RFF'rs who are currently debating "3-D pop" should check out this thread.
I’ve stayed out of that debate. I don’t think anyone can really say “why” some lenses exhibit that phenomena. Optics seems like a combination of so many different pieces that result in the photo. That being said, the Summaron is a lens I will NEVER sell. It’s perfection in a medium speed 35mm and its image quality is intangible and immeasurable IMHO.