"Its use of fluorite crystal and special rare-earth glass helps correct aberrations, especially chromatic ones." --first page above
Is this statement incorrect? Is it one more of Nikon's errors in such publications or am I reading it wrong? As I read it I read two elements as a minimum. Is there a better source of information on these models?
To the best of my knowledge (which is not perfect), Nikon chose not to use fluorite elements in their telephoto lenses since Fluorite is relatively soft, cracks more easily when subject to shocks, and is more affected by moisture and heat. Nikon was the producer of professional quality lenses, and must have felt that fluorite was too delicate for heavy use (Canon obviously chose a different path). Nikon instead developed ED glass which has much better mechanical qualities - hard enough to be used on the front element, stronger and less subject to moisture and heat. ED glass is not quite as good optically as fluorite glass, but the lens designer had greater freedom on where the ED glass could be used. This is the trade-off they made, their lenses may have been more robust than those from Canon, but Canon's may have been better corrected (although their coatings from that period were poor).
I think ED glass does use fluorite in the mix (?) along with other glasses which improves the mechanical properties. So maybe the quote "use of fluorite crystal and special rare-earth glass ..." is correct, but applies to the same ED element.
Until powerful computers became available in the 1970s, Nikon was not in the habit of redesigning the same lens every few years - ray tracing was still largely manual in those days so designing a lens was a slow and expensive task. From that point of view it seems likely the NIKKOR-P-C and K 400/5.6 are the same optically since they are only a few years apart. On the other hand, they would "tweak" existing designs to improve performance, so it is possible the design changed slightly.
One thing I just noticed, on the lens diagram at the top for the NIKKOR-P-C Auto, the third element behind the front cemented double, is a meniscus lens - the rear surface curves inwards. On the diagram of the AI lens, the rear surface of the third element curves outwards. I'm not sure how much we can trust these diagrams, or the marketing descriptions, but it offer the possibility that they are a little different.
We just have to hope that this lens is subject to one of the 1001 night articles soon...
