Colorshifts in edges of Rodagon lenses

[Patrik Roseen]

Hello, occasionally I come across Rodagon enlarger lenses which have some kind of strange green/purple/blueish color shiftings around the edge of especially the front cell. The area has varied from only a small area lika a halfcircle of 1.5mm in diameter to a 3mm wide area stretching several centimeters around the edges of the glass.

I have tried to stay away from these but since I have only seen it on Rodagons I am wondering if this is bad or not. People tell me they perform well despite this shifting color.
Any ideas as to what it might be and have you seen this on other brands?

[Michael S. Briggs]

You have probably seen lenses with separation. The optical cement has failed between two elements that should be glued together. The colors are Newton's rings -- an interferrence phenomena of the light reflecting between the two elements which now have a very thin air gap. It is common for the cement failure to start at the edge. In many cases stopping down will block the failed part of the lens from being used. A concern is that the cement failure may propagate. This problem isn't limited to Rodenstock lenses. It might be caused by improperly applied cement, or abuse during the lifetime of a used life, such as being dropped.