Pentax M 20 f4.0

At a little over 5 ounces in weight and 1 inch in depth with a 49mm filter ring, the Pentax M-series 20mm f4 lens is one of the smallest ultra-wide angle primes lenses ever made for 35mm SLRs. The Nikkor 20 f4, esteemed by Galen Rowell because of its optical quality and small size, weighs nearly two ounces more than the little Pentax lens. It’s nearly as small as the Pentax DA 21, which is regarded as a “pancake” lens. In fact, the two lenses are so nearly identical in terms of size and focal length that it’s important to remember that the DA 21 is an APS-C lens lacking an aperture ring, while the M 20 is a “full frame” lens originally designed for Pentax’s film cameras and would presumably work on a Pentax full-frame camera, assuming (per implausible) that such a camera ever saw the light of day.

The lens was originally released in 1977 and continued in production until 1984. Like many of Pentax’s older glass, this lens is full of character, with a unique color rendition and excellent contrast. It sports Pentax’s “SMC” multi-coatings,”a remarkable seven-layer lens coating process that cuts the reflection ratio down to just 0.2% per lens surface. The result is a dramatic improvement in both color fidelity and brightness and the elimination of flare and ghost images.” In the 1970s, the Pentax coatings were, along with the Zeiss coatings, generally regarded as the best. According to Mike Johnston, “Early Pentax multicoating was superior to every company's except Zeiss's, which equalled it, and even today Pentax has better lens coatings than, say, Nikon or Olympus.” Such coatings are important in a wide angle lens, as such lenses are more likely to catch stray beams of light either from the sun or some other bright light source than narrower glass.

So much for the technical side of review. How does the lens perform in the real world? I haven’t tested it on a 35mm film or FF digital camera, but on APS-C digital, it performs quite well. The M 20 has a reputation for distortion problems and weak corners. But since most of the problems are toward the edges of the frame, which are cropped away on APS-C, the result is that this lens performs quite well on digital cameras. The SMC coatings prevent both lens and sensor flare issues, the consequence being rich, contrasty images with bright, luminous, film-like colors:

Corner resolution on APS-C, at least when stopped down to f8 or so, is very fine, as we see in this 100% crop from the above photo:

While the resolution of the Pentax M 20 f4.0 lens is very good, it’s not quite in the league of other Pentax wide and normal range lenses. Nor does it quite match what the DA 12-24 f4.0 can achieve at 20mm, although it’s very close. In any case, whatever limitations the lens exhibits in terms of resolution or distortion does not prevent it from producing spectacular images. It’s unique color rendition scheme gives the lens just the sort of character and sheer beauty of rendering which many modern lenses lack. On an APS-C camera, it makes for a very convenient landscape lens which can easily be carried in a pocket and used when necessary. Here’s some more images from this fine little lens: