Pentax SMC Pentax-M 40mm f/2.8 Pancake (K)

Cross-shop this against Canon's 40mm f/2.8 STM pancake and the comparison writes itself. The Canon is autofocus, plastic, and barely thicker than the lens cap. The SMC Pentax-M 40mm f/2.8 is all metal, manual, and it pancakes onto an MX or ME so flat that the lens cap sticks out further than the barrel. People reach for the Pentax for the same reason they reach for an old SLR in the first place. It drops a real camera into a jacket pocket, and it feels like a precision instrument while it does it.

Optically it is a near-normal lens, and that focal length is the quiet reason it works. 40mm on a 35mm frame sits between the snapshot honesty of a 35 and the standard reach of a 50, so verticals hold and you do not back yourself into a wall framing a portrait. The SMC multicoating earns its initials. Flare is well controlled into a backlit sky, and contrast holds up against a bright window better than a single-coated lens of the same age. It is a simple, well-corrected design, not a fast exotic, and it behaves like one. Predictable, low-drama, no swirl, no character flaws to fall in love with.

Wide open at f/2.8 it is already sharp in the center, with a hint of softness and field curvature toward the corners, the normal tax for a compact near-normal. Stop down to f/5.6 and the corners catch up with the middle. Color is neutral and slightly warm in the Pentax house style, and the out-of-focus rendering is smooth rather than creamy. Do not expect a portrait lens that melts the background. f/2.8 at 40mm buys you a little separation and nothing dramatic, so this is a deep-of-field walkaround lens at heart. If you want subject isolation you are buying the f/1.7 or f/1.4 fifties instead.

It became the everyday lens for documentary and street shooters who wanted to carry a real camera without announcing it, and it still serves that role. The 49mm thread is shared with several other Pentax-M primes, the 50mm f/1.7 among them, so a contrast or ND filter can do double duty across the kit rather than being a one-lens purchase. When you do drop a strong ND or a deep yellow contrast filter on for daylight or black and white work, dial its factor into Zone Light Meter before you meter, so the suggested exposure already accounts for the light the glass is eating instead of you guessing a stop and a half in the field.

Today it trades cheap, which is most of the appeal. It is the budget entry to the pancake club, undercutting the rarer 43mm Limited by a wide margin while giving up the exotic build and the slightly more romantic rendering. Buy it for the form factor and the SMC coatings, not for a signature look. As a one-lens travel setup on a small Pentax body it is hard to beat, and that is exactly why people keep buying forty-year-old copies of it.