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Re: [OM] 85/2: 5/4 or 6/5

Subject: Re: [OM] 85/2: 5/4 or 6/5
From: Jim Brokaw <jbrokaw@xxxxxxxxxxx>
Date: Sun, 08 Sep 2002 23:43:49 -0700
on 9/8/02 4:50 PM, plp@xxxxxxxx at plp@xxxxxxxx wrote:

> I agree.  That suggests that the original version was 6/4, then
> they swapped a cemented pair of elements for a single element,
> resulting in 5/4.
> 
> Why would a lens designer use a cemented pair of elements in
> the first place?  That sounds like a kludge to me, a last minute
> fix.
> 
> 
> Pete

Cemented elements are used to combine two different kinds of glass, with
different refraction indexes, or to eliminate the glass/air/glass interface
in lenses where the design can accomodate it. I suspect that the use of two
elements cemented where they are the same glass type might be done to allow
cheaper manufacturing, where one large chunk shaped and polished might cost
more than two thinner pieces later joined together.

I've read that the new 'aspherical' elements often allow one piece of glass
to replace two, yielding benefits in flare reduction, weight, and
manufacturing cost (assuming that the aspherical elements are now economic
to produce, I think they mold them to a near-final shape and then polish
them just the last little bit).
-- 

Jim Brokaw
OM-1's, -2's, -4's, (no -3's yet) and no OM-oney... 


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