[tech-3d] Re: Stereo Base Calculation With a $20.00 Handheld

["Abram Klooswyk" <abram.klooswyk@xxxxxx>]

Stereo Base discussions will come up again and again, it is 
like the vote counting game which is so popular in some 
underdeveloped countries.

In fact the oldest famous protagonists, Wheatstone and Brewster, 
already quarreled on base assessment, see:
http://www.egroups.com/message/photo-3d/2236
and subsequent postings.

I always tend to laugh a little when some recent formula is supposed 
to be new, like it seems to be the case now again. 
Limiting stereo base to get a viewable picture is old hat, elaborate 
formula's go back at least to Colardeau, who incorporated near and
far points in a general formula in the 1920s. 
(Colardeau also has invented the film progression scheme used in the 
Stereo-Realist, but before that in the Homeos.)

Michael Davis wonders why I call the so-called "General Solution" 
paradoxical.
The formula is for base calculation, to get a base which results in 
a viewable deviation. Many newbies like such a rule. However, the 
formula next asks for an _input_ of the deviation which it should 
provide as _outcome_  on film, after _using_ the calculated base. 
Newbies will ask: what deviation?
You can calculate it from the base, and than get in an endless 
regression. In discussions many different values for deviation are 
mentioned, which isn't particular helpful for newbies.

David Lee, in:
www.berezin.com/3d/Tech/lens_separation_in_stereo_photog.htm
has analysed the components of a base formula in a nice way. 

I have done a similar analysis many years ago in a German journal. 
In discussing what David calls the "far point factor" I have called 
formula's with this factor "procrustean", after the mythical giant 
Procrustes, who adapted his guests to a bed by cutting of portions 
of their legs, or stretching them to fill a large bed. The latter is 
exactly what the formula does.
[An off topic remark: for the Greek treating guests badly was a 
severely violation of the divine law of Zeus, for Zeus Xenios was 
the protector of foreign travellers. Xenophobia was just against the 
law of guest rights.]

When you accept a Standard Stereo Viewing Space which reaches from 
7 feet (about 2 m) to infinity (which is the 5P standard), then you 
don't _have_ to fill up that space in every stereopicture. But in 
using the formula you nevertheless will, unless you use a different 
deviation input for every different scene depth. But then you need a 
formula to compute...   (see above).

David Lee recognizes the stretch effect, in saying that a "far point 
factor larger than 2"  "will look pretty weird".

Wheatstone has noticed the stretch and squeeze effects (1852):
"(...) M. Claudet prepared for me a number of Daguerreotypes of 
the same bust, taken at a variety of different angles, so that I 
was enabled to place in the stereoscope two pictures taken at 
any angular distance from 2° to 18° (...)".
Then Wheatstone describes the "undue elongation", "features   ... 
exaggerated in  depth", and on the other hand "undue shortening". 
"The apparent dimensions in breath and height remain in both cases 
the same."
                                                        
In the so-called "General" formula the factor:

                       far point
       ------------------------------------
                far point - near point

can become infinitely large when the difference between near and 
far gets smaller. Shallow subjects will be stretched to fill up the 
viewing space from here to eternity.
Than it becomes a matter of taste and purpose. 

When you want to emphasize tiny scratches on ancient coins, stretch 
might be your purpose. But Michael Davis says:
>It is my personal preference to use a variable base, 
>following the General Solution, for nearly every situation, 
>including the mid-field. 

When this would include a portrait of a person before a nearby 
"backdrop" I'm afraid I would call it weird, to say the least... :-).
Should I quote Sir David Brewster again?
"To add an artificial relief is but a trick which may startle 
the vulgar, but cannot gratify the lover of what is true in 
nature and in art."  (1856)

Abram Klooswyk


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