P3D Re On eye separation and viewing

[abram klooswyk <abram.klooswyk@xxxxxx>]

Hans A.J. Middendorp wrote (p3d 3750):

>>I've read this before, that interocular is not important, 
but :-) :then why can I always tell when my wife has been 
using the viewer? It is _definitely_ inconvenient viewing [or 
'frustrating', pun intended :-)] when the interocular is 
adjusted to smaller faces.

Having not really understood the frustrum discussion a couple 
of months ago on this list, I usually realise that something 
is possibly wrong with the relative spacing of the chips if I 
suddenly need to adjust the interocular from its usual 
position. The question is, has this anything to do with 
frustrums or is this something else again? <<

As Gabriel Jacob and Bruce Springsteen have pointed out 
(and before them Herman von Helmholtz over 130 year ago)
the interocular of stereoscopes should be set to the same
separation as the infinity points on the stereo image.
With large enough viewer lenses anyone can look with
parallel viewing lines (eye axes) at infinity.

However, this assumes that the focussing is set at infinity, 
by setting the distance from the images to the lenses equal to 
the lenses focal length. This is often not the case in non-
focusable viewers, where focus often is closer.

Moreover, when focusing _can_ be done, people (like Mrs 
Middendorp ? :- )) tend to set the wrong focus. They know 
there is a small picture very close to their eyes, so they 
accommodate for some close distance when looking in the
viewer, and then adjust the viewer to this wrong 
accommodation. Then again they change the interocular,
sometimes to conclude they cannot see the image.
When they do see an image, try to put a test slide in, like
a random dot picture, and you find out that they actually
see only one image in mono.

Using a stereoscope really isn't easy...

Abram Klooswyk