Pentax 18-135mm f/3.5-5.6 ED AL [IF] DC SMC DA WR

SLRgear Review
October 22, 2012
by Andrew Alexander

The Pentax 18-135mm ƒ/3.5-5.6 ED AL [IF] DC WR has been on store shelves since November 2010, sold as a kit lens with the K-5 and K-5 II bodies, but also available separately.

The 18-135mm ƒ/3.5-5.6 was designed to fit the APS-C sized sensor of the K-5 camera body, where it will provide an effective field of view of approximately 28-200mm. This lens isn't a "constant" lens, in that as you increase the focal length, both the maximum and minimum aperture sizes decrease. The following table reflects the changes as you zoom:

The lens takes 62mm filters, ships with a petal-shaped hood, and is available now for around $530.

Sharpness
You could say this lens has a bit of a personality: Sharp results across the frame are possible when the lens is stopped down, but on average you will see some soft corners.

The wider angles show the worst corner performance: when used wide open at 18mm it produces some pretty soft corners, worse at 24mm. However, this is matched with some fairly decent central performance, all the way through to 50mm. As the lens is stopped down the corners improve slightly, but never become tack-sharp; the best results are obtained at 35mm and ƒ/8, where it's almost tack-sharp.

At 100mm and above, this central region of sharpness begins to retreat, and you're stuck with soft corners regardless of the aperture. Diffraction limiting sets in at ƒ/11; fully stopped-down performance is best avoided, as the resulting images are mediocre at the wide angle (at ƒ/22) and distinctly blurry at the long end (at ƒ/40).

Chromatic Aberration
Below 100mm, chromatic aberration is tolerable - just some light color fringing in areas of contrast, and only in the extreme corners. As the lens is zoomed in to 100mm and 135mm however, chromatic aberration is quite prominent, in fact, some of the most noteworthy we've come to see in these tests. Central performance is still good, but don't expect great detail in the corners.

Shading (''Vignetting'')
Most zoom lenses tend to produce images with darker corners, at least when used at their widest aperture settings, and when zoomed to the wide-angle. The Pentax 18-135mm ƒ/3.5-5.6 WR is no exception, where using the lens at 18mm and ƒ/3.5 produces corners that are 3/4 of a stop darker than the center. Any other setting produces results with corners that aren't as dark, and when set to an aperture of ƒ/8 or smaller, dark corners aren't a factor. Curiously, when set to the 35mm focal length, there is no corner shading, even when used wide open.

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 the 50mm mark, where we note -0.6% pincushion distortion in the corners, but just +0.1% barrel distortion on average through the rest of the image.

Autofocus Operation
Pentax adopts a new focusing motor with the 18-135mm ƒ/3.5-5.6, called the DC ("Direct Current") motor. It purports to be less noisy than previous motors: it produces a light whirring when in use. Autofocus speed is pretty good, taking about one second to go from to close focus to infinity. The lens employs a "quick shift" autofocus system, meaning you can override autofocus results at any time by just turning the focus ring. The lens uses an internal focusing system, which means the front element doesn't turn, and makes life that much easier for polarizing filter users.

Macro
The lens offers 0.24x macro magnification, with a minimum close-focusing distance of 40 cm (about sixteen inches).

Build Quality and Handling
The Pentax 18-135mm ƒ/3.5-5.6 is small but substantial lens, weighing in at just under a pound (14.3 oz, 403 grams) and balancing nicely on our K-5 test body. Pentax's DA lens aren't usually outfitted with weather sealing, but Pentax has made the exception with this lens and introduced another suffix to the lens' designation: WR represents six layers of weather sealing within the lens, making it water resistant.

Despite being a consumer lens it does sport some quality touches, such as using seven rounded blades to make up the aperture and theoretically provide some pleasing out-of-focus background areas. 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, 13 elements in 11 groups, including 1 ED ands 2 aspherical elements.

The lens has no control surfaces other than the zoom and focus rings. On the Pentax K-5, auto / manual focus operation is controlled by the camera, and autofocus results can be overridden via the quick-shift system. The lens has no scales at all - distance, depth-of-field or otherwise.

The zoom ring gets all the attention with this lens. At 1 3/4 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 90 degrees. The lens extends as it is zoomed out, adding a further two inches to its length at 135mm.

Our technician Rob noticed some interesting behavior regarding the lens as it was zoomed in and out. When this lens is mounted on the camera and you zoom it, you can feel the air escaping from the rubber cover that covers the HDMI port on the camera. With the port securely closed, the zoom ring is harder to move. With it open at all, it zooms very easily. It may creep in if you leave that rubber port open, but may not if the port is closed.

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 autofocus system uses soft stops to indicate that you've reached the threshold of focus on either side.

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 17-70mm ƒ/4 AL IF SDM SMC DA ~$600
It may not provide the same telephoto reach of the 18-135mm, but it is a much sharper lens, complete with a consistent ƒ/4 aperture across its focal range. It also has its issues with chromatic aberration.

Pentax 18-270mm ƒ/3.5-6.3 ED SDM SMC DA ~$800
Slated to be released in November 2012, the 18-270mm will be Pentax's answer to the Tamron 18-270mm.

Sigma 18-125mm ƒ/3.8-5.6 DC OS HSM ~$350
We haven't yet tested the Sigma 18-125mm, but it is available in the Pentax mount; as Pentax camera bodies include in-body image stabilization, you would want to ensure that the OS stabilization in the Sigma lens is turned off.

Conclusion
As economical consumer lenses go, the 18-135mm ƒ/3.5-5.6 WR performs fairly well: it has some problems in the corners of the images it produces, namely, they're soft in the corners at certain apertures, and have high chromatic aberration as well. I can't help thinking that a directive came down that the Pentax kit lens needed to have more reach, and cost less, so to get there, some corners had to get cut. For the majority of users who will buy this lens - as the companion lens to a K-5 II purchase - it will no doubt meet their needs. But for the discriminating user who demands the sharpest and the cleanest of images, be prepared to buy up to the DA* line of lenses.

Product Photos

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.