The Pentax SMC DA 18-135mm F/3.5-5.6 ED AL (IF) DC WR ($476.95) is a compact, long range zoom with a weather-sealed design to match the Pentax SLR lineup. It's price isn't bad, especially when you consider its build quality, but image quality drops off significantly as you zoom, delivering soft results at 70mm and beyond. Coupled with the distortion that's typical of this type of lens, we don't recommend buying it, which is a shame as there aren't any third-party alternatives available with an all-weather design. The Sigma 18-200mm F3.5-6.3 DC Macro OS HSM ($389.00 at Amazon) is a fair-weather lens, but one that delivers a longer zoom range and better optical performance, all for less money.

Design

The 18-135mm ($449.00 at Amazon) is a sturdy, compact zoom. It measures 3.0 by 2.9 inches (HD) when set to the 18mm position and weighs about 14.3 ounces. It's finished in black, with a green accent ring, and its barrel is a mix of metal and polycarbonate elements. A reversible lens hood is included, and the front element supports 62mm lens filters.

A large zoom ring occupies most of the barrel. It's covered in textured rubber, and has markings at 18, 24, 35, 50, 70, 100, and 135mm. The lens does extend when zooming, just about doubling in height when set to 135mm. The manual focus ring is narrower, positioned at the base of the lens, and is finished in the same texture.

Internal seals, and a red o-ring around the lens mount, make the 18-135mm an all-weather lens, as long as it's paired with a Pentax SLR that includes similar protection from precipitation. Most models do, with a few budget options like the K-S1 ($388.00 at eBay) and K-500 ( at Amazon) notable as rare exceptions that aren't designed for all-weather use.

At its widest angle it covers a field of view that approximates a 28mm lens on a full-frame system, and its 7.5x zoom ratio mimics a 200mm when zoomed all the way in. Its can focus on subjects as close as 1.3 feet (0.4-meter), for a decent 1:4 reproduction ratio at 135mm. That's not really macro lens territory, but it does make it a versatile option when you want to hone in on small details.

The lens omits image stabilization, but that's true of all Pentax lenses. The system puts image stabilization in the camera body, not the lens.

Image Quality

I tested the 18-135mm with the 24-megapixel K-70 ($545.58 at Amazon) . Its optical performance isn't the best we've seen, especially at the telephoto end of its range. But its wide-angle performance is decent. Imatest's center-weighted sharpness test shows the lens nets 1,962 lines per picture height at 18mm f/3.5, with solid performance at all but the edges of the frame. We want to see an image with 1,800 lines at a minimum, and the outer border of photos is a blurry 1,157 lines at its widest angle and aperture.

Stopping down to f/4 gives the overall resolution a modest bump (2,101 lines) and boosts edges to 1,431 lines. You get better peformance from edge to edge at f/5.6—the average score is 2,229 lines and the periphery is quite decent at 1,758 lines. At f/8 there's a slight drop in fidelity (2,189 lines), while f/11 nets the strongest overall quality—2,240 lines on average with edges that top 2,000 lines. Diffraction sets in at very narrow apertures, dropping the score slightly at f/16 (2,109 lines) and more noticeably at f/22 (1,753 lines).

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Performance remains strong at 35mm. At f/4.5 the average score is 2,080 lines, with edges that are soft, but not terrible, at 1,626 lines. You get better performance from edge to edge starting at f/5.6—the average score is 2,485 lines, and edges only lag behind by about 100. The story is the same as you stop down—image quality remains strong at f/8 (2,424 lines), f/11 (2,304 lines), and f/16 (2,138 liens), with diffraction once again reducing quality at f/22 (1,815 lines).

It's at 70mm where problems start to show. At f/4.5 the lens scores just 1,687 lines, with edges that are blurred at 899 lines. Stopping down to f/5.6 nets little improvement—1,697 lines. You need to go down to f/8 to get a decent image (2,172 lines). Image quality is also strong at f/11 (2,221 lines) and f/16 (2,111 lines), before taking at dip at f/22 (1,829 lines).

At 135mm, images are soft, no matter what f-stop you use. The lens shows just 1,280 lines at f/5.6, giving photos at blurry look. It's a bit better at f/8 (1,629 lines), but it doesn't approach our target average until f/11 (1,787 lines) and f/16 (1,782 lines). Resolution dips to 1,605 lines at f/22.

Distortion is a common issue with long zoom designs, and the 18-135mm is not immune. At 18mm it shows about 3.6 percent barrel distortion, an effect that makes straight lines appear to curve outward. It gives way to an inward curve, pincushion distortion, at 35mm (1.2 percent), 70mm (1.6 percent), and 135mm (1.3 percent). Distortion is less of an issue than image softness, as you can correct it. The K-70 and other recent Pentax SLRs can automatically remove it when shooting JPGs, and you can apply a lens profile in Lightroom ($9.99/Month at Adobe) to nix it from Raw images.

Likewise, JPG shooters can correct dim peripheral illumination in-camera, and the same lens profile that removes distortion also corrects for uneven lighting across the frame. I used an Expodisc to shoot a flat gray image with the lens, and analyzed it using Imatest's Uniformity tool. At 18mm f/3.5 corners lag behind the center of the frame by 2.2 f-stops of brightness (-2.2EV), giving photos a natural vignette. The deficit is cut to -1.6EV at f/4, and is negligible at -0.9EV at f/5.6 and narrower apertures.

Illumination at 35mm is even at every f-stop, but at 70mm f/4.5 the corners lag behind the center by -1.6EV. Stopping down to f/5.6 barely cuts into that difference (-1.4EV), but at f/8 and narrower the corners are within a stop of the center. At 135mm f/5.6 we see corners that are dimmer by -1.5EV, but at f/8 and narrower the frame is evenly lit.

Conclusions

The Pentax SMC DA 18-135mm F/3.5-5.6 ED AL (IF) DC WR is a very enticing option for Pentax SLR owners. It boasts a solid 7.5x zoom range, an excellent build, an a weather-sealed design. But at the end of the day, it's all about image quality. And while the zoom does a fine job at wider angles, its telephoto performance is quite disappointing. Pentax owners that want good amount of zoom coverage are better off pairing a shorter lens like the tiny, weather-resistant 18-50mm ($296.95 at Adorama) with a telephoto companion, or opting to forgo protection from the elements and choosing a third-party zoom. The Sigma 18-200mm F3.5-6.3 DC Macro OS HSM, for instance, gets you a bit longer coverage range and images that are more consistently sharp from its widest angle to its telephoto extreme.

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