Re: bubbles

[P3D John Bercovitz <bercov@xxxxxxxxxxx>]

WJC writes:

> The sharpest lens I've ever owned was an f/4.5, 300mm., (I forget). Not so 
> hot at f/4.5, probably because of the bubbles in the glass near the rim o-) 
> But, at f/5.6, it was very near the theoretical limit of resolution. 
> It was the product of the less modern, more time consuming method of 
> annealing. They used to let the glass cool and the bubbles rise out of it. 
> My point is that "bubbles" may be a good thing.

The bubbles were a bug that was a feature 8-) of the best glasses of the day.
If the bubbles weren't too big, they weren't a real problem.  I have a 36"
f/8 that has the problem but it doesn't seem to kill anything.  The bubbles
served as a sort of marker to tell you that the best glass was being used.
You won't see it in little pieces of glass - just the honkers.  What we're
talking about here is bubbles in the glass but what Allan C was talking about 
was bubbles between the elements of a cemented doublet.

John B


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