I use my D5100 all the time with manual focus lenses. However as Marty pointed out, the camera now has to be used as a fully manual camera only, with no light meter. The lens will still automatically stop down to whatever aperture you set the lens to, just like your FA. There are times I use a handheld lightmeter, but generally over the many years of using a hand held meter with meterless cameras like a Rollei TLR or a Hassy, I became pretty good at guessing the correct exposure. While I have a few bodies that meter with manual focus lenses, the D5100 allows me to use the few non-AI lenses I have without breaking the camera while still shooting very high resolution photos. You will be amazed at how good those older mf lenses perform on the D5100.