Why is lead "faster"?

Vixen actually came in ahead of the wave in the fast, not hugely expensive apo class with the ED102SS f/6.5, which was discontinued because the manufacturing process involved lead.

This little scope is dear to my heart because I own one. It's an ED doublet to be exact. In any case, after only a few years Vixen had to discontinue the line due to environmental factors that were apparently related to lead. They then moved to f/7.7, f/8, around there. Given that "the market" continues to abound in f/7-point-O's, they undoubtedly had a reason for this choice.

My question is: why does "getting the lead out" make for a somewhat slower scope?

In all the many learned debates about fluorites and FPLs and such I never see hide nor hair of lead. Personally I think of lead as kinda-not-very transparent, you know, Superman can't see through it, and stuff.

thanks
Greg N

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Lead oxide has been used in glass production for a very long time, particularly in crystal. It helps to make a glass very transparent.

I don't know anything about Vixen's production but the glass industry in general has been moving away from glasses produced with lead. The lead in the glass is not a problem as it is bound pretty tightly but there are environmental concerns with the manufacturing processes and the waste products. Many of the old "standard' glasses have been replaced with lead free types. Perhaps the optical quality was not available for Vixen design with new glasses.

hope this helps,

dan

There is no lead in ED glass. There may be lead oxide in some flint glasses, but there are plenty of candidates for a mating element that have no lead oxide in them. Lead doesn't make a lens "faster". The Vixen ED doublet was not some sort of special design using "unobtainium". It was a very basic ED doublet with ordinary glasses.

Rolando