I have a Nikon FA with a 50mm f1.2 lens. I've been told that it can be used on my D5100. However, when I fire the 5100, the camera send an electrical signal to the lens telling it to stop down to whatever the selected f/stop was selected in the camera. With the FA, the f/stop is set on the lens, and when that camera is fired, a finger inside the lens opening of the camera pushes on a rod protruding from the back of the lens and mechanically causes the lens to stop down to whatever was selected on the lens. How would this work with the 5100? Or wouldn't it?