"Correct me if I'm wrong, but Canon was marketing man-made fluorite
lenses way
before that. Isn't that an extra low dispersion (ED) "glass?""
Fluorite had lower dispersion than the glasses in use at that time,
making it especially useful for longer lenses, where color errors are
more visible. But fluorite is a crystal, not a glass. It was also more
expensive, more fragile and more temperature-sensitive than glass, which
is why you no longer see lenses using it.
"Multicoating was around in the 1950's. I believe that View Camera
magazine just had an article on this. It took the marketing types till
the late 1970's to get us thinking we had to have it."
I'm not sure about this. I remember when Pentax introduced multicoated
lenses in the early '70s (I think). The process had been developed by an
American company, Optical Coating Laboratories. Pentax must have thought
it useful, or they would _never_ have licensed it from an American
company.
One of the reasons multi-coating was so late in coming was that many
designers didn't think it would work. In order for the cancellation to
work properly, the beam of light would have to "know" about the presence
of the layers it hadn't yet reached! But mathematics overrides common
sense, and multi-coating does, indeed, work.
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