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

Pentax built the M-series to shrink the camera. The ME and MX bodies arrived in the mid-1970s as a deliberate answer to the size race, and the lenses had to match. The SMC Pentax-M 50mm f/1.4 is the fast normal redrawn to fit a smaller body without surrendering the f/1.4 speed that defined a serious system lens. It is a double-Gauss derivative, seven elements in six groups, the basic layout every maker reached for when they wanted a fast fifty, with an added rear element that keeps it from being the strict symmetrical six-in-four of the textbook. Pentax wrapped it in their Super-Multi-Coating, the SMC that was genuinely ahead of the pack on flare control when it shipped.

Wide open it surprises people who expect the usual fast-fifty mush. The center is already crisp at f/1.4, sharper than the reputation of the speed class suggests, but there is visible spherical aberration glowing around bright highlights and the corners stay soft until you stop down. By f/2.8 it tightens fast, and from f/4 to f/8 it is sharp across the frame with the kind of contrast that holds up on slide film. Stopped down it competes with anything Nikon or Canon made in the same class. At full aperture the out-of-focus highlights render as soft round discs, the falloff into the background reads smooth rather than nervous, and skin gets a flattering low-contrast wash that portrait shooters go after on purpose.

Point this into a backlit window and you get far less veiling haze than a single-coated lens of the same vintage would throw. Color comes back neutral and slightly warm, never the cool cast some German glass leaves on film. It is not flawless. Shoot a bright point source just inside the frame and you can still pull a ghost or two, but you have to work to make it misbehave.

It is the cheap entry into fast glass on the K mount, which is a large part of why so many people own one. Street shooters like the compact body it was built for. Portrait people park it at f/1.4 and f/2 for the rendering and the soft skin. It adapts cleanly to mirrorless because the K mount has the flange distance to spare, so a second life on digital keeps demand alive. The honest weakness is that spherical glow at f/1.4 and the corners that lag until you close down. If you need clinical edge-to-edge sharpness at full aperture this is not your lens, and the later SMC Pentax-A or a modern Sigma Art will bury it on a test chart. People buy it anyway because of how it draws and how little it costs against those alternatives.

It cross-shops with the Nikkor 50mm f/1.4 AI and the Canon FD 50mm f/1.4, holding the line on optics while usually undercutting both on the used market. One metering note. The spherical glow at f/1.4 costs you real contrast, so a wide-open reading will not hold once you stop down. Meter at the aperture you actually intend to shoot. In a dim room where you are wide open by necessity, take a Zone Light Meter spot reading off the subject's lit cheek and place it where you want skin to fall, rather than letting an averaged reading sink the frame.