T3D Re: TECH-3D digest 340

["Norman DAVID Scholefield" <ndavids@xxxxxxxxx>]

John,

I was wondering if the fact that the new focus point, using the .25 diopter
lens, is 27 1/4 feet (8.3M), instead of a focus point of infinity, would be
a problem?

David

-----Original Message-----
>
>Date: Fri, 31 Jul 98 17:04:02 PDT
>From: bercov@xxxxxxxxxxx (John Bercovitz)
>To: tech-3d@xxxxxxxxxxxxxxxxx
>Cc: SHIELDS@xxxxxxxxxxxxxxxxxx
>Subject: T3D Re: P3D negative lens for distance
>Message-ID: <9808010004.AA10499@xxxxxxxxxxx>
>
>> I've not gotten any response yet about a distance lens for a Nimslo--it
>> did occur to me that, since glasses have already been used as closeup
>> lenses for the Nimslo, why not talk to an optician? So I did, and the
>> first one seemed willing to try making something for me. The trouble is,
>> even though I know it needs to be a negative lens, I don't know how
>> strong I should ask for. I would think not very--the distance lens for
>> my 16II wasn't very strong. The Nimslo has prime focus at 9 ft.
>
>                     1   1   1
>For this I would use - + - = -
>                     S   S'  F
>
>If I read you correctly, you want the Nimslo to focus at infinity
>instead of 9 feet (2.7 meters). -snip-  Plug and crank:
>
>       1      1    1
>  ------- + ---- = -
> infinity   -2.7   F
>
>So F = -2.7 meters.  -snip-  Generally opticians grind in 0.25 diopter
-snip-
>so ask for a -.25 diopter lens.
>
-snip-


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