Table of Contents
Pentax-M SMC 40mm f/2.8
Summary
The SMC Pentax-M 40mm f/2.8 is a legendary pancake-style prime lens for Pentax K-mount 35mm and full-frame digital SLRs, produced from 1976 to 1984. At just 18mm in length and weighing only 110g, it holds the title of smallest SLR lens Pentax ever made. Despite its diminutive size, it features a 5-element/4-group optical design, a 5-blade automatic diaphragm, and Pentax’s Super Multi-Coating (SMC) for improved contrast and flare resistance. With a 40mm focal length—slightly wider than the standard 50mm—it offers a natural perspective ideal for street, travel, and everyday photography. The lens focuses down to 0.6m, uses a 49mm filter thread, and delivers punchy, contrasty images from f/4 onward. While it’s a bit soft wide open at f/2.8 and has some vignetting and chromatic aberration on full-frame sensors, stopping down improves sharpness significantly.
Tech Specs
| Manufacturer | Pentax |
| Series | M Series Prime |
| Model | SMC |
| Focal Length | 40mm |
| Max Aperture | f/2.8 |
| Min Aperture | f/22 |
| Format | Full-Frame |
| Image Circle | - |
| Minimum Focus | 60cm |
| Focus Rotation | ° |
| Iris Blades | 5 |
| Optical Design | 5/4 |
| Squeeze | none |
| Front Diameter | 63mm |
| Filter Thread | 49mm |
| Focus/Zoom Rod | none |
| Native Mount | Pentax K |
| Data Protocol | none |
| Length | 18mm |
| Weight | 110g |
| Body Telescope | - |
| Body Material | - |
| Focus Drive | - |
| Iris Drive | - |
| Zoom Drive | - |
History
Country of Origin:
Release Date: 1976
Launch Price:
Design Revisions: