Re: Star D & lenses

[crowell@xxxxxxxxxxxxxxxxx (Jim Crowell)]

At 6:08 PM 2/9/96, SCHUB@xxxxxxxxxxxxxxxxxx wrote:

>     So why _does_ it strain the eyes so much to look through pre-Holmes-Bates
>stereoscopes that use two complete lenses (rather than half lenses)?  The
>feeling I get when I do this is that my eyes have to work hard on convergence,
>not focus or other things.  And why do plastic stereo lorgnettes and molded
>splastic stereoscope lenses mimic the shape of the split-lens originals?

You've basically got it; you can think of a half-lens as a
combination of a regular spherical lens and a base-out
triangular prism.  The effect of the prism is to shift the two
images laterally inward (towards your nose), so you don't have
to diverge your eyes as far to fuse them.

I just found out fairly recently that my eyes have a very strong
tendency to diverge (equivalent to 3 or 4 prism diopters), which
is why I find wall-eyed free-viewing so easy.  Supposedly this
contributes to eyestrain (I have to fight the divergent tendency
to look at things close up), so they tried putting in a prism
component in my glasses to correct it.  Which made my eyes start
to strain outdoors, unfortunately.  Back to the ol' drawing
board, I guess.

-Jim C.


------------------------------------------------
Jim Crowell
U.C. Berkeley School of Optometry
360 Minor Hall, Berkeley, CA 94720
(510) 642-7679
jim@xxxxxxxxxxxxxxxxx



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