[tech-3d] Re: Stereo Base

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

Hi Joseph,

>From: Joseph Valvo  <jvalvo@xxxx>
>Date: Sat Nov 18, 2000 1:57pm
>Subject: Stereo Base
>
>
>Mike, 
>
>
>I read with interest your dissertation on stereo base calculations and
>programming the HP30s. You have made an interesting contribution. 

Thank you, but it's just an implementation of the Bercovitz/Spicer/whomever
formula, but having it on a $20.00 handheld calculator is a great way to
use it in the field.  I appreciate your comments.

>
>However, I feel the formulas somewhat cumbersome and procedure to program
somewhat lengthy. Further, with these low end calculators, sometimes
pressing the wrong key screws up the whole string. That can be disaster,
particularly half way through. 

Actually, this one is pretty easy to recover from mistakes with.  Pay
attention to this section of tips for editing the formula (quoting the
original article):

"Notes:  To move the cursor left or right, use the arrow pad.
            It's safest to tap an arrow, moving one character position at a
time, 
            rather than holding it down.
        To delete everything to the right of cursor position, use 'CL'
        To delete only the character at the cursor position, use 'DEL'
        To insert a character at the cursor position, use '2nd INS'
        To cancel INS mode, issue '2nd INS' again"

You should also take confidence from the fact that I have documented EVERY
keystroke necessary to get the formula into the calculator and to use it in
the field.  Try it!

>
>Is there some way you could condense the formula for those of us new at
Stereo, and are satisfied taking a spread sheet into the field. 

First, the formula is as condensed as it can be, and still be accurate.
I'm using this one, as seen on the Victorian 3D Society page at:

  http://home.mira.net/~kiewavly/bases.html 

             af*an      1         1
   b0 = d * (-----) * ( -  -  --------- )
             af-an      f        an*af
                              2*(-----)
                                 an+af

That's it if you want to handle all situations. 

>I would want to put the formula in an Excel spread sheet. also would
results be skewed to much if a guy used a constant for the far point? 

Have a look at the link noted above for special cases where shorter
formulas can be applied.  There are several.  

Oh, look what I just found, quoting that page:

"Special Case No. 2 - Maximise the Depth of Field, with the Far Point at
Infinity"

It's a much shorter formula - here it is (Eqn 4) without the accompanying
text:

               an         1       
   b0 = d * ( ---- ) - ( --- )
               f          2

Where d is deviation, an is the Near point, and f is the camera lens focal
length

I can tell you that this formula will be accurate until the Near-to-Far
ratio shrinks to about 1:25 - even there, the base will already be in error
by about 5%.  So, for example, you could use this abreviated formula with a
Near distance of 10 feet and a Far distance anywhere from Infinity down to 
250 feet.  As the Far gets closer than 25 times the Near, the calculated
base will become shorter than it should - the images will look increasingly
flatter as your Near:Far ratio approaches 1:1.
You can use this formula as long as the Far point is at least 25 times
further than the Near point.

Just remember that the Maximum Allowable On-Film Deviation is equal to f/30
where f is the camera focal length.  Thus, for a 36mm lens, you would
specify a value NO LARGER THAN 1.2mm for deviation.
This is NOT A CONSTANT.  Feel free to reduce this value to something LESS
THAN 1.2mm (something less than 100% of MAOFD).  Shoot several rolls that
at 80%, for example.  If you want more or less depth in your scenes, try
something different for awhile.  By sticking to one percentage of MAOFD,
you will have a consistent on-film deviation across every stereo pair you
produce.   

You might also want to throw in another multipler to come up with the final
value for d.  That would be to adjust for any mismatch in viewer focal
length vs. camera lens focal length.  For example, I use a 78mm viewer with
65mm lenses, so the value I use for d looks like this:

Percent of MAOFD  *  MAOFD  *  VFL/CFL = d 

0.80 * 65/30 * 78/65 = d

>Please respond to me direct if you choose. 

I don't have your e-mail address.  I'm new to the list - is there a way to
obtain an author's address?   Here's mine if you'd like to take this offline:

zilch0@xxxxxxxxxxxx

Have fun with your spreadsheet!

Mike Davis





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