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."
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