Pentax faced a challenge when it released its long-awaited full-frame K-1—a dearth of modern lenses to go with the high-resolution camera. The Pentax HD D FA 24-70mm F2.8 ED SDM WR ($1,099.95) doesn't skimp on build, matching the weather-sealed design of the K-1, and it's much more affordable than alternatives for other systems. Its optical performance is in line with its price, though, with distortion and weak edge performance at wider apertures, so we're not giving it top marks. But it's a solid option for full-frame Pentax shooters, even if it doesn't perform as well as $2,000 lenses for other systems.

Design

The 24-70mm ($959.95 at Amazon) is a squat, heavy lens. It measures 4.3 by 3.5 inches (HD) at its shortest position, and extend when zoomed beyond 24mm. It's a heavy lens at 1.7 pounds, but one that's solidly built. It's sealed against dust and moisture, just like the K-1. A lens hood is included, as is a soft carrying case, and the front element supports the 82mm filter size.

The barrel is black polycarbonate, with few adornments. The zoom ring is hard, textured rubber, and turns smoothly and comfortably. There's a lock switch to keep it set at the 24mm position. The zoom ring is marked at the 24, 35, 50, and 70mm positions.

An internal motor drives autofocus. The lens supports quick shift adjustments, so you can adjust focus using the narrow manual ring at any point, without having to adjust camera settings.

The lens can focus as close as 15 inches (38cm). It's a fine working distance for general snaps and portraiture, but isn't quite macro territory. The maximum magnification at the closest focus distance and 70mm position is 1:5, which means that subjects are projected into the image sensor at one-fifth actual size. For a zoom lens, 1:3 is typically considered macro territory.

The lens omits optical stabilization, but that's not surprising. Pentax builds stabilization into its camera bodies, not its lenses. You'll benefit from the sensor-based stabilization system with the K-1 and every current APS-C body as well.

Image Quality

I tested the 24-70mm with the 36MP K-1 ($1,335.00 at Amazon) and Imatest software. At 24mm f/2.8 the lens scores 2,284 lines per picture height on a center-weighted sharpness test. It's better than the 2,250-line average we want to see at a minimum from a 36MP sensor, but just barely. The central area of the frame puts up 3,000 lines, which is in very good territory, but it gives way to acceptable levels as you move away from center (2,338 lines), and the outer edges of our test shot are noticeably blurred (941 lines).

To improve edge performance, and overall resolution, you need to narrow the working aperture. At f/4 the lens scores 2,556 lines, with resolution that touches 3,000 lines through most of the frame. Edges are still soft, at 1,386 lines. To get better results for landscape shots where edge sharpness is important you'll want to shoot at f/5.6—the lens shows 2,858 lines on average and about 2,500 lines at the periphery—or at f/8, where you get 2,925 lines on average with even performance throughout the frame.

Resolution remains strong at f/11 (2,896 lines), but diffraction cuts into resolution at f/16 (2,687 lines) and at the minimum f/22 setting (2,292 lines).

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Image quality is in the very good range at 35mm f/2.8, where the average score is 2,516 lines and that level of quality shows through most of the frame. The edges are noticeably soft, 1,759 lines, which can be an issue for landscape shots, but will be hidden by shallow depth of field in many other instances.

At f/4 the resolution improves to 2,697 lines and edges are crisp, 2,247 lines. We see similar results at f/5.6 (2,711 lines), f/8 (2,660 lines), f/11 (2,612 lines), and f/16 (2,642 lines). There's some loss of resolution due to diffraction at f/22, but the lens still manages 2,357 lines.

There's a drop in resolution at 50mm f/2.8. The lens shows 2,125 lines on average. The center of the frame tops 2,500 lines, dropping to an average of about 2,220 lines as you move toward the edges. The periphery is noticeably blurred; 1,189 lines. Depth of field is going to hide this for many images, of course.

There's a modest bump in edge clarity at f/4 (1,469 lines), with an overall improvement to 2,575 lines on average. But for crisp results across the frame, you'll want to shoot at f/5.6; the average score is 2,710 lines and the periphery shows a decent 1,969 lines. Things are better at f/8, with the average score hitting 2,843 lines and the edges showing a strong 2,357 lines. Resolution remains good at f/11 (2,778 lines) and f/16 (2,646 lines) before taking a step backward at f/22 (2,393 lines).

At 70mm f/2.8 the lens delivers results similar to its 50mm f/2.8 output. It manages 2,153 lines on average, with soft edges (1,335 lines). Stopping down to f/4 improves the overall resolution to 2,726 lines, with edges that are acceptably crisp (2,203 lines). At f/5.6 the average score jumps to 2,988 lines, and you get even performance across the frame at f/8, where the average is 3,054 lines. Resolution is still strong at f/11 (2,927 lines) and f/16 (2,732 lines), before taking a slight step back at f/22 (2,464 lines).

Imatest also checks photos for distortion. The lens shows a strong amount of barrel distortion, 4.4 percent at 24mm, but relatively little (0.8 percent) at 35mm. We see some modest pincushion distortion at 50mm (0.4 percent), and just enough that you'll notice if it if you're looking for it at 70mm (1.1 percent). If you shoot JPGs you can tell the K-1 to remove distortion automatically. But Raw shooters will need to apply correction manually. If Lightroom is used to convert images you can apply a lens profile for one-click adjustments.

The lens casts a noticeable vignette around the edges when shot at wider apertures, resulting in dimmed corners. At 24mm we see dimness throughout the f-stop range, starting at -3.7EV at f/2.8 and dwindling to about -2EV at f/5.6 and smaller settings. At 35mm it's overt at f/2.8 (-2.8EV), f/4 (-1.9EV), and f/5.6 (-1.4EV), before dropping to to a reasonable -1EV at smaller f-stops.

The story is similar at 50mm f/2.8 (-2.5EV), but we cut the deficit to -1EV by f/5.6. At 70mm f/2.8 the deficit is -2.7EV; it drops to -1.7EV at f/4 and is less than -1EV at narrower settings. Again, K-1 owners who shoot in JPG can correct for dimmed corners automatically, with four different levels of correction available in camera. Raw photographers will have to make adjustments manually or via a lens profile if they find the effect distracting, though it's not always detrimental to an image. Some may prefer a bit of a natural vignette, especially for portraiture.

Conclusions

Because Pentax was exclusively APS-C for so long, full-frame owners don't have a ton of lenses from which to choose. The Pentax HD D FA 24-70mm F2.8 ED SDM WR is one of a pair of standard zooms available for the system—the other is a less expensive 28-105mm F3.5-5.6 design. Going with the shorter 24-70mm nets wider wide-angle coverage, a brighter aperture throughout the zoom range, and image quality that's quite good. It's not without its flaws—it doesn't perform as well as 24-70mm designs that sell for more than $2,000, and it shows some distortion and vignetting. But Pentax owners who want a modern, f/2.8 standard zoom will certainly be happy with its price, and for shots where edge-to-edge clarity is paramount it's simply a matter of narrowing the aperture a bit. If you don't need an f/2.8, however, don't count out the 28-105mm—it's less than half the price and is also a very good performer.

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