Pros & Cons
While Pentax had a few legacy full-frame lenses in its catalog when it released its first full-frame DSLR, the K-1, it was without a modern standard zoom. It soon developed a pair: the premium HD D FA 24-70mm f/2.8ED SDM WR ($1,299.95) and the more affordable HD D FA 28-105mm F3.5-5.6ED DC WR ($499.95). The 28-105mm is a fine choice if you're moving to a K-1 and want a compact, crisp zoom, but it exhibits the same optical shortcomings as inexpensive starter zooms for APS-C cameras—its aperture is unambitious, edges show weak performance, and there's some visible distortion.
Design
The 28-105mm is finished in black polycarbonate with a green accent ring and an o-ring seal at the lens mount to prevent moisture or dust from getting inside your camera. It's light at 15.5 ounces, measures about 3.4 by 2.9 inches (HD) at its shortest position, and supports 62mm front filters. The barrel extends when zoomed, and a reversible lens hood is included.
The zoom ring is covered in textured rubber, so you can grip and turn it comfortably, even when wearing gloves. It occupies most of the barrel, and is joined by a narrow manual focus ring covered in the same textured rubber. The focus ring is active at all times, even when the camera body is set to autofocus mode, so you can fine-tune focus to your liking.
Focal length is marked at 28, 35, 50, 70, 90, and 105mm. The lens can focus to 1.6 feet (0.5-meter) throughout its zoom range, delivering a maximum 1:4.5 macro magnification when focused to its closest distance at 105mm. The internal DC autofocus motor is fairly quiet, and quick to adjust focus.
Image Quality
I tested the 28-105mm along with the 36MP full-frame K-1. At 28mm f/3.5 it scores 2,555 lines per picture height on Imatest's center-weighted analysis, better than the 2,200 line we want to see at a minimum. Most of the frame gets good marks, but the outer third is on the soft side at 1,916 lines.
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Stopping down to f/4 delivers a modest bump in performance (2,641 lines), but edges are still soft. At f/5.6 the average score jumps to 2,915 lines, and the periphery is crisp at 2,610 lines. Peak resolution is at f/8 (3,037 lines), with a negligible drop at f/11 (2,925 lines), and more significant dips at f/16 (2,688 lines) and f/22 (2,297 lines).
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At 70mm the maximum aperture is f/4.5, and the lens resolution is 2,346 lines on average, with blurry edges (1,266 lines). Things even out a bit at f/5.6, with the average score improving to 2,699 lines, and edges improving dramatically to 2,021 lines. At f/8 you get crisp results from edge to edge—2,999 lines on average with edges that lag behind by 19 lines. Image quality is still strong at f/11 (2,981 lines) and f/16 (2,813 lines) before dropping to 2,372 lines at f/22.
The aperture opens to just f/5.6 at 105mm, where it scores 2,440 lines on average with blurry edges (1,312 lines). At f/8 the average score jumps to 2,702 lines, and edges are much crisper at 1,934 lines. For the best image quality, shoot at f/11, a setting at which the lens scores 2,749 lines on average with edges that reach 2,300 lines. Performance takes a small hit at f/16 (2,676 lines) and a bigger one at f/22 (2,345 lines).
There's some noticeable distortion throughout the zoom range, typical for a lens of this type. At 28mm it shows 3.5 percent barrel distortion, giving straight lines the appearance of a distinctly outward curve. It transitions to pincushion distortion as you zoom, which makes lines curve inward, showing 1.3 percent at 70mm and 1.4 percent at 105mm.
Corners are dim at wide apertures. I used an ExpoDisc to shoot a flat gray image and ran it through Imatest's Uniformity analysis tool to determine how close the center and edges of the image are in brightness. At 28mm the edges and corners show a drop from the center at all tested apertures. It's the worst at f/3.5, with corners showing a 4-stop (-4EV) drop. That deficit is cut to -3EV at f/4 and -2EV at f/5.6. At f/8 and narrower apertures it's not as pronounced (about -1.5EV), but still noticeable.
It's still an issue at 70mm, with corners showing a -2.7EV drop at f/4.5 and a -2EV dip at f/5.6. At f/8 and beyond the deficit is cut to less than -1EV, which is barely noticeable in the field. Results are similar at 105mm. At f/5.6 there's a -2.7EV drop, which is cut to -1.4EV at f/8, before illumination evens out at f/11.
Both distortion and illumination can be corrected. If you shoot in JPG format you can enable in-camera correction to compensate. If you opt for Raw capture, a lens profile in
Conclusions
The Pentax HD D FA 28-105mm F3.5-5.6ED DC WR is a pretty standard example of a starter zoom lens. It delivers strong sharpness throughout most of the frame, but sacrifices edge performance and distortion control in order to achieve a 3.75x zoom range in such a compact package. It offers some features you don't typically get with a starter zoom, such as weather sealing, and is less expensive than the Canon EF 24-105mm f/3.5-5.6 IS STM, a similar lens for the Canon SLR system. The Canon lens has a few marks in its favor, including more even image quality across the frame and a wider angle at its short end. But if you're getting started out with the Pentax K-1 and want of a zoom that won't break the bank, the 28-105mm is going to be on your radar—just be aware that its design isn't one of optical perfection.
Final Thoughts
