Just bought a used 20mm f/2.8D lens for my D750. Each shot causes the mirror to lock up. I have to release the lens and turn it twenty degrees to get the mirror to fall again. I love the lens' look but this is unacceptable. Tested my AFS 35mm f/1.8, a 28mm f/2.8D and a 50mm f/1.8D to see if mirror locked up -- maybe the notorious D750 shutter problem -- but no, just this one new lens causes it. Anyone have any idea what is wrong with the lens that would cause this? Thanks for any help.