Re: BOGUS - 1/30 Rule!

[bercov@xxxxxxxxxxx (John Bercovitz)]

Great definitions, Bob!  I wish I could write so clearly.

I agree with Bob Mannle that the 1:30 rule, as usually
stated, is bogus.  However, I'm going to have to disagree 
on one point.  IMHO the 1 in 30 does apply to the lens 
focal length:

Normal lens for stereo camera = 36 mm
maximum allowable on-film deviation = 36/30 = 1.2 mm 

Normal lens for full frame = 45 mm
maximum allowable on-film deviation = 45/30 = 1.5 mm

Normal lens for medium format = 80 mm
maximum allowable on-film deviation = 80/30 = 2.6 mm

These are the maximum allowable on-film deviations usually 
used in stereo work and no, it's not a coincidence that
the figures come out that way.

As you can see, the 1 in 30 rule does work if it's restated.
The problem is that if you include infinity in the scene, 
the rule degenerates to the commonly-stated version which is 
correct only when infinity is in the picture and is incorrect 
at all other times.  My reading of Ferwerda and Waack leads 
me to infer that the early workers knew the correct form of 
the 1 in 30 rule but that the correct form got lost in an 
attempt to make the rule easier to remember and calculate and 
so all we have left today is the degenerate form.

There... I feel much better now, too.  8-)

John B


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