Re: [photo-3d] better viewers

["Dr. George A. Themelis" <drt-3d@xxxxxxx>]

Mike Galazin writes:

> So it got me wondering, what is the result if one tries to view a Realist
> format slide through one of these MF viewers (assuming that the slide were
> centered properly)?  Is the magnification wrong?  

Yes, the magnification is too small.  The FL of the lenses
(which is what determines the magnification, according
to the formula M = 250mm/FL) is I think about 80mm.
This makes the image appear almost half in size, compared
to the 44mm FL of the red button viewer lenses.

> Even if it were, I have a
> feeling that a new Realist format viewer with large lenses of appropriate
> magnification and with a hood around the eyes, like that on the new viewer
> Alan is showing, would be a marked improvement.  What say ye all?

Such viewers already exists.  Haven't you heard of the deWijs
or Ekeren viewers?

There are physical and manufacturing restrictions when
you try to make a single two-element achromatic lens.
If you look at the different lenses in the market, you 
will notice that the ratio of the diameter/FL is about
0.5 or smaller.  If you want a large lens with a short
focal length (for high magnification) then you want
this ratio to be high.  But it does not get higher than 0.5
in the common two-element achromatic lenses
Examples: 20mm dia., 44mm FL.  This is the red
button, Revere, etc. spec.  25mm dia, 50mm FL.
This is the case for a Airequipt and other lenses.
I think that if you attempt to make a two-element
achromatic lens with large diameter and short FL,
you will end up with something that has too much
distortion and other aberrations, making it unusable.

To get over this restriction, people have used two
achromatic lenses per eyepiece.  If these lenses
have the same focal length, then their combination
has 1/2 the FL.  This way  you can pick two large
lenses, say 30mm in dia. with 80mm FL each and
by putting them next to each other you get one
lens (eyepiece) with 30mm in dia. and 40mm in FL.

This is exactly what deWijs and Ekeren (in his
5XR viewer) and also Alan Lewis (in some of
his designs) have done.  One drawback is the
expense because now you need 4 large achromatic
lenses instead of two smaller ones.

George Themelis




 

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