Long journey to a decent 50x/0.8 microscope lens

Some time ago I wrote that my old used 50x/0.8 Carl Zeiss Jena microscope lens appeared to be worse than my original Chinese one. This did not stopped me - I bought more modern used Olympus UMPlanFL 50x/0.8 lens - and it appeared to be even worse : it was probably dropped once or twice, so it was completely misadjusted and it's resolution was several times worse than difraction limited. This lens was sucesfully returned to the seller.

Finally I got older Olympus MSPlan 50x/0.8 - and this one appeared to be nearly perfect. It took me 3 trials to get a decent 50x lens - yet another remainder that buying optical goods on ebay require some caution and that good optics visually does not differ at all from the terrible one...
See below quality comparison of all 4 lenses.

Olympus MSPlan 50x/0.8 (most recent one, keeper):

Chinese 100x/0.8 Planachromat (my old Chinese lens - too high magnification and slightly lower resolution):

Olympus UMPLanFL 50x/0.8 (resolution below any standards - likely dropped/misadjusted):

Carl Zeiss Jena 50x/0.8 Planachromat HD (vertical chromatic trail hints on misadjustment):

It looks like this lens closes the page of inexpensive upgrades of my microscope via ebay. Further upgrades are much more exotic, so that no one sells these on ebay (like 100x/1.45, 10x with water immersion or modern apochromatic lenses).