Yesterday I wanted to use my 150mm f/4 C on my 501CM for a 1-second exposure and noticed that when I hit the shutter pre-release the shutter gets tripped. However the shutterblades then don't close until I click the shutter and when I do they close very slowly. What's the problem here? The lens or the body? I would guess it's the lens since I don't have that problem with my 80mm, however, I had the body CLA'd a while ago and it's still under warranty until the end of this month so I want to make sure it's ok.