I have used my 400 f/5 with extension tubes and bellows for close up work. The main camera that I use it on is a Bessa R2, built in meter makes calculating exposures a thing of the past. There is nothing like having a 5'-12' working space when taking pictures of butterflies and bees on flowers. I have found the lens to be very high contrast and sharp. I had used a Nikon set up in the past, but the lack of working distance limited what I could take pictures of.
karl