[GrahamS]

Robert, you cannot mount a Nikon non-Ai lens on a D7100 or higher body unless the lens has been Ai converted. It is possible to convert the lens yourself if you have the skills (search Google for "Nikon Ai lens conversion" ) Some professional repair shops may still offer the service if they still have a conversion kit available. Personally, I wouldn't bother converting a 50mm lens as a 50mm f1.8 Af-D can be bought for little money and it is a superior lens.