P3D Re: Suddenly, I can freeview..

["Greg Wageman" <gjw@xxxxxxxxxx>]

(This message was bounced back in January when Calcite apparently took a
little break...)

From: Robert Cruickshank <robcruic@xxxxxxxxxxxx>


>I have always been disappointed that I can't freeview (or rather can
only
>do it crosseyed, with much eyestrain) Today, I realized that I can
>freeview very easily, if I take off my glasses- I'm very nearsighted,
so I
>can only see clearly to a distance of 7 cm (I just measured) so  this
>limits the size of things I can see; realist slides are fine, keystone
jrs
>are fine, but fullsized Holmes cards are out.
>So whats the deal, optically?


I am not quite in the same boat as you, Robert, but I am a nearsighted
eyeglass wearer.  Here's what I understand of the problem.

The problem that all parallel freeviewers have, is to decouple vergence
and accommodation.  That is, our eyes are aimed parallel for viewing
an object at infinity, but since the actual distance of the print we're
viewing is quite close (like 12"-15" or so), they must "accommodate", or
focus, at that near point.  This is an unnatural situation, and so takes
some training.  Hand viewers overcome this by optically making the image
appear at infinity, matching vergence with accommodation.  This is also
why us nearsighted folks generally have to wear our corrective lenses
when using viewers, particularly non-focusing viewers, even though the
slide is physically only a few inches away from our eyes.

Us myopic folks have an additional problem.  Our eyeglasses are designed
to correct for our nearsightedness by, in effect, making "infinity" be
optically at our point of near focus.  With an object actually *at* our
point of near focus, combined with eyeglass correction, the plane of
focus is practically at our corneas.  Combine that with parallel
vergence,
and there's no way we are going to get our already-inadequate
eye muscles to focus in that situation.  By removing our eyeglasses, our
eyes only have to focus at approximately their natural near-focus point
while remaining parallel.

     -Greg W. (gjw@xxxxxxxxxx)



------------------------------