The Pentax A 50mm f2 is the third generation of Pentax’s K mount basic model standard lens and also the last one with a f/2 maximum aperture. The first one is the K 55mm f2 released in 1976 and was later replaced by the lighter and simpler M 50mm f2 introduced in 1979. In 1985, Pentax introduced a newer version (this one) which incorporates electronics to support automatic aperture control for shutter priority and programmed auto-exposures on the A-series (e.g. the Super Program/Super A) and P-series (e.g. the P5/P50) bodies. As far as I have read from the Internet, the A 50mm f2 has the same optical formula as the previous M version but is built with more plastic. Two years later, Pentax released the F 50mm f1.7 as their basic model standard lens to accompany the first Pentax AF SLR, the SFX/SF1.
Optically, the Pentax A 50mm f2 is not superbly outstanding for a 50mm of the era. However, it still retains the “optical reputation” of Pentax SMC primes. After I deliberately tested for its sharpness and contrast, I found out the following. Below are all 100% crops from 2400 dpi scans of Kodak Ektar 100 film (ASA100 color negative). They were cropped off-center about half way to the corner of the frame.
As shown in above, the lens performs best from f/4 to f/8 with f/5.6 being the sweet spot (only very slightly better). It is worth noting the difference between f/8 and f/11, which could be the result of camera shake or the image quality at f/11 really being inferior to f/8. Nevertheless, the performance at f/4 – f/8 did impress me enough to reconsider to use the Pentax system as a light, carry around, always ready street shooter. Finally, let’s see one more sample photo taken with this lens using Ilford HP5+.