Re: [photo-3d] Re: focus and convergence

["John A. Rupkalvis" <stereoscope@xxxxxxxxxxxxx>]

Well stated.  Very often a different form makes something seem different
than something that really is the same.

For example, the lorgnette type of stereo viewer is often simply a pair of
magnifiers side-by-side in a single holder or frame that can be hand-held.
A stereo viewer for prints is essentially the same thing with an extension
arm and a holder for stereo cards, etc.  A stereo slide viewer is
essentially the same thing enclosed in a box so that the slide can be
backlit without front light hitting it and washing out the image.

A slight variant is the prism type of viewer.  However, since these are
usually used close to the item to be viewed, these often have magnification
power as well.  Which mean that they are actually lenses.  An off-center
section of a lens functions the same as a wedge prism (it shifts the image
to one side), so it is not as much a variant as it might seem.  This
(off-axis lens section) is incorporated in the most common design of the
stereo card viewers.

One time I made a stereo viewer with an adjustable interocular out of just a
pair of off-center lens sections mounted in a pair of approximately 65mm
diameter plastic gears.  I bored large holes in the centers of the gears,
into which the lens sections were mounted.   The two gears meshed with each
other.   Thus, the optical centers of the lenses remained constant, while
the convergence changed as the gears were rotated.  The entire board that
the lenses and gears were mounted to could be moved back and forth to focus.
It worked quite well.

JR

----- Original Message -----
From: "Dr. George A. Themelis" <drt-3d@xxxxxxx>
To: <photo-3d@xxxxxxxxxxx>
Sent: Saturday, October 14, 2000 6:56 AM
Subject: Re: [photo-3d] Re: focus and convergence


> >I had him put on my reading glasses and try again.  It worked.
>
> This is like giving him a viewer.  Reading glasses are like
> viewer lenses with longer focal length.
>
> >His eyes thought he was looking at a distance but I had caused them
> >to focus close so his convergence then matched the focus.
>
> He still has to decouple accommodation (focus) and convergence
> because as he explores the picture moving from front to back
> his convergence changes while the accommodation is fixed.
> This is true anytime we look at stereo slides through a viewer.
>
> George
>
>
>
>