Re: Star D and lenses

[SCHUB@xxxxxxxxxxxxxxxxxx]

     OK, I now believe the argument that you don't need interocular adjustment
in a viewer if you have large-diameter lenses whose center-to-center
distance is the same as the infinity-point separation of the slide.
But what if you encounter a slide that's not mounted very well, and has
and infinity-point separation that doesn't match the viewer-lens separation?
You have to diverge or converge your eyes to fuse it, right?
It seems to me that to view it in comfort, you want to be able to shift the
lenses so that their centers have the same separation as the infinity-points
on the slide, so that you can look straight ahead as before.  So I still think
that interocular adjustment is a valuable thing to have.
     I did a little checking on my own focusing habits, and found that
the settings I usually use put the slide noticeably less than 1 focal length
from the lens--maybe I need new glasses?  On my Revere viewer, I habitually
have the focus racked all the way in (!), which puts the slide roughly 2 mm
closer to the lens than infinity focus.  (The lenses on the Revere are 50mm
focal length, as near as I can measure).   I also checked a stereoscope that
I built that has a Reel-3D lorgnette (one of the rectangular ones) for lenses,
and found that I tend to keep the stereo card at about 6.25" from the lens,
while the focal length of the lens is about 10.25" (!).
    It occured to me that a cheap source for homemade viewer lenses might be
those cheap 8x loupes you can buy at photo stores for $6-10.  They aren't
achromats, but they do use two-element lenses, so they should beat a single-
element lens.  I checked, though, and unfortunately, the ones I have don't
allow me to see even a whole Realist frame at once, let alone a 7p slide.
This is partly because I wear glasses.  I suppose a dedicated optical hacker
could cut the thing apart and bring the lenses closer together, which (I think)
should both shorten the effective focal length and widen the field somewhat.
But I guess it's probably not worth the trouble.


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