Pentax's classic pancake — the M 40mm f/2.8 that defined compact K-mount.
The SMC Pentax-M 40mm f/2.8 is one of the smallest and flattest SLR lenses ever produced, a true pancake design measuring only 15mm from flange to front element. Part of the compact M series introduced in 1976, the 40mm focal length is a near-normal view slightly wider than the conventional 50mm, and it pairs particularly well with the Pentax ME and ME Super for an ultra-compact manual SLR setup.
40mm focal length, f/2.8 maximum aperture, seven-element SMC (Super Multi Coated) formula, Pentax K bayonet mount, 15mm body depth (pancake), 49mm filter thread.
The defining attribute of the 40mm pancake is its minimal footprint — at 15mm depth it makes any Pentax K body genuinely pocketable. The SMC coating delivers good contrast and flare resistance. The 40mm perspective is slightly wider than a standard 50mm, giving a natural wider framing that suits street and documentary photography. Optically very good from f/4, acceptable at f/2.8.
Inspect all elements for fungus, haze, and coating condition. The very short flange-to-element distance means scratches on the rear element are particularly harmful. Confirm K-mount bayonet tab operates correctly. Aperture blades should be clean and oil-free.