Pentax SMC DA18-135mm F/3.5-5.6 ED AL (IF), DC WR (476.95): This compact zoom has a weather sealed design that matches the Pentax SLR line. Although the price tag is not too high, considering its build quality, image quality suffers as it zooms in.
This results in soft images at 70mm and beyond. We don't recommend this lens because of the distortion it produces. Unfortunately, there isn't any other all-weather options. Amazon's Sigma 18-200mm F3.5 6.3 DC Macro OS($449.00) is an all-weather lens that offers a wider zoom range, better optical performance and a lower price.
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Design
Amazon's 18-135mm zoom is sturdy and compact. It costs $476.95. When set at the 18mm position, it measures approximately 3.0x2.9inches (HD). It weighs in at 14.3 oz. The barrel has a mixture of polycarbonate and metal elements and is finished in black. The front element can hold 62mm lenses filters and a reversible lens cover.
The barrel is dominated by a large zoom ring. The barrel is covered with textured rubber and marked at 18-24, 35, 50 and 70. 100 and 135mm. When zoomed, the lens will extend slightly. It can reach a maximum height of 135mm. It is located at the bottom of the lens and has a narrower manual focus ring.
The 18-135mm is an all-weather lens. As long as you pair it with a Pentax SLR, which offers similar protection against precipitation, internal seals and a red-o-ring around its lens mount make it an all-weather lens. Most models have them, but there are a couple of budget choices like the K-S1or K-500 that offer similar protection. These rare exceptions don't make sense for use in all weather conditions.
It covers an area of approximately 28mm on a full frame system. The 7.5x zoom ratio simulates 200mm when zoomed in all the way. The lens can focus at 1.3 meters (0.4 meter) and has a good 1:4 reproduction rate of 135mm subjects. Although it is not really a macro lens, it can be used to focus on fine details.
However, the lens does not have image stabilization. This is true for all Pentax lenses. Camera body is responsible for image stabilization, but not lens.
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Image quality
The 18-135mm was tested with the K-70 24-megapixel ($616.00 on Amazon).The optical performance of the 18-135mm is not the best, particularly at the telephoto end. However, its performance at wide angles is good. Imatest's center weighted sharpness testing shows that the lens produces 1,962 lines per image height at 18mm F/3.5. It also has solid performance around the edges. At its broadest angle and aperture, we want an image that has at least 1,800 lines. The outer edge of photos should be blurry at 1,157 lines.
The overall resolution is slightly increased to 2,101 lines by lowering to f/4. This also boosts the edges up 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. At very small apertures, diffraction can occur, which decreases the score at f/16 (2.109 lines), and even more at f/22 (1.753 lines).
At 35mm, performance is strong. The average score at f/4.5 is 2,080 lines. Edges are slightly soft but still not bad at 1,626. 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).
Problems start at 70mm. The lens has a f/4.5 score of 1,687 lines. It also blurs edges at 899 lines. Stopping down to f/5.6 nets little improvement—1,697 lines. To get decent images (2,172 lines), you need to reduce the aperture to f/8. The image quality at f/11 (2.221 lines) is excellent, as well at f/16 (2.111 lines), but you should dip to f/22 (1.829 lines).
Images are soft at 135mm regardless of what f stop you choose. At f/5.6 the lens only shows 1,280 lines, which gives photos a blurry appearance. The lens is slightly better at f/8 (1 629 lines), but doesn't reach our target average until the f/11 (1 787 lines) or f/16 (1 872 lines). At f/22, resolution drops to 1,605 lines.
Long zoom lenses can cause distortion. The 18-135mm lens is no exception. It shows a barrel distortion of 3.6 percent at 18mm. This makes straight lines seem to be curved outwards. This gives rise to pincushion distortion at 135mm (1.2%), 70mm (1.6%), and 135mm (1.3%). You can fix it, so distortion is not as significant as image softness. You can remove distortion from raw images using the Pentax SLRs K-70 or other Pentax models.
JPG photographers can also adjust dim peripheral lighting in-camera. The same lens profile which corrects for distortion also corrects uneven lighting throughout the frame. I shot a gray flat image using the Expodisc and then analyzed it with Imatest's Uniformity software. The 18mm f/3.5 corners are 2.2 f stops brighter than the center, giving the photos an natural vignette. At f/4 the deficit drops to -1.6EV, while it is minimal at f/5.6 or narrower apertures.
At 35mm, illumination is equal at all f-stops. However at 70mm F/4.5 the corners trail the center by -1.6EV. The difference is barely reduced to f/5.6 (-1.4EV), however, the corners at f/8 are still within one stop of the center. We see corners at 135mm, f/5.6 that are darker by -1.5EV. However, the frames evenly lighten with f/8.
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Conclusions
Pentax SMC DA18-135mm F/3.5-5.6 ED AL (IF), DC WR is a tempting option for Pentax SLR users. The Pentax SMC DA 18-135mm F/3.5-5.6 ED AL (IF) DC WR has a strong 7.5x zoom, a sturdy build and a weather-sealed construction. It all comes down to image quality. While the zoom is excellent at wide angles, it's very poor in telephoto.
Pentax owners who want good zoom coverage should pair a smaller lens, such as the 18-50mm F3.5-6.3 DC Macro OS HSM ($139.49 on Amazon). Or opt to not protect the camera from the elements by choosing a third party zoom. For example, the Sigma 18-200mmF3.5-6.3 DC Macro OSHSM gives you more coverage and sharper images from its widest angle up to its telephoto extreme.