In 42mm mount: a Super Takumar 24mm, look for lens with a hood, most of the screw mount wides are prone to flare. For cheap with fair to good performance stopped down maybe a Vivitar 20mm. I dont recall if Pentax made a 20mm in 42mm. I have the Super 24mm Takumar and at one time had a 20mm Vivitar, the the Takumar is very good wide open, the vivitar was ok wide open and very sharp stopped down, but like all 20mm of the time easy to distort, I sold it at some point as 20mm is not a lens I used or needed very often.