Brian, it sounds like the rear group on someone's Xenar or Tessar are starting to separate. Canadian balsam is the optically clear cement holding the two rear elements together as a group.
If it is slight, which would be only towards one edge, then the lens may be ok when stopped down.
- Andrew in Austin, Texas -
35mm Gear Bessa R, Leica II, - IIIf RD/ST, - IIIg, - M3
Medium Format Fuji GW 690III / Minolta Autocord
MF Folders Agfa Record III and Super Isolette / Voigtlander Perkeo II and Bessa II
Digital a D300 with a some primes
"Who spilled the Dektol on the bathroom carpet?"