P3D Re: Setting up for stereo pairs?

[Michael Watters <mwatters@xxxxxxxxxxxxx>]

>Is there a formula for determining the distance I need to move the camera
>in relation to focal length of my lens and or camera to object distance?

The full-blown version of the formula is this:

B=p*S1*S2
     ______
     F*(S2-S1)

Just in case that looked like garbage on the screen:

B=pS1S2/F(S2-S1)  in linear form

B=Base seperation
p=deviation on film, 35mm max is around 1.2mm or so.
F=focal length of lens
S1=distance to near object in picture
S2=distance to far object in picture.

I like the following simplification:
Let's assume 2 things:  First, we'll set p at 1mm.  The max is close to 1 and setting it to 1 allows it
to drop out of the equasion (as long as we figure F in mm that is) AND gives a little room for
error.

Second, we'll assume S2 is infinity.  If you're taking pictures outdoors this is basically true 

anyway so it's not a big assumption.  If S2 is set to infinity, S2-S1 is roughly equal to S2 (infinity  
minus a fixed number is still infinity) so the S2's cancel each other out.  We're left with the 

following:

B=S1/F

ie.  The optimum base seperation is the distance to the near object divided by the length of the
lens you're using (in mm).   If you're using a 35mm lens, that becomes a 1 in 35 rule.  If it's a
50mm lens, it's a 1 in 50 rule.  Sound familiar?   Course if you're using fisheye's or telephotos
it's different.  That's the only time it gets really useful.

My habit with twin rigs is that the cameras RARELY are mounted to a different base seperation.
The formulas are only useful to let you know what the nearest allowable distance is so you
don't stretch it when you're shooting.

mike


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