P3D Re: mounting to infinity (pt 2 of 2)

[roberts@xxxxxxxxxxxxxxxxx (John W Roberts)]

>Date: Mon, 8 Dec 1997 20:34:40 -0700
>From: Larry Berlin <lberlin@xxxxxxxxx>
[John R]
>>The angular
>>convergence of the eyes to converge on any particular object in the field
>>of view would then be much closer to what it would be in "real life" than it
>>would for a stereo-window mounted photo for which the real and virtual
>>infinities didn't happen to match up.

>>As noted before, mounting to infinity can cause significant viewing problems
>>if the image is *projected* onto a large screen, causing the disparity of
>>the projected image to become greater than the normal human interocular.
>>Also, I don't believe that stereo photos should always have as much
>>orthoness (be as orthoful?) as possible - but for those who really want
>>ortho, I expect that mounting to infinity would be useful.

>*****  Mounting to infinity, as I understand it from your discussion here,
>then is like taking pictures and ignoring the lens of the camera. 

I have no idea what you meant by that. (I deleted part of the quote, but I
couldn't see any connection in the deleted part to this statement.)

>Mounting to infinity such that a projection of
>the image causes too great a disparity is the equivalent to having the
>picture be out of focus by being too far or close to the lens. 

No, mounting to infinity (or "ortho mounting") can cause problems for
projection because the photographer often does not have control over what
magnification will be employed in the projection to screen at the stereo
club, etc. If the scene photographed doesn't contain the full range of depth
from the stereo window distance to infinity, then shifting the apparent 
distance by mounting to the stereo window moves the apparent distance of
the nearest object closer (thus reducing the risk of excess disparity in
projection with excessive magnification). If the scene photographed uses
the full depth range from the distance of the stereo window to infinity,
then there's no difference between mounting to the stereo window and mounting
to infinity.

The *worst* combination is photographing a scene which includes details at
infinity, and with the nearest object closer than the stereo window, then
trying to mount that photo to the stereo window - this will cause unacceptable
disparity of the most distant objects even when using a viewer.

>I would propose that it's incorrect to classify mounting to infinity as
>automatically ORTHO, since any ORTHO reconstruction could not occur unless
>the window is properly positioned relative to all image content, both near
>and far objects. 

I tried to make that clear.

>If an image has too great a total depth for instance, NO
>position of mounting will give 100% Ortho results. 

John B conducted some actual experiments a year or two ago, that would seem
to contradict that statement (depending on how you define "too great").
If I recall correctly, John took stereo pairs with too much depth for mounting
to the stereo window, and was able to mount them so that this excess depth
could be used.

>Since my eyes can allow
>for infinity at a wide range of actual positions and my mind interprets the
>scene relative to all these inner relationships, it seems mounting relative
>to the window is the more accurate method. 

Not if by "accurate" you mean "accurate reproduction of reality".

>If images are mounted with
>awareness of the window in mind, like getting a photo focused relative to a
>lens, it should work better for either a hand viewer or projection.

Well, that's what I would generally be inclined to do - but I don't consider
myself to be an "ortho-fanatic". The topic of this discussion was to clarify
some points relating to orthostereo photography, not to argue that all (or
even most) stereo should be ortho.

John R


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