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

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

Subject: Re: Stereo Base Calculation with a $20.00 Handheld

Dear Mike Davis,
Quite a defense you wrote (22 Nov 2000) in favor of
the "General Solution"!     
Just a few comments. 

Of course the equation solves for base, but, supposing a certain 
camera and a certain scene, some of the right hand variables are 
more or less settled in the practical shooting: focal length, near 
and far point distance. 
Then only the deviation value should be _chosen_ as input, and 
newbies tend to find this difficult. Of course test rolls will 
give most people experience, but then, why a formula at all and 
not just testing different bases?

I wrote:
>>When you want to emphasize tiny scratches on ancient coins, 
>>stretch might be your purpose. 
You answered: 
>It is apparent that your intention here is to suggest that in a 
>situation where stretch might be desirable, I would use the 
>General Solution and this would somehow prohibit that goal. 

I'm sorry not to have been clear enough on this point. 
My intention was just the opposite: the so-called General 
Solution tends to give stretch in many cases, but that should be 
a purpose only in selected cases, like with the ancient coin. 

>The formula matches the geometry perfectly.  It is both valid 
>and useful.  
As said before, the matching to geometry is a matter of taste. 
I like the concept of Viewing Space, where the difference in 
convergence of the eye axes is about 2 degrees between 2 meter 
(7 feet) and infinity. 

Base formulas which include far point distance as well as near 
point distance (in object space), tend to squeeze, or mostly 
stretch, the extent of the photographed scene to _cover_ the 
_entire_  viewing space from infinity to (mostly) 7 feet. 
Remember that taking pictures is not the end, mounting, viewing and 
perception follow. Several years ago I have put the whole process 
from object space to viewing space in a spreadsheet
(its gone now in the dust of MS-Dos 2 and 5 1/4'' floppies), 
showing how a test object (cube) would be deformed by the variables 
of the process. 
That the "general solution" squeezes or stretches follows directly 
from the math. That I call the Procrustean approach. 
Moreover, the effect will vary from slide to slide, when the 
formula would be used consequently. 

Of course you can use different percentages of deviation. 
>John Bercovitz and others have preceded me in communicating that 
>one should select a DESIRED on-film deviation...
But then sooner or later the newbie will ask for a formula :-) 
to help him chose the deviation... My "Carrolleschery" was just a 
pun, but such formulas have been published. 

>It is my personal preference to use a variable base, following 
>the General Solution, for nearly every situation, including the 
>mid-field.
A funny thing is that John Bercovitz, who started the original
Tech-3d 
list but has not posted here since long, used to call himself 
"Orthoman", meaning that he was (is?) in favor of orthostereoscopy 
all the time. His site is still online:
        http://www.angelfire.com/ca2/tech3d/
Orthostereoscopy is an orthodoxy which condemns using anything 
other than a normal (about 6.5 cm) base for the midfield.

I understand that you don't use the "General Solution" always, so 
is it too ambiguous (:- )) or is it not so general after all? 
"Useful for all but macro work" you write, and "I abandon the 
General Solution at Near:Far ratios less than 1:2". 
(Your original post is also at:
http://www.egroups.com/message/tech-3d/48    )

The "truth embodied by the General Solution" which you defend so 
"vigorous"ly  just has not been accepted generally as useful for 
general use, especially not for the mid-field (which still is the 
subject in a majority of stereopictures).
The precursors of the formula in this 20th century (we are almost 
in its last month now) have been disputed by many well known 
stereographers since before you and I were born. 
Some other time I have said that "Private" Solution would be a 
better name than "General" Solution .:-)

>the Maximum Allowable On-Film Deviation is equal to f/30.
This again is disputable. Read what Ferwerda and Krause wrote in 
their books on "double depth",  which for the 5P system means 
2.4 mm deviation, about 1/15 * camera focal length. 

>Useful for all but macro work.
In my experience a general formula like you mean _can_ be used 
for macro, if only (bellows) extension is included in the math, 
accounting for "working" focal length. Also, as in any stereo 
photography, it must be known if a more or less normal way of 
mounting and viewing is used.
(Surprised that I defend a "general solution" against you? :-))

The bottom line is that any formula or rule will work for the 
knowledgable person who understands stereogeometry and 
perception of stereo images, and that personal taste, not 
mathematics, will be the final judge of what is acceptable.

Abram Klooswyk



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