P3D Re: Replacement Lenses for KI

[Greg Erker <erker@xxxxxxxxxxxxxx>]

>but how about putting two Achromats in each one?  What should I consider
>before I go out and buy them.
>
>I'm taking into consideration the lenses in combination formula "Ftotal =
>(F1xF2) / (F1 + F2 + distance between the
>lenses)".  And am assuming that the resulting focal length is measure from
>the center of the two identical lenses,
>or proportionately between differing focal length lenses.  That means if I
>used two 85mm Achromats, or an 85 and a
>75, I could put them in my Kodaslide and it would work.
>
>Is there something I'm missing?  It would still be less expensive than the
>Combi viewer.
>
>Grant

  Double achromats should work well if the
mechanicals work out. IE the thickness of
two lenses plus airspace has to fit in the
viewer's holders. You have to hold the
lenses in place, parallel to each other and
at the same spacing on L and R.

  One thing you can get from double achromats
is a larger diameter short focal length optic.
With a single lens you could never (never say
never) get a 40mm diameter by 40mm FL lens
(even if it is an achromat). But a pair of 40x75's
might do the job.

  I have built a viewer using two pairs of 36mm dia
by 94mm achromats to get a 36x50mm FL viewer optic.
Works very well. Alan Lewis sells a viewer that
uses 32xsomething double achromats to get 32x47mm
resulting optic.

  My viewer is on my page at:
http://www.angelfire.com/ca/erker

  Using double achromats cancels some lens abberations
but I believe it makes barrel or pincushion distortion
worse. You will have to test or decide if you can live
with the resulting distortions you get.

  Finally, with double achromats you probably want
coated lenses since you now have 4 surfaces per side
which would reduce contrast if uncoated.

Greg E.


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