The recently discontinued 50 2.8 is a fantastic lens. But if you need F2.0 it will not work for you. It works for me 98% of the time.
Basically as you move through the Summicrons different versions, they get more contrasty, and the area of maximum sharpness gets larger at the more wide open settings. The DR/Rigids are only sharp in the center 8 mm, the 1969/79 model this expands to 12/16 mm, and the current one it is fully sharp almost over the entire frame.
There were two versions of the DR/rigids, the first being more like the collapsible, the second being more advanced. Within the early/late models, the DR/Rigid have identical optical formulas, just the mounts differ. This probably accounts to why Tom A says his are different. They may be, but not because one is DR and one is rigid. That or it is simply lens to lens variation. Those optical units that became DR were selected from those whose focal length tested to precisely 52 mm or something close. Otheres became Rigids and the mounts selected to match the exact focal length.
A well cared for Summarit 1.5 is a beautiful lens, but it will not test up to standard. On real subjects, it is unique. Most are fogged and scratched and do not qualify.
3.5 and 2.8 Elmars are also very nice. Again a decent sample is required.
