Welcome to Nikonians! The D5100 requires the lens to have a CPU Chip built into it in order to provide metering. All AF lenses have a Chip so they will meter on the D5100. The older Nikkor P series MF lenses and some of the newer Third Party Manufacturers MF lenses have a Chip (check for electrical contacts on the lens) that will allow metering. The older MF lenses and the new ones without CPU Chips (no electrical contacts on the lens) will not meter on the D5100. They can be used in Manual exposure mode and Manual ISO. You will have to use a hand held Light Meter, the Sunny 16 Rule, or the Trial and Error method using the Histogram to set the correct exposure. Given that most fisheye lenses have a very larger DOF even wide open focusing should be very easy. Good Luck and Enjoy your Nikons!