Re:Stereo Depth Formulas

[bercov@xxxxxxxxxxx (John Bercovitz)]

In some situations, you may wish to go to the trouble of using the 
exact formula for on-film deviation to decide the maximum 
separation for the two perspective points of the stereo pair.  
(The word for this separation of perspective points is 
"stereobasis".)  If you have a really close-up shot, this formula 
is worth using.  The formula came about when Steve Spicer rescued 
me from some floundering I was doing a year ago.

          an*af 
b0 = d*[ ------- ( 1/f  - 1/a) ]
          af-an 


b0: This is the maximum tolerable separation of the camera's 
lenses.  If this value is exceeded, the limits of on-film 
deviation will be exceeded.  b0 will also be the on-film 
separation of image points of objects located at infinity.  
 
d = Maximum allowable on-film deviation.  You can calculate the 
on-film deviation of a stereo slide by subtracting the separation 
of two image points from the farthest object in the scene from the 
separation of two image points from the nearest object.  (The two 
images points are located, one each, on the two frames of the 
stereo slide.)  Bob Mannle gave the following values for the 
maximum allowable on-film deviation:

>              Realist format  = 1.2 mm
>              35mm Full Frame = 1.5 mm
>              Medium Format   = 3.0 mm


f = The focal length of the camera's lenses.
 
a = The distance at which the camera is focussed.
  
af: distance from the camera lens to the farthest object in the 
scene.
 
an: distance from the camera lens to the nearest object in the 
scene.
 
John B

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The inevitable PS:
Note: As we can see, the maximum on-film deviation is the true 
"one in thirty" rule.  Realist format uses 35 mm lenses and 35/30 = 
1.2 mm.  Full frame miniature format uses 50 mm lenses, and 50/30 = 
1.7 mm.  Medium format uses 80 mm lenses, and 80/30 = 2.7 mm.

The nearly-inevitable PPS:
Yes, I've tested the formula by shooting closeups and it is more 
accurate for closeups while giving the same answer for far shots 
that Waack's formula does.



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End of PHOTO-3D Digest 1058
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