[tech-3d] Re: Different F.L.s

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

Hi David!

In tech-3d@xxxx, siggyblue@xxxx wrote:
>> I have these stereo viewer/books I made.  The lenses I put in 
>> them are the inexpensive plastic holmes print viewer glasses 
>> sold by reel-3d.  At the time I made them I didn't know that viewer 
>> focal lengths should match the camera's.  The only camera I 
>> have now is a 35mm. Is there an adjustment I should make if I'm 
>> using the Bercovitz formula?  The each view print is 60mm wide.
>> David

Given that you are using the same formula Bercovitz uses to calculate
stereo base here's the answer to your question:

This is the formula you are currently using (which, by itself, makes no
accomodation for a mismatch between viewer FL and camera FL):

Stereo Base = deviation * [ (far pt * near pt) / (far pt - near pt) ] * [
(1 / camera FL) - (1 / focused distance) ] 

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

This is the same formula modified to accomodate a mismatch between viewer
FL and Camera FL):

Stereo Base = deviation * [ (far pt * near pt) / (far pt - near pt) ] * [
(1 / camera FL) - (1 / focused distance) ] * (viewer FL / camera FL)

This will decrease your stereo base as you increase your camera FL.

I am very pleased with the consistent, predictable results I get using the
latter formula.  (By the way, I have settled on use of a deviation equal to
70% of MAOFD, where MAOFD=Camera FL/30.  I also abandon this formula when
the Far:Near ratio drops below 2:1, using the equation Base=Near/15 instead.)

This same modifier (viewer FL / camera FL) can be applied to any method one
might be using to determine base (if that method expects viewer FL to equal
camera FL.)  David Lee, for example, does not use the same formula we use,
but he does apply this same modifier for mismatched FL's.  

See: http://www.pauck.de/archive/mailinglist/mf3d/mhonarc/msg00467.html

Mike Davis



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