The Pentax SMC FA 645 55-110mm f/5.6 ($1,699.95) is a zoom lens for the company's 645D ($2,175.00 at Amazon) and 645Z medium format cameras. Its zoom ratio is a modest 2x, but it captures a slight wide angle to medium telephoto field of view, and considering that it covers a medium format image sensor, it's surprisingly light and compact. There are some compromises—the lens isn't sealed against dust and moisture like the digital 645 bodies are, it's not as crisp as pricier prime lenses, and it doesn't capture as much light as a prime. But given its price, and the versatility that a zooming design delivers, it's a solid choice for Pentax medium format owners.

Design
The 55-110mm ($698.00 at Amazon) is a sturdy lens, with a metal barrel, manual aperture ring, and rubberized zoom and focus control rings—but it doesn't have the o-ring seal at its base that completes the weather-resistant design of Pentax 645D and 645Z ($5,023.51 at Amazon) cameras. It measures 4.1 by 3.5 inches (HD) at its shortest, weighs 1.1 pounds, and supports 82mm front filters. It's at its most compact at about 75mm, as the barrel extends slightly when zooming to 55mm or to 110mm. A reversible lens hood, with a removable cutout window so you can adjust a circular polarizer with the hood mounted, is included.

When used on a digital body it's best to keep the aperture ring set to its A position, as that allows the f-stop to be adjusted via the camera body. If you're still shooting film, manual aperture control (from f/8 through f/32 in half-stop increments, along with full-stop jumps to f/45 and f/5.6) is available via the ring. Digital bodies are able to adjust the aperture with third-stop precision through its entire range. The zoom range is rather modest—2x—matching the field of view of a 43.5-87mm field of view mounted to a full-frame 35mm camera.

The lens can focus as close as 2.6 feet (0.8 meter). That doesn't give it a huge magnification capability—it captures objects at about 1:6.25 life size at 110mm—but is a solid working range for portraiture and casual photography. It's easy to switch between manual and autofocus—the focus ring is a clutch that is pushed forward to engage the autofocus system and pulled back to switch to manual focus control. There's no internal focus motor; instead the lens is driven by the body's autofocus screw drive.

Because the maximum aperture is a mere f/5.6, you'll notice that the viewfinder is dimmer than when shooting with an f/2.8 lens like the SMC D FA 645 55mm F2.8 AL (IF) SDM AW ($769.95 at Amazon) . It's not a huge deal in sunlight, but could be a concern for photographers working indoors, either in an event space or in a studio without modeling lights.

Image Quality
I tested the 55-110mm with the 50MP 645Z. At 55mm f/5.6 it scores 3,170 lines on Imatest's center-weighted sharpness test. That's better than the 2,200 lines we hope to see in a high-resolution camera like the 645Z, but it lags behind other medium format lenses at the same f-stop. The HD D FA 645 35mm F3.5 ($1,599.95 at Amazon) hits 4,308 lines at f/5.6.

Edge performance at 55mm is a bit weak as well, with the periphery of the frame coming up soft at 1,769 lines. Stopping down to f/8 improves things all around, bumping the overall score to 3,683 lines with crisper edges (2,361 liens). At f/11 the lens scores 3,689 lines, and the edges put up an excellent 3,124 lines. Peak resolution is at f/16 (3,902 lines), with edges that barely lag behind the center of the frame in crispness. Diffraction reduces sharpness at narrower settings—3,501 lines at f/22, 2,766 lines at f/32, and 1,848 lines at f/45.

See How We Test Digital Cameras

At 80mm the edges of the frame are much sharper, keeping up with the center even at f/5.6, where the average score is 3,447 lines. At f/8 it improves to 3,828 lines, and peaks at f/11 (4,034 lines). Numbers remain solid at f/16 (3,982 lines), but dip at narrower apertures—3,578 lines at f/22, 2,880 lines at f/32, and 1,891 lines at f/45.

Sharpness drops off at 110mm. At f/5.6 the lens shows just 2,626 lines, although with even performance across the frame. Stopping down to f/8 gives it a boost to 3,291 lines, and it improves again at f/11 (3,694 lines). Peak performance is at f/16 (3,743 lines), before giving way to diffraction at f/22 (3,442 lines), f/32 (2,883 lines), and f/45 (2,076 lines).

There's a little bit of barrel distortion at 55mm, just about 1.3 percent. That adds a slight outward curve to straight lines, but is something that can easily be removed using processing software like Adobe Lightroom ($9.99/Month at Adobe) . At 80mm and 110mm distortion is a nonissue.

I also used Imatest to evaluate how even light falls on the image sensor using an ExpoDisc to shoot a flat gray frame and the Uniformity tool to analyze it. At f/5.6 the corners lag behind the center by a little less than 1 f-stop (1EV), which is just within the tolerance of what we like to see. At narrower apertures corner illumination is within a third of a stop of the center, which isn't something you're going to notice in the field. The 55-110mm benefits from its design here; it covers a full 645 format film frame, and the sensor that Pentax uses in its digital medium format line is slightly smaller.

(Photo Credit: Elisa D. Keller)

Conclusions
The Pentax SMC FA 645 55-110mm f/5.6 is a solid workhorse lens for medium format photographers who have invested in the Pentax 645 system. It's not a standout performer like the company's excellent HD D FA 645 Macro 90mm F2.8 ED AW SR ($500.00 at Amazon) , but its price tag is more down to earth. It captures images that pass muster on our sharpness test, don't show a lot of distortion, and are evenly illuminated. It's a relatively inexpensive zoom lens, so don't expect it to keep up with a prime like the excellent 35mm F3.5, which can take full advantage of the 645Z's 50MP image sensor. The narrow maximum aperture is an issue, presenting a dimmer image in the viewfinder than a prime lens or a pricier zoom like the SMC FA 645 45-85mm f/4.5 ($2,399.95), but as long as you're working under the sun or in a studio with modeling lights, you'll be able to see your subject without issue. Overall, the 55-110mm is a solid choice if you want the convenience of a zoom lens, but don't want to break the bank.

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