P3D Re: negative lens for distance

[erker@xxxxxxxxxxxxxx (Greg Erker)]

>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.
>
>Any thoughts, formulas, etc. would be appreciated!

  Is the 9 ft official or just an approximation?

  Do you want to shift the focus point exactly to
infinity or just move it back so that infinity
is sharper when shooting wide open? Say to 20 feet?

  9 ft is about 3 meters. So you could approximate
the Nimslo as a camera focus at infinity with a
+1/3 diopter closeup lens built it. Thus adding
a -1/3 diopter lens from the optician should push
the focus back to infinity.

  This is a very rough approximation though.

  Glasses prescriptions normally go in 1/4 diopter
increments AFAIK so a -1/4 might be the weakest
non-window-glass lens you can get. But it may
just shift the focus point out far enough that
the infinity focus is acceptably sharp.

  If you had some different strength lenses to
play with you could perhaps lock your Nimslo
into the Bulb mode and check the focus at the
film plane using some ground glass (or plain
glass with Scotch magic tape on it) and a 10x
loupe.

Good luck - Greg E.

(I see that Bob W just posted your question
on Tech-3d so take their answers over mine
any day :)



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