P3D The "PerPax principle" Challenge!

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

For example, I can explain the PerPax principle (I will do
it in my series when we discuss the effects of focal length and
stereo base on the sense of depth) without any complicated 
geometrical concepts.  Just intuition.

This principle was brought up by H. C. McKay and it simply says
the following:  "If you increase the focal length of the recording
lenses you should increase the stereo base at the same ratio, to
get an orthoscopic view."

For example, if you use 45 mm lenses in your camera and viewer
and 2.5" stereo base (ortho stereo) and one nice day you decide
to triple the focal length of your recording lens to 135 mm to
take a portrait, then you will have to triple your stereo base
to 7.5" to get a good stereo.

There has been some controversy regarding this principle.  It
has been proven with simple geometric principles that these
two actions do not cancel each other and will result in a stereo
that is not equivalent to ortho stereo (i.e. there are problems
and I person can see them).

McKay was a practical man: "It you don't see it, it's not there."
Yet, others insist that in the PerPax principle the problems
are there and you can see them.

During last year's PSA convention in St. Charles IL, legendary
stereo photographer Otto Walasek (EFIAP, APSA, Diamond+ :-))
showed a stereo pair of a model's face taken with 600 mm lenses
(!!!!) separated quite a bit from each other (following the 
PerPax principle of ratios) and said:  "Some people say that
you cannot do this.  Well, I did it, and it works!!!"  He was
kind enough to let me borrow and copy the slide because I wanted
a stereo pair with two eyes.

So, here is a list of questions:

- Why when you increase the FL you need to increase the stereo base?
- What happens if you don't do it?
- Is increasing them in the same ratio (PerPax) a good idea?
- What are the problem with the PerPax principle?
- Under which conditions can these problems be seen?
- Can McKay, Walasek and the critics, ALL be right?  

The answer to this last question is, for me: "Yes!"  

I'll explain how and why (using simple intuition) when we reach the 
appropriate installment in the Stereogram series.  Also I will
give recommendations for nice, naturally looking portraits with
twin SLR cameras.  In the meanwhile, you are welcome to take up the 
challenge here in photo-3d! (I think we have done it in the past.... 
Result:  Chaos!)

-- George Themelis


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