Test examination needs also a look for stress induced birefringence or in german Spannungsdoppelbrechung. This test is very simple and very necessary for longer focal lenses like Dagor-(Protar, Collinear etc-) types, specially the Reproductionversions. On a light desk you lay a pola-filter, test with another polafilter, that these two are really dark when crossed. Then pose the optik on the first polafilter and lay the other polafilter top of the optik, turn for dark. Not understandable blurry picturequality results from stress from the cemented parts. In rare cases but also from not cemented lenses. In polarisationmikroskopie had been methods developed, to reduce this errors in the lenses by experienced craftmen for repair. Once thuch a men from Leica told me, that he had order from his company to shut the door with the key, nobody of the clients in the institutes was allowed to see how this repair functioned.