When reading "Carl Zeiss Jena", people in fact read "Carl Zeiss" and forget "Jena".
For a pre-war lens, no problem as Carl Zeiss was located in Jena and lenses were very well made. In fact they were at this time among the best not to say the best.
Then, things were sharply down the drain after WWII when Carl Zeiss split in 2. The western part in Oberkochen continued the quality and innovation tradition but the Jena part, well... Spotty quality control, poor mechanical quality (diaphragm), poor lubricant, you name it, one step above the soviet production, no more. When I see now people paying top dollars to buy a '60 or '70 CZJ lens and all the buzz about these lenses on various forums, I wonder if people would have the same attitude if lenses were wearing a different name... To me CZJ means pre-war lenses, superb craftsmanship, precise engraving, not these mass-produced low quality products.