I just purchased a 50mm Distagon for a 500CM. Wow, these are finicky! The slow speeds are anybody's guess and switching from self timer and "X" is also questionable. How does one get into the green speeds for long exposures? This particular lens won't let me turn the shutter speed either coupled or uncoupled from the aperture (EV values). Also, the DOF preview is slow and sticky. I understand these are older lenses, but i'm not sure I got what I paid for regarding this 50mm Distagon. Any hints?