Convergence and deviations; some examples

[P3D Dr. George A. Themelis <fj834@xxxxxxxxxxxxxxxxxxxxx>]

Here is a list of situations where I used convergence to record stereo
images (in reverse chronological order):

- Portrait of my daughter Lea with twin SLRs mounted bottom to bottom.  I
used Dr. Bernard Stell's bar and demonstrated both the equipment and the
picture in the last Cleveland club meeting.  The bar will let you control
the camera separation and convergence.  Convergence is needed when you use
long lenses for a close up in the short direction of the film.  If you do
not use convergence, you will not be able to mount the pair in a Realist
mount.

- Hand-held hyperstereos including the Manhattan Cityscape.  I try to
maintain the central object in the viewfinder and this is usually the main
object.  The end result is that the lens' axis converges to this object.

- Scanning Electron Microscopy stereo pairs.  All high magnification pairs
(that I am familiar with) are taken with convergent axes. 

All these situations reflect either convenience or necessity and the
vertical deviations are small.  But the author of this article in the
"In Depth" newsletter (I don't have it handy right now) insisted that are
other (good) reasons to use convergence.  I'll try and find the article and
post a summary.

In another case, I viewed pictures of a sample at different stages of
deformation (I treated pair of pictures as a stereo pair, even though the
camera was stationery, only the sample was being deformed).  With the major
deformation axis in x-direction (the direction of the eyes), I was able to
visualize the strain field.  Needless to say, there was also deviation in
the y-direction, but that did not stop me from using the method.  

What's my point?  Don't let unwanted deviations stop you from recording or 
viewing a stereo pair if that's the only way to do it.  But, obviously, in 
most cases of twin or single camera stereo photography, that's not the only
way to do it.

-- George Themelis


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