This video shows how one can disassemble, repair, and reassemble these Promaster MC and Marexar CX MC 28mm 2.8 lenses. The problem is not always the aperture but more likely just the mechanism in the read mount that gets sticky from old grease.
There are some extremely amazing lenses out there and this Pentax-A 100mm f/2.8 SMC Macro is a prime example of mechanical and optical perfection. However this is a hard to find lens and still costs just as much as a newer auto focus 100mm macro lens even today. For perfect focus I would check Ebay to see if you can get a split focus screen for your body.
This Sigma 85mm f/1.4 EX DG HSM is a fantastic lens for the price with quick focus, sharp photos and really nice bokeh producing some really nice portraits. For a while I went crazy telling everyone around me to go out and buy one because they are so good.
This Bell & Howell 85mm f/1.4 Aspherical IF MC is a fantastic lens for the price if you don’t mind manual focusing. I would get a split focus screen from Ebay for your DSLR to make focusing easier. This lens comes in many names other like Samyang, Rokinon, Bower, and Pro Optic but are all made by Samyang so look around Adorama or Ebay for deals under the other names.
I feel bad for the people who bought their nice new Ricoh camera with this Ricoh Riconar 55mm f/2.2 kitted with it. This lens is very poor. However if you want some strange and artsy spherical aberration, this lens is interesting to play with.