T3D Lens testing (was Re: conflict resolution)

[erker@xxxxxxxxxxxxxx (Greg Erker)]

>Does anyone have any references (preferably online) that deal with
>the physics of lenses, etc?  I have had one of those Revere lens/shutter
>boards lying around for some time now and I want to build a camera
>from it.  I guess my first question is how far do the lenses need to
>be from the film plane?  But I would really like to know how I can
>calculate this for any lens.

  If your shutter assembly has a bulb or T position
it might be easier just to measure it (heresy on this
list I know :)

  Rig up a ground glass the lens can focus on
(or plain glass with Scotch Magic tape stuck on it).
Aim the lens at a far away bright object (like a
street light) and move it until the image on the
GG is sharp (with lensboard parallel to the GG).

  Measure the distance from the ground side of the
GG to some well defined point on your lens board
and you are off to the races.

  For calculating you need to know the lenses focal
length and where the rear nodal point is located.
The latter is the diffult one in many cases. For a
single element double convex lens the nodal point is
in the middle of the lens. For an SLR lens (especially a
wide angle one) it may be out in space behind the lens.
For a 3 or 4 element (non-SLR) camera lens it's
probably somewhere inside the glass. Where exactly is
hard to know I suspect without have the prescription
data for all the lens elements.

Good luck - Greg E.



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