The early symmars had 3 aperture scales for each of the two cells alone and for the full assembly, those those were of (related) Dagor design, IIRC. I tested the front cells alone of the plasmat type at it looks as good as the rear cell alone, but then the aperture scale has a shift ...
This is the dagor type, showing that the scale for the front lens alone is shifted some 1/3 stop, so we should expose 1/3 stop longer to compensate. I gues that the plasmat type needs similar compensation if using the front cell alone, at least this would be the starting point.
This has been said, but C.Perez tests include the Symmar 150 conversion with and without a yellow filter, showing no benefit from the filter usage. This was not a lab test, but it's good enough to deliver interesting information.
For the converted configuration, results in that test suggest that chromatic aberration is not the problem, but spheric aberration in the corners when wide open that lowers performance to 20 lp/mm in the corners, reaching 30lp/mm in the corners by f/22. ...while the center and mid of the conversion is always very good at around 50 lp/mm, not bad.
So IMHO the conversion it's pretty operative, with corners a bit softer, which it's irrelevant in many images. For portraits sure it's irrelevant. For landscape we may have the sky in the two top corners, and we may have water in motion in the botton corners...
Then, if we use for example the symmar 210 converted to 370mm (because of the larger circle) for 4x5 we don't take the outer boundary, so the image it's really good even in the 4x5 corners, being a very lightweight choice.
Those old symmars are a piece of gear.
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