Re: [ATM] ATM: Opthalmic blank magic-

["David Harbour" <scarab2@cox.net>]

But Richard-

This is great! This is probably it! Thanks for you generous help, my
friend!!-

Dave


----- Original Message ----- 
From: "Richard F.L.R. Snashall" <rflrs@rcn.com>
Cc: "ATM List" <atm@atmlist.net>
Sent: Thursday, May 05, 2005 5:38 PM
Subject: Re: [ATM] ATM: Opthalmic blank magic-


> David Harbour wrote:
>
> >Hey, guys-
> >
> >I am 100% certain that I saw an article in Sky and Scope, one solar
eclipse
> >year (not too long ago; perhaps a decade or so) where a fellow explained
> >that one could calculate how to obtain an ENORMOUS effective focal
length,
> >by applying a certain formula, to calculate which two opthalmic blanks,
each
> >of a given diopter, would do when taped together, spaced considerably,
back
> >to back. Do any of you guys remember this? It seemed enormously clever to
> >me; he was able to project an image of the eclipse an enormous number of
> >feet away. He gave the formula for picking the apppropriate blanks, and
> >providing their spacing. Does no one remember this?
> >
> >
> >
> Couldn't find that article in the S&T archives, so your memory is
longer...
>
> I did, however, gin up the following "on the back of an envelope" (so I
> don't know how accurate it is;-):
>
> If a positive lens is followed by a negative lens of power equal in
> magnitude
> but opposite in sign by a distance equal to "k" part of the focal length
> of the positive lens, then distance of the final focal plane from the
> second lens (for infinite distance objects) is equal to (1-k)/k times
> the focal length of the first lens.  The effective focal length is 1/k
times
> the focal length of the first lens.
>
> For k < 0.5, the distance is more than the focal length of the first lens.
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