I enjoy collecting the Nikon F with the standard prism. I have two Nippon Kogaku bodies and was comparing them to decide which one to keep. One is a 64x and the other is a 65x. I don't know if I will physically chase the serial number as far back as I can - sometimes it is just fun to watch others do it or examine eBay offerings of old serial numbers. While I enjoy intellectual Nikon F collecting, I am more interested in building a practical clean model with old lenses, and take pictures with it. My sample 65x prism appears different than the 64x prism and later prisms that I own. Using Richard de Stoutz's Nikon F Collection and Typology site as a reference, the prism is Type 3 and "correct" to the serial number. The black prism housing inner walls appear to contain a perfect pattern of white paint spatter in slightly variable sizes. If this is commonly regarded as fungus, the pattern is suspiciously perfect on all inner surfaces. A happy fungus family? The mirror itself is clean and does not reveal deterioration. Any experience with this observation? Switching the subject, I use a circular diopter for the later Nikon F model, but I am at loss finding a diopter for the older square eyepiece. Any ideas for better luck? Given such a fondness of the Nikon F, I have become curious about its older RF sibling as well, although my conversation with a Leica repair expert indicates that Nikon RFs are failing these days (I assume he means that it is difficult to replace or repair them). No-one will question the reputation of Leica M, Leica R, and Contax lenses, but I grew up with Nikon and I can have more fun sampling vintage Nikon bodies and lenses. I just seem to settle into them better than other cameras regardless of quality. I am an artistic person that relies on aesthetics, so it can frustrating to subdue film and digital Leica RF collecting, but that's life. We all have our priorities for saving and spending, and we can shuffle our priorities a bit, but having fun with Leica RF really is a different stratosphere. I do not own a Nikon Df, but but I note that reviews and opinions have been heated on the Internet more than usual for a digital camera. It would be fun to see a digital Nikon Ds as homage to the original Nikon RF.