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. Pentax HD D FA 24-70mm ED F2.8 ED SDM WR (1,099.95) is a sturdy camera that matches the K-1's weather-sealed design. It's also much cheaper than other options. However, its optical performance is comparable to the price. We don't give it high marks for distortion or weak edges performance when using larger apertures. It's still a good option for Pentax full-frame shooters even though it isn't as efficient as other $2,000 lenses.
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Design
Amazon's 24-70mm is a heavy, squat lens that costs $1216.75. At its narrowest point, it measures 4.3x3.5inches (HD), and can be extended to 24mm when zoomed in. Although it is a very heavy lens, at 1.7 pounds it's well-built. Like the K-1, it is protected against moisture and dust. The lens hood and soft case are included.
With few embellishments, the barrel is made of black polycarbonate. The rubber zoom ring turns easily and smoothly thanks to its hard texture. The lock switch allows you to set the zoom ring at 24mm. You can mark the zoom ring at 24, 35, 50 and 70mm positions.
An internal motor drives autofocus. You can quickly adjust the lens' shift settings, so that you can focus at any time using the narrow manual dial.
Focusing distances of up to 15 inches can be achieved (38cm). This distance is fine for portraiture and general photography, but not macro. Maximum magnification for the 70mm position and closest focal distance is 1:5, meaning that subjects can be projected onto the image sensor at one fifth of their actual size. A zoom lens with a magnification of 1:3 will typically be considered macro territory.
The lens does not have optical stabilization. This isn't surprising. Pentax integrates optical stabilization in its cameras, but not into their lenses. The sensor-based stabilization system will be available with both the K-1, and all current APS-C bodies.
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Image quality
I used the Amazon 36MP K-1 for testing the 24-70mmwith Imatest software. The 24mm f/2.8 lens scored 2,284 lines per image height in a center-weighted sharpness check. This is slightly higher than the minimum 2,250 line average from a 36MP sensor. Although the central portion of the frame has 3,000 lines which is very impressive, it becomes less acceptable as you get further away (2,338 line), while the edges of our test shot become noticeably blurred (941 line).
You can increase edge performance and resolution by reducing the working aperture. The lens has a resolution of 3,000 lines at f/4, scoring 2,556 lines. The edges are 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.
The resolution is strong at f/11 (2.896 lines), however, diffraction reduces resolution at f/16 (2.687 lines) as well at minimum f/22 (2.292 lines).
The 35mm f/2.8 image quality score is very high. It averages 2,516 lines. This quality shines through the majority of the frame. Although the edges appear soft at 1,759 lines (which can pose a problem for landscape shots), they will not be noticeable in other situations.
The resolution increases to 2,697 lines at f/4 and the edges become crisper, 2,247. Similar results are seen at f/5.6 (2711, 2,660, f/11 (2612 lines), 2/3 (2,642 line), and 1/16 (2,642 line). The lens can still produce 2,357 lines despite some loss in resolution from diffraction at F/22.
At 50mm F/2.8, there is a decrease in resolution. On average, the lens displays 2,125 lines. Averagely, the center of the frame has 2,500 lines. As you get closer to the edges, the average is about 2,220 lines. It is noticeable that the periphery of the frame has become blurred at 1,189 lines. This is going to be hidden by depth of field for many images.
A slight increase in the edge clarity is seen at f/4 (1.469 lines), and an overall improvement of 2,575 lines. For crisp results, shoot at f/5.6. The average score for the frame is 2,710 lines. The periphery has a respectable 1,969 lines. The average score for f/8 is 2,843 lines, while the edges show 2,357 lines. The resolution is good at both f/11 (2.778 lines), and f/16 (2.646 lines), before going backwards at f/22 (2.393 lines).
The lens' 70mm F/2.8 aperture produces results comparable to the 50mm F/2.8 equivalent. The average line count is 2,153 with soft edges (1.335 lines). The overall resolution is increased to 2,726 lines by reducing the aperture to f/4. Edges are acceptable crisp at 2,203 lines. The average score at f/5.6 jumps up to 2,988 line. At f/8 the performance is even across all frames, which averages 3,054 lines. The resolution is strong at f/11 (2.927 lines) as well at f/16 (2.732 lines), but it starts to fall slightly at f/22 (2.464 lines).
Imatest can also check photos for distortion. Barrel distortion is evident at both 24mm and 35mm. However, it's very noticeable at 4.4 percent (at 24mm) but only 0.8 percent (at 35mm). Pincushion distortion is visible at 50mm (0.4%), but not enough to be noticeable at 70mm (1.1%). You can use the K-1 program to automatically remove distortion from JPGs. Raw photographers will have to manually correct the images. You can make one-click corrections to images converted by Lightroom using a profile lens.
When shot at larger apertures the lens creates noticeable blurring around edges, which results in dimmed corners. Dimming is evident at 24mm. It starts at -3.7EV with f/2.8, and drops to a mere -2EV when you use f/5.6 or smaller settings. It's at 35mm that it is overtight at f/2.8, f/4 (-1.91EV) and f/5.6(-1.4EV). Then, at smaller settings, the value drops to a more reasonable -1EV.
Similar story at 50mm (f/2.8): -2.5EV. However, we reduced the deficit to 1EV using f/5.6. The deficit at 70mm f/2.8 is -2.7EV. It drops to -1.7EV with f/4, and it is lower than -1EV when using narrower settings. K-1 users who use JPG to correct dimmed corners can do so automatically. There are four levels of correction in the camera. If they feel the effect is distracting or unnatural, raw photographers can make manual adjustments to their images. For portraiture, some people may like a little natural vignette.
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Conclusions
Pentax has been exclusively APS-C since the beginning, so full-frame photographers don't have a lot of options. 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. The 24-70mm zoom provides a wider coverage and brighter aperture across the entire zoom range.
It also gives you a better image quality. 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. Pentax owners looking for a modern, standard zoom at f/2.8 will be pleased with the price. For shots that require edge-to-edge clarity, it is simply a matter narrowing your aperture. 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.