T3D Re: TECH-3D digest 244 Viewer lenses

[John Slivon <frogs@xxxxxxxxxx>]

It has been suggested that camera lenses might be used as viewer lenses. I
have a variety of camera lenses so I thought I'd try a few. Here are my
results. With the Nikkor 35mm f 2.8 the total aperture was too small to
permit viewing the entire slide at once through the side that would permit
focussing on the slide. With the 50mm f 1.4 Nikkor the entire 7p image could
be seen when viewed through the rear. However, viewing became difficult when
the eye was not absolutely centered on the lens axis. In  addition, the
image was smaller than one would expect from a 50mm achromat. The slide was
also just beyond the focal point which in this case is almost at the surface
of the front element. The remainder of the lenses I tried are large format
lenses. The view through a 135mm f4.5 Tessar is restricted and allows only a
small (60x60mm) viewing area. This is woefully small for a lens of that
focal length. A single achromat of 63mm x 35mm aperture (f 1.8) covers the
same area but gives twice the magnification. It seems to me that an
effective aperture of at least f2.5 is needed when using single achromats
but when using camera lenses, the smallest effective aperture needs to be
around f1.4. 
The 47mm Wollansak viewer lenses with an effective aperture of f2.5 have
great coverage and magnification as do the 44mm achromats in a Red Button
viewer. I'm not a lens expert but it seems to me that camera lenses are
designed to do one thing very well and that is to place an image on a flat
surface with as much definition as possible. Magnifiers they are not! Not
all achromats are suitable for viewer applications. Good viewing is
dependent on lens design. I recently ordered a pair of achromats from Edmund
Scientific to put into my new stereoscope that I recently made. Up until now
I have been using some Revere achromats that I got from Dr. "T". These have
been excellent but since I wear "specs" I thought that the new larger lenses
(28x47mm f.l.) would make viewing easier. Imagine my surprise when I tried
to view one side of a mounted pair just to see if the lenses were OK! They
were definitely not OK! There was extreme pincushion distortion and much
unsharpness at the edges. Turning the lens so that the side with more
curvature was toward the eye made it worse.( of course). Another set of
achromats that I received from Edmund Scientific (35x63) for use in a future
medium format viewer have no such problems. I noticed that these lenses are
flat on the side nearest the eye. The 28x47 lenses have two curved surfaces,
on side being more curved than the other. I have also noticed that simple
plano-convex and meniscus lenses give very good results but have a wee bit
of chromatic abberation at the extreme edges. I believe that these good
viewing properties are the result of having either a flat or negatively
curved surface nearest the eye. If one is ordering achromats from a catalog
, one can check the radii of the lens surfaces and choose the one that has a
very long radius for one of the outside faces, as close to a meter or more
as possible. If the radius has a negative value, ie., concave rather than
convex, all the better. I would be interested in other opinions and
experiences on this matter.
JS 


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