P3D Re: PS on "why the 1/30 rule does not work"

[Tom Deering <tmd@xxxxxxxxxxx>]

On 8/29/98, Dr. George A. Themelis wrote:
>I am sorry if I sound too harsh on my good friend Tom.   He has
>done an excellent job in presenting complicated formulas for
>us to use right away.
>
>My only reservation is on the nature of these formulas. I think
>they predict a limit, not an optimal situation.  Still, it is
>good to know the limits. The 1/30 rule appears to be a conservative
>approach.  It works but tells you very little about why it works
>and how far off the limit it is.

We play a trick on our brain when we look at a stereo slide.  We can trick
our brain into seeing stereo if we show each eye slightly different views.
It the slides are too similar, we see almost no stereo.  If the slides are
too different, the trick breaks down and the slide is uncomfortable to
view.  How much is "too different?"  That's what the formulas try to
predict.

Turns out that pretty simple geometry can predict how much is "too
different".  It's just that most of us are unable to do geometry in our
heads.  For this reason, the 1:30 rule-of-thumb was developed.  It's not as
accurate, but you can do it in your head.

The 1:30 estimate is close enough for many situations, but it doesn't work
very well for closeups.  You can compare the 1:30 estimate with the full
geometric solution by looking at http://www.deering.org/basis.html.  You
can decide whether you need the simplicity or the accuracy for yourself.

Cheers,

Tom



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