P3D Re: rules

["Dr. George A. Themelis" <DrT-3d@xxxxxxxxxxxxxxxxxxxxxxxxxxx>]

>McKay's excellent writing is marred by the completely illogical statement
>that in stereo photography, 'if you can't see it, it isn't there'.

I quoted this from McKay's introduction in his book "Three-Dimensional
Photography" (1953).  The point that McKay was trying to make is to
pay more attention to stereo*scopy* (seeing stereo pictures) versus
stereo*grammetry* (measuring stereo pictures).  What he is saying
is, I believe, to not let small details, theoretical considerations 
with little practical significance, etc., distract us from the main 
purpose of enjoying stereo photography.  He was promoting worry-free 
and guilt-free stereo photography.  He is also promoting experimentation 
by encouraging people to "break the rules" and "see what happens".

Peter mentions the issue of stereo mounting.  McKay was very aware of 
the importance of accurate mounting and he has written on that too.  
He believed that faulty mounting was giving stereo a bad name.  He 
appears to have promoted the EMDE mounting system, which, in my opinion 
too, was (is) a solid, fool-proof system.  Once you follow the 
recommendations, i.e. use a "Distant" mount for scenes from 7 ft
to infinity, a "Medium" mount for scenes from 5 ft to 20 ft (don't
recall the exact distances) etc., there was very little that could
go wrong.  The chips are held in corner pockets, making rotation
or other errors very unlikely (unless intentional).  The stereo window
is positioned automatically.  A great system for beginners! 

Another area where McKay's "principle" applies, is the area of lens
toeing in.  We are told that we should not converge the lenses
but point them parallel.  We know (and can derive mathematically)
that toeing in causes keystone distortion.  Yet, there are many
cases where toeing in makes sense.  When shooting hand-held
hyperstereos it is easy and safe to point to the main subject
(this results in toeing-in).  There are cases where having the
appropriate stereo window (at the point of convergence of the lenses
is very important.  There are cases where without toeing in there
would be no picture.  So, we toe-in.  Can we see the distortion?

Some time ago Boris asked us to look at his images and identify
the ones with keystone distortion (part of his early work).  We
took our rulers out and measured carefully.  We could not see
anything.  Boris told us that all of them had keystone distortion
because he was toeing in the lenses in his twin SLR rig, before
he knew better.  Well, it was there but no one could see it.
Applying the McKay principle "if we cannot see it, it is not there".
And we are all happy!

George Themelis


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