The Pentax HD D FA 645 35mm F3.5 ($1,599) covers a wide-angle field of view when paired with a Pentax 645Z ($5,023.51 at Amazon) or 645D ($2,175.00 at Amazon) medium format camera. Considering that it's designed to cover an image sensor larger than you'll find in full-frame SLRs, it's surprisingly light and compact. Although it shows some barrel distortion, the lens is very sharp. It's a solid option for Pentax medium format photographers.

Design
The HD D FA 645 35mm ($1,599.95 at Amazon)  measures 3.5 by 3.5 inches (HD), weighs 1.3 pounds, and supports 82mm front filters. It wouldn't be considered overly large when paired with a full-frame SLR, and is downright svelte in the world of medium format, especially when you consider its wide-angle coverage. The lens captures a field of view that's roughly equivalent to a 28mm prime on a full-frame 35mm camera.

The lens barrel is metal and free of toggle switches. It does have a physical aperture ring (you'll want to set it to the A position to use it with a digital camera) and a manual focus ring. The focus ring is wrapped in knurled rubber and doubles as a clutch; pulling it back toward you changes operation to manual focus, and pushing it forward activates autofocus. The lens doesn't incorporate an internal focus motor in its design; instead it's driven by your camera's screw drive, part of the lens mount. The focus speed is quick, but is on the noisy side.

The 35mm is by no means a macro lens, but it can focus pretty close. At its closest focus distance, 11.8 inches, it projects objects at one-quarter life size onto the image sensor. The close focus capability is a plus for capturing subjects nearby, while showing some of the surrounding environment at the same time.

The lens does omit weather sealing, which is included in both of Pentax's 645 digital cameras. You'll want to take care to cover the lens and mount when working in rain or snow. If you want a weather-sealed lens that covers this angle of view for the 645, Pentax does have the HD DA 645 28-45mm f/4.5 ED AW SR zoom. It's larger, heavier, and a lot more expensive—$4,999.95—but does offer image stabilization as well, another feature the HD D FA 645 35mm doesn't include. Image stabilization isn't nearly as common in the medium format world as it is in 35mm and APS-C cameras.

Image Quality
I used Imatest to analyze the image quality that the HD D FA 645 35mm is able to muster when paired with the 50-megapixel 645Z. At f/3.5 the lens shows excellent sharpness, well in excess of the 2,200 lines we aim to see from a high-resolution camera. It scores 3,778 lines on a center-weighted test, with image quality that is dead even from center to edge.

Stopping down to f/4 delivers a significant jump in resolution, 4,046 lines, and at f/5.6 the lens shows 4,308 lines. Its peak resolution is at f/8 (4,376 lines), but it's still a solid performer at f/11 (4,168 lines). Diffraction takes a more noticeable toll at f/16, reducing the resolution to 3,757 lines, so you should avoid stopping down that far unless it's absolutely required to get ample depth of field for a shot. You can also shoot at f/22 (3,091 lines) and f/32 (2,178 lines), but image quality isn't as good at those narrow settings as it is at wider apertures.

See How We Test Digital Cameras

There's a bit of barrel distortion, about 2.3 percent. This gives straight lines the appearance of an outward curve in photographs. It's noticeable in field conditions, especially when photographing architecture, but is corrected with ease when editing photos in software like Lightroom ($9.99/Month at Adobe) .

Corners are a little dim when the lens is shot at wide apertures. I used an ExpoDisc to shoot flat gray images, and analyzed them using Imatest's Uniformity tool. At f/3.5 the corners are noticeably dim, about 2.2 stops (2.2EV) darker than the center of the image, and sides lag behind the center by about 1.3EV. Stopping down to f/4 brightens the corners to -1.7EV and the sides to -0.9EV. At f/5.6 and beyond the corners are within 1EV of the center, barely noticeable in the field, and the sides are within a half-stop, completely negligible. Periphery illumination, like distortion, can be corrected with ease in Lightroom.

Conclusions
The Pentax HD D FA 645 35mm F3.5 is a strong performer, capturing images that are evenly sharp from edge to edge, an important consideration for landscape photographers who want detail at the edge of the frame to be just as crisp as it is at the center. It's also quite compact for a medium format lens, especially when you consider its wide field of view. It's not without its drawbacks—there's some barrel distortion, and corners are dim when shooting at wider apertures. It's also pricey when compared with similar lenses for full-frame systems, though not at all out of line with medium format lenses of this type—similar lenses from Hasselblad, Phase One, and Leica range in price from $3,000 to $7,000. Pentax 645D and 645Z owners in want of a wide-angle prime should take a close look at the HD D FA 645 35mm F3.5.

Best Lens Picks

• The Best Fujifilm Lenses for 2020
• The Best Leica M Lenses for 2020
• The Best Micro Four Thirds Lenses for 2020
• The Best Pentax SLR Lenses for 2020
• More Lens Reviews
• More from Ricoh

Further Reading

• Tamron Details Affordable 70-180mm F2.8 Zoom Lens
• The Best Sony Mirrorless Lenses for 2020
• The Best Nikon Lenses for 2020
• The Best Canon Lenses for 2020
• Oops! $13K Camera Lens Sells for $95 Due to Prime Day Error