a filter will introduce some additional corner shading at 17mm.

The zoom ring gets all the attention with this lens. At 1 1/2 inches wide, there's plenty to grab, and the rubber ring uses a series of deep ribs for texture. The resistance on the ring is just right, turning smoothly through its range of 45 degrees, and there's no evidence of zoom creep. The lens extends as it is zoomed out, adding a further 1 3/4 inches to its length at 70mm.

The focusing ring leaves a lot to be desired, if you have any need to focus manually. The ring is just a quarter-inch wide, with very small ribs. The short focus throw (around 25 degrees) provides fast autofocusing results, and accuracy is quite good, but this short throw means that to get any kind of manual focus accuracy you will be nudging the ring in tiny, tiny movements. The ring also moves very easily, meaning the slightest movement can disrupt even autofocus results. The focus range ends in hard stops on the close-focus and infinity sides, and the lens can focus past infinity.

The PH-RBM petal-shaped lens hood is the standard Pentax affair - reverses and mounts for easy storage and does well to reduce lens flare. Pentax makes it easy for polarizer use by providing a removable window in the lens hood, so you can turn the polarizer from outside the hood. The interior of the hood is a smooth matte black finish.

Alternatives

Pentax 18-55mm ƒ/3.5-5.6 SMC P-DA ~$170
The odds are Pentax users already have this lens, as it came as the kit lens with pretty much every digital Pentax body. The 17-70mm makes a worthy upgrade to this lens, offering improved sharpness at wide angle and a constant ƒ/4 aperture. Results for chromatic aberration are about the same, perhaps slightly better with the 17-70mm, and light falloff is also reduced. Distortion is perhaps better on the 18-55mm as there is a point of parity at 35mm where there is virtually no distortion.

Pentax 16-50mm ƒ/2.8 ED AL IF SDM SMC DA* ~$750
We haven't yet tested this lens, but for a just a few more dollars you get a constant ƒ/2.8 aperture at the cost of reduced telephoto distance, as well as the refined features of the DA* lens designation.

Sigma 17-70mm ƒ/2.8-4.5 DC Macro ~$390
The Sigma makes a worthy competitor, offering a slightly wider aperture of ƒ/2.8 at the wide-angle setting. Image sharpness is quite similar to the Pentax, though at ƒ/2.8 it's slightly softer (of course, the Pentax doesn't offer the ƒ/2.8 aperture). Chromatic aberration results are much more tightly controlled thanks to the SLD element and distortion is slightly better. Corner shading is a bit more prominent on the Sigma. This Sigma doesn't feature HSM focusing, so it'll be slower and noisier than the Pentax. The Sigma is much less expensive, and offers significant macro performance (0.43x).

Tamron 17-50mm ƒ/2.8 Di II LD Aspherical IF SP AF ~$
A very popular Tamron model, the 17-50mm offers a constant ƒ/2.8 aperture in a light and compact design. At the cost of some telephoto range, the Tamron is sharper, and offers improved resistance to chromatic aberration. Distortion results are similar and light falloff is minimal. However, autofocusing is going to be slower and noisier than the Pentax, with the lens relying on a body-driven screw motor.

Conclusion
Optically, we can't find a whole lot to complain about with the Pentax 17-70mm ƒ/4. It's decently sharp even wide open; light falloff and distortion aren't much to be concerned about. Outside of the middle of the zoom range, the numbers for chromatic aberration are a little high for our taste: I'd assume that in order to keep costs down, Pentax opted not to include ED glass elements. However, the cost is already at the $600 level, which makes it sort of a loss-leader to the Pentax 16-50mm ƒ/2.8, which can be had for as little as $700, and offers pro-level performance at the cost of a little telephoto distance.

In the meantime, the Sigma and Tamron alternatives are strong contenders, offering similar (if not better, in the case of the Tamron) performance for significantly less money. But for Pentax purists who want to stay loyal to the brand, and want a better-performing kit lens than the 18-55mm, the 17-70mm ƒ/4 becomes a worthy upgrade.

Sample Photos

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.

Still Life shot

VFA target

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.