The SMC Pentax 20mm 1:1.4 is an ultra wide-angle lens that was originally exhibited by Asahi at Photokina 1976.
At the time, its wide F1.4 aperture made it the fastest 20mm lens of its kind. Despite this, the lens managed to be quite compact with barrel dimensions of just 80mm long x 65mm wide. It is also rumored that despite Asahi's exploration of aspherical lens designs at the time (such as in prototype version of the 15mm F3.5), this 20mm has an optical formula based solely on spherical elements.
This 20mm is among the rarest Pentax K-mount lenses in existence, as it was never commercially produced. Only seven prototypes (each slightly different optically) are said to have been produced. An example of one of these prototypes—otherwise a fully functional lens—sold at auction for $6900 in 2022.
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