Re: [ATM] ATM: Opthalmic blank magic-

["Richard F.L.R. Snashall" <rflrs@rcn.com>]

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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