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Re: [photo-3d] prescription "Holmes glasses"


  • From: Linda Nygren <lnygren@xxxxxxx>
  • Subject: Re: [photo-3d] prescription "Holmes glasses"
  • Date: Tue, 09 Jan 2001 09:07:51 -0600

I am not sure exactly what you are saying about the "prismatic"
component usually found in Holmes viewer lenses, or how you are
measuring the optical center. I understand that Holmes viewer lenses are
(or can be) made by cutting a "normal lens" in half. I presume this is
done and then the halves reversed so that the thick edge is on the
outside of the pair not the inside. [An individual closeup + diopter
lens is thick in the middle, unlike - diopter lenses as for myopia
correction which are thick on the edges.] The prismatic effect thus
created is what moves the visual axis outward and makes it easier to see
each image with each eye, rather than having the person have to
"unconverge" his eyes to look straight ahead at a card as is done with
parallel freeviewing. Right? On my Holmes viewer the lens centers are
actually only about 54 mm apart; the edges are 80mm apart, but I suppose
the "optical center" is really at the edge rather than in the middle of
the lens (which is really a half lens), right?

So you are saying that by moving two normal closeup lenses far from each
other so that you are using the inner half of each, that achieves the
same effect? You just ignore the outer half of each lens? So nonprimatic
lenses at an 80 mm separation would give the same effect as prismatic
lenses as in a Holmes viewer do with a closer "interocular"? So the myth
is that special prismatic lenses are necessary, just change the lens
spacing?

In Bob's case (or is it Bill?), the eye doctor apparently added a
prismatic component to the prescription as they might for someone with
an eye muscle deviation problem that does not allow them to align their
visual axes normally. This might be achieved in a similar way to what is
described for Holmes viewer lenses; I really don't know how they do it.
But it should result in the outside edges of the lenses (by the
temples)  being thicker than the inside edges (by the nose), and and
should change the visual axes as he describes. The prism component is
different and additional to the rest of the eyeglass prescription as I
understand it, with its spherical component (standard near- or
farsightedness correction) or cylinder component (for astigmatism, which
is just a variation in the above correction so that the cross sectional
curve of the lens may be greater in a horizontal than a vertical axis,
e.g.). 

I think that the wide interpupillary distance you describe would be
necessary for nonprimatic lenses, but it sounds like Bob's work just
fine with the "prism" component. Does that sound right? -Linda



Abram Klooswyk wrote:
> 
> In a thread on Stereoscopes,  Bob Wier  wrote Jan 7, 2001:
>  >(...) on my last eye exam, I took along a card and got a
>  >prescription (which included prisms) to allow me to wear
>  >them as a viewer - the major problem is that since they
>  >focus at about 20" (...)
> 
> and  Jan 8, 2001:
> >the prisms involved make it possible for me to position my
> >eyes more or less normally (ie, somewhat converged) at 20",
> >while still fusing the images.
> 
> The actual prescription is not quite clear from this
> description, but I suppose that it involved lenses with a
> cylindrical component, as used for astigmatism. I also suppose
> they where prescribed for Bob's interpupillary distance.
> 
> If this is true, it is not the best possible solution.
> On all regular prescriptions for correcting eye glasses the
> interpupillary distance of the patient is given. Few eye
> doctors would give you a prescription with an interpupillary
> much larger than your own! First you would have to discuss
> stereocard viewing optics. (Don't take for granted that all
> eye doctors know about Holmes stereoscope optics.)
> 
> The key feature of "Holmes glasses" is that the separation of
> the optical centers should _not_ be equal your interpupillary,
> but should be at the (infinity) separation of the stereo
> photographs you want to view, with antique cards this is
> about 85 - 90 mm.
> 
> Following this principle I have constructed "Holmes glasses"
> from cheap reading glasses, the plastic type sold in various
> places, here even at gas stations. I used the strongest pair
> available, focal length about 25 cm (10''). These are not
> quality lenses but they are satisfactory.
> 
> (Because I am myopic I have to use these glasses on top of my
> regular glasses with are made for my interpupillary.)
> 
> A maybe superfluous remark on the myth of "prismatic lenses":
> all normal ("spherical") lenses resemble prisms when used
> eccentrical. Oliver Wendell Holmes has cut normal lenses for
> his stereoscope.
> 
> Abram Klooswyk