T3D Re: stereo math

[john bercovitz <bercov@xxxxxxxxxxx>]

> With John Bercovitz in the audience, I'm encouraged to bring up a math
> question.  Forgive me!

Sheesh!  What do you think this is?  A technical list?  

> I visualize 1.2 mm of onfilm deviation as being related to the amount of
> convergence the eyes can comfortably do.  Would it be just as correct to
> measure it as an angle?  And is that why the 1.2 mm is actually f/30?

It's not actually convergence which is the limit but in all other respects
your conjecture is correct.  As an example, if your eyes converge on some-
thing which is 250 mm away and your eyes are 65 mm apart, the included angle 
is about 15 degrees which is a whole lot more than 1 in 30.  

For reasons not solidly known, by anyone I think, in stereo pairs all 
people can tolerate a total on-film deviation of 1/30th the focal length 
and most people can tolerate 1/15th under decent conditions.  (I'm using 
"focal length" loosely here.  When you shoot a closeup, you move the lens 
away from the film so the "focal length" increases for the purposes of this
calculation.)

> And is any of this related to the reason why perfectly mounted slides 
> don't look right when projected?

I'm not sure what you mean but I'm limited by the internal concept that 
"perfectly-mounted" means "projects well".  Can you give me an example?  
I'm sure that would clear things up for me.

Thanks,
John B


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