OK, time to render my judgement on the Soligor CD 20mm (Mine is Soligor with the Ricoh mount, I had to remove the "pin". Thanks for the heads up Frederic, that could have ruined my day!)
Stopped down just a little, I think it is darn near sharp as my much more modern Irix 15mm I posted last week. Wide open, the Soligor is still silly sharp on the focal plane. This really surprised me.
But for starter comparisons, the first shot here is with the DFA100WR stopped down to F6.3!
smc PENTAX-D FA MACRO 100mm F2.8 WR ƒ/6.3 100.0 mm 1/200 ISO100
And the next 2 shots are the Soligor 20mm wide open. I was blown away how sharp it still is at F2.8...
Soligor 20.0 mm F2.8 1/1600 ISO100
Soligor 20.0 mm F2.8 1/1600 ISO100