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

["Michael K. Davis" <zilch0@xxxxxxxxxxxx>]

Abram,


>Date: Mon, 20 Nov 2000 15:52:01 -0000
>From: "Abram Klooswyk" <abram.klooswyk@xxxxxx>
>Subject: Re: Stereo Base Calculation With a $20.00 Handheld

[snip]

>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.

There is no paradox.  You could apply your argument to the push-pull
relationship between aperture and shutter speed.  But unlike the
relationship between aperture and shutter speed, where we quite often
toggle between wanting to fix one and solve for the other, any hope of
calling the General Solution a parodox vanishes when we acknowledge that
Base is on one side of the equal sign, ALL BY ITSELF, and deviation is one
of several variables on the other side.  

The General Solution is an equation.  The author had a reasonable
expectation that users would understand they are *solving for base*, NOT
solving for deviation, not solving for focal length, not solving for near
point or far point, etc.  I have yet to see the General Solution expressed
as a solution for deviation - with a  d  all by itself on one side of the
equal sign and base specified as a variable along with all the others, on
the other side of the equation.  

I can't imagine any newbies actually doing the math to rearrange the
General Solution in the first place, much less finding themselves in an
endless regression, solving for deviation, then for base, then for
deviation, then for base, etc.  Your argument is quite simply fantastic.

Everyone I've read about or spoken to who uses the General Solution, is
completely content to use it for one purpose - solving for base.  Base is
the output.  

The formula is used to deliver a base value that will PRODUCE the on-film
deviation you have CHOSEN to specify as a variable.  The General Solution
can deliver numerous bases, each unique to the scene described by the Near
and Far variables, but these bases, though different from one scene to the
next will CONSISTENTLY produce the CHOSEN on-film deviation, within the
range of bases permitted by your equipment and technique, of course.

It is not at all difficult to select an on-film deviation value that
satisfies your personal taste for how much depth your images will have. 

My original post, as seen at 

http://www.pauck.de/archive/mailinglist/tech-3d/mhonarc/msg01919.html

suggests shooting with bases that are calculated using a deviation equal to
80% of the Maximum Allowable On-Film Deviation.  I also explained that
MAOFD is just the camera focal length divided by 30.   80% of MAOFD is just
a place to start.  Shoot a few rolls at 80% of MAOFD to get a feel for it.
It's subjective.  It's up to you to determine if you find it appealing.  If
80% of MAOFD is a little too strong for you (too much "bite" for your
taste), try a few rolls at a lesser percentage.  The General Solution will
deliver the exact same deviation on-film, every time.  And even if you've
found that something like 75% MAOFD suits your needs most of the time,
there's nothing stopping you from CHOOSING more or less depth for a
particular scene you are about to shoot.  You are in control, but it's your
responsibility to get a feel for the numbers.

[snip]

>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... :-).


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.  This assertion is incorrect on two
counts.  First, using the General Solution, I can add stretch at will by
increasing the deviation used to calculation base.  The General Solution
does NOT have to be calculated using the Maximum Allowable OFD or any other
fixed value for deviation.  Second, the article from which you quote
clearly states that I abandon the General Solution at Near:Far ratios less
than 1:2.  I discuss that at length across several paragraphs. (See the
link above.) Did you miss that portion of my article or are you just
spinning my words? 

I will vigorously defend your right to prefer more or less on-film
deviation than I do.  I will support your right to select and use equipment
and methodology that YOU find satisfying.  But I can not refrain from an
equally vigorous defense against absurd attacks on the truth embodied by
the General Solution.  The formula matches the geometry perfectly.  It is
both valid and useful.  

Why must there be a steady onslaught of condemnation for something so
unambigous?   I'm convinced it's mostly just a matter of:

ignorance (I don't know how to use it so I hate it.)
arrogance (My many years experience have canonized my techniques.) 
pride (I will never admit I'm wrong about other people's techniques.)
laziness (I'll just burn a lot of film and see what I get.)
contentiousness (I live to stomp on other people.)

or some combination of the above.  

Mike Davis


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