The Pentax SMC FA 31mm f/1.8 Limited ($1,299.95 direct) is a fast aperture lens that performs as a standard-angle prime when used on a Pentax APS-C D-SLR, as it delivers a 46.5mm equivalent field of view. This lens and the two others in the FA Limited series stand out from other current offerings from Pentax thanks to the presence of a large manual focus ring and all-metal build quality. And because the lens delivers fully manual aperture control, you can use it on any Pentax K-mount SLR camera—film and digital alike.
The lens itself is pretty big and heavy, due to its full-frame design and all-metal build. It measures 2.7 by 2.6 inches (HD) and weighs a hefty 12.2 ounces. The hood is built-in and the included felt-lined metal lens cap slides over it to protect the front element when not in use. It uses 58mm filters and uses an older screw-drive system for autofocus—so the lens is a bit slower and noisier when focusing than newer optics with silent internal focus motors.
As there isn’t a full-frame Pentax D-SLR at this time, the FA 31mm is more aptly compared to standard-angle lenses, even though it can act as a wide-angle lens on a 35mm body. As such, it’s a tough sell in the value department. Nikon makes a DX 35mm f/1.8 lens that only covers the APS-C sensor, and although its build quality is not on the same level, it sells for just under $200. Pentax owners may find that the SMC DA 35mm F2.4 AL lens is a better fit—it’s one f-stop slower, which means it can capture only half the light as the 31mm, but is priced at a mere $220 and is intended only for use with APS-C digital bodies.
There are a couple of full-frame digital platforms on which you can mount the FA 31mm (and other full-frame Pentax lenses like the FA 43mm Limited ). The easiest is with the yet-to-be-released Leica M camera, which features live view focusing for adapted lenses—you just need to buy a $40 adapter from Amazon—but that camera is set to sell for around $7,000. You can also adapt Pentax glass to Canon EOS bodies, so pairing the 31mm with a full-frame body is an option there, but you’ll have to remove the aperture lever from the lens and shave off a sliver of the Canon’s mirror—it’s not a project for the faint of heart. Both solutions will require you to focus manually.
The lens is priced at a premium, but delivers performance to match. According to Imatest, even at its widest f/1.8 aperture it scored 1,868 lines per picture height, which is a little bit better than the 1,800 lines required for a sharp image. Stopping down to f/2.8 increased the score to 3,015 lines, and it tops 3,200 lines at f/4, and approaches 3,400 by the time you hit f/5.6. This lens can grab a sharp photo at any aperture setting—so you can freely adjust the aperture to control depth of field without having to sacrifice image quality. It’s a much, much better performer than the Sigma 30mm F1.4 EX DC HSM lens, a similar lens that is available for Pentax cameras, but doesn’t cross the 1,800-line mark until f/4.
The FA 31mm has been on the market long enough that it generally sells for less than its $1,300 list price, but you should still expect to part with around $1,000. If you do make that purchase, you will be anything but disappointed by the FA 31mm’s optical performance—it has rightfully earned a reputation as a top-notch lens. But it suffers due to the lack of a full-frame digital Pentax body, and the screw-drive focus experience seems antiquated compared to newer lenses with focus motors. If you are still shooting film and want the best wide-angle for your Pentax system, the FA 31mm is it—but the majority of folks are now shooting digital, and will likely find that the $220 DA 35mm F2.4 AL to be an easier purchase to justify, even though it isn’t viewed with the same level of reverance as the FA 31mm.
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