Pros & Cons
-
- Crisp optics, especially when stopped down.
- All-metal build quality.
- Wide aperture.
- Integrated hood.
- Mechanical aperture control ring.
- Excellent manual focus experience.
-
- Expensive.
- Dated screw-drive autofocus system.
- Dim corners and soft edges at wide apertures.
- Some barrel distortion.
- Omits weather-sealing.
Pentax SMC FA 31mm F/1.8 Limited Lens Specs
| 35mm Equivalent (Telephoto) | N/A mm |
| 35mm Equivalent (Wide) | 31 |
| Dimensions | 2.7 by 2.6 inches |
| Lens Mount | Pentax K |
| Stabilization | None |
| Type | Lens |
| Weight | 12.2 |
Despite only offering APS-C digital SLRs for the past decade, Pentax has continued to keep the full-frame SMC FA 31mm f/1.8 Limited ($1,099.95) in its catalog. The wide-angle prime lens, which can now be used digitially in all its glory along with the full-frame Pentax K-1, is the definition of a classic. Its all-metal build, premium finish, and character have made it the darling of many a photographer over the years. It's certainly the best 31mm lens ever made—the oddball focal length is part of its charm—and while it has some optical drawbacks that modern lenses sidestep, it remains an excellent option for any Pentax owner, regardless of whether they opt to pair it with a full-frame or APS-C SLR.
Design
The 31mm ($896.95 at Amazon) features an all-metal build and is available in your choice of silver or black finish. Older versions of the lens are stamped Made in Japan, but newer ones come off a factory line in Vietnam—you'd be hard-pressed to spot the difference between the two, and by all accounts they feel and perform identically, within the realm of the sample variation. It's the widest of the three lenses in the FA Limited family, the others being the FA 43mm Limited ($409.00 at Amazon) and the FA 77mm Limited ($899.95).
The lens measures 2.7 by 2.6 inches (HD), is heavy for its size at 12.2 ounces, and supports 58mm front filters. The hood is built in, but only blocks incoming light from the the top and bottom. A slip-on metal cap, lined with felt, protects the front element when it isn't in use.
The autofocus system is driven by your camera, rather than an internal focus motor. A screw turns to adjust the focus ring, which can be a bit noisy, but a short 90-degree focus throw allows the 31mm to move from its closest 11.8-inch (0.3-meter) focus distance to infinity rather quickly. When focused at the minimum distance the lens is by no means a macro—it reproduces objects at about 1:6 life-size—but it's a decent working distance for a lens of this type.
If you opt for manual focus, you'll be pleased by the throwback feel of the knurled ring. The textured metal ring turns readily, but not freely. There's still some resistance there, which is helpful if you want to make a minor focus adjustment. The focus distance scale is marked in feet and meters, and pairs with a corresponding scale that gives you an idea of depth of field and facilitates scale focus. It's marked from f/4 through f/22 in full-stop increments.
Aperture control with a modern digital Pentax body is automatic—you just need to make sure the contorl ring is set to the A position and locked in place. The 31mm is fully compatible with bodies that predate electronic control. If you're pairing it with a classic like the Pentax LX, you can use the mechanical ring for manual aperture control. The presence of the ring also makes it possible to pair the 31mm with a mirrorless digital camera like the Sony Alpha 7 II ($898.00 at Amazon) using a simple mechanical adapter.
As is the case with most of the lenses in the HD DA Limited series (for APS-C Pentax cameras), the FA 31mm Limited is not sealed against dust and moisture. It's been on the market for fifteen years at this point, so it predates Pentax's first weather-sealed digital SLR. Likewise, the lens isn't optically stabilized, but that's a moot point for Pentax owners as the company builds stabilization into its camera bodies.
Image Quality
I paired the 31mm with the 36MP full-frame Pentax K-1 to see just how well it performs when used with the company's first full-frame camera. At f/1.8 it falls a little shy of the 2,200-line mark we want to see a lens hit when paired with a high-resolution camera, notching 2,118 lines per picture height on Imatest's center-weighted sharpness test. The central area of the frame is crisp (2,608 lines), but the mid parts are on the soft side (1,918 lines) and the periphery drops all the way to 1,439 lines.
Similar Products
Our Current Picks for The Best Pentax SLR Lenses for 2021
Performance improves as you stop down, even a little bit. At f/2 the lens delivers a solid average score (2,336 lines) with nearly 3,000 lines in the center, 2,110 lines in the middle, and 1,504 lines at the edges of the frame. At f/2.8 it's a stellar performer, scoring 2,705 lines across the frame, with superb performance in the center, strong performance in the middle third (2,616 lines), and just a hint of softness at the very edges (1,980 lines).
At f/4 results are crisp from edge to edge. The average score hits 2,933 lines, with most of the frame hovering around 3,000 lines and just the edges falling short of that mark (2,359 lines). The lens peaks at f/5.6 (3,139 lines, with 2,900 lines at the edges) and f/8 (3,108 lines with dead even performance through the entirety of the image).
There's a slight drop in fidelity at f/11 due to diffraction, but the lens still manages to hit 2,970 lines, so feel comfortable stopping down that far if a shot calls for it. You should avoid f/16 (2,693 lines) and f/22 (2,357 lines), as there's no advantage in resolution when shooting at very narrow apertures.
See How We Test Digital Cameras
I used an ExpoDisc to shoot a flat gray frame and ran it through Imatest's Uniformity tool in order to see how even illumination is from corner to center. At f1/.8 there is a noticeable vignette effect, with corners that lag behind the center by 2.9 stops (-2.9EV). It's lessened at f/2 (-2.3EV) and f/2.8 (-1.4EV), but you'll need to stop down to f/4 before the effect is negligible in field conditions (-0.9EV). Dim corners and edges are easily corrected for when editing images on a computer; Adobe Lightroom ($9.99/Month at Adobe) includes a one-click profile for the 31mm to even out illumination.
Barrel distortion is about 2.4 percent, which is noticeable in field conditions. It gives straight lines a slightly curved appearance, like the hoops of a barrel. If it's distracting—you'll tend to notice it more when shooting architecture—it too can be removed via software. Lightroom's lens profile also corrects for distortion.
Conclusions
The word classic is overused, but I can't think of a better one to describe the Pentax SMC FA 31mm f/1.8 Limited. We live in a society where it's common to trade-in a perfectly good smartphone every two years, drive a car for three of four, and just look at technology in general as disposable rather than lasting. Pentax has kept the SMC FA 31mm f/1.8 Limited in produciton for 15 years, an eternity in the digital age, and many photographers have waited patiently for the company to deliver a full-frame digital body to use it as it was intended—as a wide-angle lens.While other camera makers have released lenses with similar angles of view and light-gathering capabilities for less in recent years—the Nikon AF-S Nikkor 28mm f/1.8G ($799.00 at Amazon) is one example—the fact that the FA 31mm is able to hold its own against much newer competition is a testament to just how far ahead of the pack it was when first released. But if you're shopping for a lens today, you do have to consider modern competition, which is why we're withholding an Editors' Choice recommendation from the FA 31mm. But it's still an excellent performer, and a fine choice for Pentax owners, especially those who have moved to the full-frame K-1.
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 Pentax