Table of Contents
Pentax-M SMC 20mm f/4
Summary
The SMC Pentax-M 20mm f/4 is a compact ultra-wide-angle prime introduced in 1977 and produced until 1984, designed for full-frame K-mount film cameras. It was the smallest 20mm lens Pentax ever made, offering a 94° diagonal field of view in a body barely larger than a standard 50mm—making it a favorite for landscape, architecture, and travel photography where portability matters. It features an 8-element - 8-group optical design, Super Multi-Coating (SMC), and a 5-blade automatic diaphragm. The lens stops down to f/22, focuses as close as 25cm, and delivers 0.12× magnification. It uses a 49mm filter thread, weighs just 150g, and measures 63 × 29.5mm. It includes a dedicated rectangular clip-on hood, plastic cap, and was often sold with a hard case. There’s no “A” setting on the aperture ring, so it’s best used in manual or aperture-priority modes.
Tech Specs
| Manufacturer | Pentax |
| Series | M Series Prime |
| Model | SMC |
| Focal Length | 20mm |
| Max Aperture | f/4 |
| Min Aperture | f/22 |
| Format | Full-Frame |
| Image Circle | - |
| Minimum Focus | 25cm |
| Focus Rotation | ° |
| Iris Blades | 5 |
| Optical Design | 8/8 |
| Squeeze | none |
| Front Diameter | 63mm |
| Filter Thread | 49mm |
| Focus/Zoom Rod | none |
| Native Mount | Pentax K |
| Data Protocol | none |
| Length | 29.5mm |
| Weight | 150g |
| Body Telescope | - |
| Body Material | - |
| Focus Drive | - |
| Iris Drive | - |
| Zoom Drive | - |
History
Country of Origin:
Release Date: 1977
Launch Price:
Design Revisions: