Quote Originally Posted by ridax View Post
I also doubt it would work with a 135° lens that is not retrofocus nor is made according to the Roosinov's aberrational vignetting principle (like all the Biogons, Grandagons, etc., etc.). A spherically curved filter may probably be a solution.... but certainly not a cheap one.
Please explain further. I ask because Rodenstock's Pantogonal, a 3 element lens and neither retrofocus nor Russar type, was offered with a flat center filter.

"This consists of a plano-convex lens of yellowish green glass which absorbs very strongly blue and violet light, and a plano-concave lens of a colorless, very transparent material. The optical constants of both these lenses is exactly equal so that the cemented lenses act as a plane parallel plate which is dark in the center and transparent at the sides."