>> Of what is it composed? I understand that it used to include calcium >> fluorite, but apparently that stuff caused other problems and something >> else is used now. Anyone know?
>I don't know what glass formulations Pentax used in it's ED elements >however I >doubt that they would have been fluorite based as use of this material >was >pretty much the domain of Canon. It was difficult to implement flourite >lenses >but I don't remember if it oxidized or was hydroscopic. 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
