There is a dof preview button on the body on the left side near the bottom of the lens,this must be unlocked to mount or unmount a lens. The button is in the slide switch, push it in and slide the switch to the left side(awayfrom lens) of the camera(when facing the rear as you would look thru the finder). It's the switch in the forground of your pic, right next to the place on the lens that's cocked out.