Don't know if it was true. I have read that Nikon was the first to develop
ED element and used on 300/2.8 in 1972 to compete against the Canon fluorite
element. Pentax & Olympus have licensed from it since. Unlike fluorite
element, ED element has the weakness of producing slightly cooler colour,
but with the advantage of being much cheaper.
Alan Chan
http://www.pbase.com/wlachan
Someone suggested "marketinium", which is essentially true in that
"ED" glass is merely a name and doesn't have to mean anything specific, or
even consistent.
However, it usually refers to glass with a very low (high?) index of
refraction and such, and there are very few ways to make such glass.
Most of them involve exotic elements. The actual formulation of them is
apparently a deep dark secret.
Pentax has used quartz and I think fluorite (flourite it primarily used in
baking...) in the old ultra-apo Takumar lenses. Canon may be the only
company to use pure fluorite, because as I understand it there are some
practical difficulties such as it being very soft, temperature-sensitive,
and hard to make in the lab in large bits. Apparently even Canon has
moved away from pure fluorite in many cases.
Rumor has it that Nikon ED glass is fluoro-crown, which is I think
fluorite bits on top of good glass bits somehow. What Pentax uses I don't
know, although it pretty much has to be similar to what everyone else uses
if it in fact has the same properties.
DJE
