T3D Re: P3D negative lens for distance

[bercov@xxxxxxxxxxx (John Bercovitz)]

> 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).  Object rays come in from infinity 
and you want them to focus at 2.7 meters from the camera which, 
since it is on the same side of the lens as the incoming rays, is 
a -2.7 meters.  Plug and crank:

       1      1    1
  ------- + ---- = -
 infinity   -2.7   F

So F = -2.7 meters.  The optician will measure in diopters which 
is just the inverse of the focal length in meters.  1/(-2.7) .36 
diopters.  Generally opticians grind in 0.25 diopter increments if 
I remember right, so ask for a -.25 diopter lens.

I would try to put the additional lens at the front focus of the 
camera's lens.  You can find the front focus by running light from a 
distant object backwards through the camera lens and noting where 
the distant object's image comes to a focus.  The advantage of 
placing the lens at the front focus is that it then lies in 
"zero-power space" and so its accuracy is less important.  This is 
why spectacles are set 16 mm in front of the cornea.  That is where 
the front focus of the eye is located.

Generally, I think "prime focus" means the focus of the prime lens
or mirror of a telescope.  Then there are other foci according to 
what attachments are used.  In the case of the Nimslo, the prime 
focus would then be located very slightly in front of the film 
causing the sharply-imaged object to lie closer than infinity.

John B


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