Astronar 400mm f6.3

I have a vintage lens, Astronar 400mm f6.3 "girl watcher" I believe it's from the 50's. It's obviously one hundred percent manual and it is a c mount. I have an adapter to fit it onto my D90.

Problem: it takes extremely soft pictures.

What I have tried: every f stop from 6.3 to 32. Every shot on a tripod.

I have a 75-240 and even cropped the images look sharper than the 400mm.

Is it the lens? Am I supposed to do something with my camera when using a manual lens?

I am not familiar with this certain lens but you do realize that the lens is 60 years old. Glass made in the 1950's just does not have the clarity and coatings that modern lenses have.
I see it as putting a single barrel carburetor on the new Camaro and expecting it to perform as advertized. It just won't happen.
Good luck and have fun.