An all-black metal, manual Pentax lens.
1977 - 84
f/4 - 22
Performance / Hotspot
The hotspot performance here is good, but never quite reaches stellar levels even when wide open. This will present itself when shooting bright foliage around the outside of the frame while there's flat dark tones in the center. If shooting colour infrared the issue most obvious when the colour of the foliage can be seen in the sky.
For such a small and light lens the Pentax-M 20mm f/4 is pretty sharp, extremely so in the center and not bad in the corners, but what was most impressive was how good the corner performance was when shooting colour infrared (590nm) - see samples below. On the bad side - the geometry of this lens seems rather all over the place