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Interesting.

One of the many fun side benefits of acute  bacterial meningitis,which I have had a number of times,the first at six months of age,is hemianopia,or blindness in one eye.As a result,I have never been able to see in any thing,but 2D.

                                   Roger

George Brock-Nannestad <[log in to unmask]> wrote: From: Patent Tactics, George Brock-Nannestad

Hello, just a few words on stereo:

1) stereoscopy
I am a not-very-active collector of stereoscopic representations, and I 
regard it as the poor man's holograms and extremely fascinating. I have a 
number (puny as collections go) of the old standard stereoscopic images, such 
as Underwood & Underwood, and I built my own Holmes-type viewer with variable 
inter-ocular distance for them. I also have a number of examples of the uses 
of stereo photography, scientific, mapping, and of the methods used for 
recreating the impression, i.e. the coding of the images.

There are several factors involved in good stereoscopy. First of all, each 
image must obey the rules of perspective, which means that the angle 
subtended by an object must be close to the natural angle of observation. For 
an ordinary telephoto image that means that you must see it from such a long 
distance that you really do not draw it closer, and a wide-angle image must 
be seen with an eye so close to the image plane that you may not be able to 
focus and hence you need to use a lens - usually called a magnifier. All of 
this makes for some simple geometrical calculations. In other words, for each 
image you come across (obviously without barrel or cushion distortion) there 
is only one particular viewing distance where the eye will regard it as 
natural. This is how a trompe l'oeuil is made. A famous example is used in 
Dorothy Sayers' short story "The Haunted Policeman".

A good stereo image pair is taken with an inter-objective distance 
corresponding to the average distance between the eyes, but that also has 
only one correct viewing distance for each enlargement. Otherwise your eyes 
tell you that something is wrong, the illusion partly disappears, and you get 
the cardboard cutouts that were mentioned. Aereal photography is like that: 
the roofs of houses appear much steeper than they are, because the base 
between photographs is so much wider (hundreds of feet) than the inter-ocular 
distance.

Now, if I look at many stereo pairs (it comes in bursts) I have until now 
been able to train my eyes to be wide-eyed, and then I can see without a 
viewer. However, John R.T. Davies once taught me that by crossing your eyes, 
which is considerably easier and may be trained to painlessness, you can 
instantly see many stereo pairs in 3D. However, a pair of zoom lenses would 
be ideal for a Holmes or Brewster viewer.

In 1997 I gave 3 papers at the AES Convention in New York, and one of  these 
was on acoustic recording, with a lot of documentation. As an inlay to the 
preprint (as they were called then) I had prepared a sheet with three 
stereoscopic pairs of original VTMC/GC recording horns, exactly the size to 
fit an old viewer. I had made the images by controlled sideways shifting of 
the camera between exposures. To my knowledge, they are the only stereoscopic 
representations of recording horns, but I would like to hear about any 
others.

The best stereoscopic illusion I have ever had was watching an Underwood & 
Underwood picture from the Eiffel tower in Paris. It was taken straight down 
from the top, and holding the viewer vertical while standing up almost gave 
me vertigo. Even though it was in b/w.
 

George


> --- Don Cox  wrote:
> 
> > It is odd that "stereo" for images means having depth, from near to
> > far,
> > while "stereo" for audio means having width. All visual images have
> > width, while few audio recordings have depth.
> 


 
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