Pentax SMC Pentax-FA 31mm f/1.8 AL Limited (K)

Pentax built the FA Limited trio at the turn of the millennium as a love letter to its own K mount, and the 31mm was the one that arrived last and cost the most. The 43mm and 77mm came first, normal and short-portrait, jewel-like little things in machined aluminum. The 31mm joined them in 2001 as the widest and the only member with an aspherical element, which is what the AL in the name tells you. This was Pentax planting a flag. Autofocus SLRs were going plastic and zoom-first, and the company answered with three solid-metal primes that felt like they belonged on a Leica counter.

What it renders is the thing people actually mean when they say "Pentax color." Stopped down to f/4 or f/5.6 it is bitingly sharp across the frame with deep, saturated contrast that leans warm without going orange. Skies come back a believable blue, greens stay separated, and skin holds a healthy tone instead of going waxy. Wide open at f/1.8 there is a faint glow and the corners soften, but the center is already usable and the out-of-focus rendering is smooth rather than busy. The aspherical surface keeps coma in check at the edges, so night streetlights stay round instead of smearing into wings.

The focal length is the quiet genius of it. 31mm on full-frame 35mm is wider than a normal lens but not so wide that it distorts faces or fights you in tight rooms. It is the one-lens-walk-around length, close enough to reportage that documentary and street shooters reach for it, wide enough for environmental portraits and landscapes where you want context around the subject. The built-in sliding hood and the hard-stop manual focus ring make it pleasant to use deliberately, which suits the people who buy these: photographers who slow down on purpose.

The honest weakness is cost and weight relative to what it is. This is a slow-ish standard-wide prime, f/1.8 not f/1.4, and it has always commanded a price that makes people flinch. It is heavy for its size, all that aluminum, and the manual focus throw is short enough that critical wide-open focus by hand takes practice. If you only ever shoot stopped down, a cheap FA 35mm f/2 gets you most of the sharpness for a fraction of the money. You pay the Limited tax for the rendering, the build, and the bragging rights.

It still sits near the top of the K-mount want-list, cross-shopped against the Sigma 35mm f/1.4 Art for people who care more about speed and against the Voigtlander 40mm for people who want manual character. Pentax kept it in the catalog for two decades and reissued it with HD coatings, which tells you how little they wanted to change. Wide open in a dim interior, meter the highlights you care about and let the f/1.8 maximum aperture do the gathering. In Zone Light Meter, set the working aperture to f/1.8 and place a face on Zone VI, then watch where the shadows fall before you trust the glow.