Pentax SMC Pentax-A 50mm f/1.4 (K)

Buy a 50mm f/1.4 for one reason above all others: the extra light. In a dim room with no flash, the SMC Pentax-A 50mm keeps a handheld shutter alive when slower fifties have already quit on you. That is the whole pitch for this lens, and on the right Pentax body it delivers.

It is a double-Gauss, seven elements in six groups, the layout almost every fast fifty has used since the 1950s. Performance follows the usual pattern for the type. Wide open at f/1.4 most reviewers describe it as soft and glowy with some field curvature, the kind of rendering that suits faces more than test charts. Stop down to f/2.8 and it tightens up considerably. By f/4 to f/5.6 it is sharp across the frame with good contrast, helped by the SMC multi-coating Pentax used across the A-series barrel. Shoot toward a window or a streetlight and the coating holds contrast where cheaper glass goes flat.

Bokeh is smooth rather than showy. Near wide open the out-of-focus areas stay calm and rounded. By f/2.8 the diaphragm blades start to show and point sources go slightly polygonal. It does not swirl and it does not ring its highlights. The background just falls back quietly, which is what you want for a portrait and not what a bokeh chaser is hunting for.

The A in the name is the practical detail. These ran from 1984 to 1989, the K-mount generation that added the A position on the aperture ring so the lens could feed program and shutter-priority bodies. That makes it the version to want if you shoot an LX, a Super Program, or a modern Pentax DSLR, because you keep the aperture ring and gain the auto modes. The earlier M and pre-A K versions cannot reach the A position, so they lose those program and shutter-priority modes.

The honest weakness is that wide-open softness. If you need critical sharpness at f/1.4, this is not your lens; none of the Pentax fifties in this family are truly crisp from full aperture, so plan to stop down. (Worth noting for K-mount shoppers: the Sigma 50mm f/1.4 Art people often cite was never made in native Pentax K mount, so it is not a real cross-shop here.) People still buy the A 50mm because it is cheap, solidly built with a metal barrel and a smooth focus ring, and it carries the SMC look. For the money it is hard to beat.

One metering note. Wide open at f/1.4 you can put the actual low-light scene into Zone Light Meter rather than guessing: set the app to f/1.4 and read the shutter it returns, then decide whether to stop down a third for safety on faces. The 49mm filter thread is small and common, so an ND or a polarizer for daylight work costs almost nothing.