Table of Contents
Pentax-A SMC 20mm f/2.8
Summary
The SMC Pentax-A 20mm f/2.8 is a manual-focus ultra-wide prime introduced in 1985 for the Pentax KA mount, offering full automation via its “A” setting while retaining classic manual handling. It shares its optical formula with the later FA version—10 elements in 9 groups—and delivers expansive coverage on full-frame and APS-C bodies. With a 94° diagonal field of view, it’s ideal for landscapes, architecture, and environmental scenes. The lens features a 5-blade automatic diaphragm, stops down to f/22, and includes a 67mm filter thread. It focuses as close as 25cm, offers a 0.12× magnification ratio, and weighs 245g, measuring 70 × 43.5mm. Build quality is solid, with a metal core beneath a plastic shell, and the lens balances well on both film and digital bodies. Unlike the FA version, the A 20mm f/2.8 is fully manual focus, but it supports all exposure modes on modern DSLRs thanks to its KA mount and aperture automation. The front element does not rotate, making it friendly for polarizers and graduated filters. There’s no internal focusing, but the focus ring is smooth and precise. The lens originally shipped without a hood, though a dedicated MH-RB 67mm screw-in hood was available separately.
Tech Specs
| Manufacturer | Pentax |
| Series | A Series Prime |
| Model | SMC |
| Focal Length | 20mm |
| Max Aperture | f/2.8 |
| Min Aperture | f/22 |
| Format | Full-Frame |
| Image Circle | - |
| Minimum Focus | 25cm |
| Focus Rotation | ° |
| Iris Blades | 5 |
| Optical Design | 10/9 |
| Squeeze | none |
| Front Diameter | 70mm |
| Filter Thread | 67mm |
| Focus/Zoom Rod | none |
| Native Mount | Pentax K |
| Data Protocol | none |
| Length | 43.5mm |
| Weight | 245g |
| Body Telescope | - |
| Body Material | - |
| Focus Drive | - |
| Iris Drive | - |
| Zoom Drive | - |
History
Country of Origin:
Release Date: 1985
Launch Price:
Design Revisions: