#1. "RE: Mounting AIS lens on D5100" In response to In response to 0 Sun 03-Feb-13 01:53 AM by Leonard62
When you mount the AIS 50mm f1.8 to the D5100, the lens aperture dot should fall directly under the K in Nikon on the front of the viewfinder housing. It doesn't matter what the aperture is set to, the dot is under the K when it clicks in.
Just make sure the aperture lever on the back of the lens springs back nicely when the lens is set to f22 so while looking at the blades they open to wide open from the nearly closed position when you press it with your finger. Also look for any damage to the mount.