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

["Greg Wageman" <gjw@xxxxxxxxxx>]

From: Tom Deering <tmd@xxxxxxxxxxx>


>On 8/30/98, Dr. George A. Themelis wrote:


>>No, I am sorry, I disagree Tom.  We have already decided how much
>>is "too different"  We arbitrary set this to 1.2 mm on film deviation.
>>This is an assumption.  There is no realy physical or experimental
>>evidence to support this assumption.
>
>If you want to learn about the 1:30 rule from someone who actually
>understands it, try http://www.werple.net.au/~kiewavly/bases.html.
Note
>the graphs near the bottom.  Compare the formula that Bercovitz calls
"the
>most accurate method" (#1), with George's 1:30 rule (#4).


Well, I've been following this thread, and I went and looked at the
above-mentioned page myself.  First, in the definitions section, we
find:

"d: this is the on-film deviation, and is equal to the difference
between bf and bn (ie: d=bf-bn). The practical maximum allowable value
of d when using the 35mm format about 1.2mm."

Note the use of the words "practical maximum" and "about".  This
suggests to me that the number is an empirically-derived figure that
accommodates a wide range of potential audiences.  Just like we know
that not everyone's interocular is 65mm, and depending on what
population you sample, it may not even be the "average"!

Second, in the section labeled "Aim", it says very clearly:

"The aim is to make use of the available 1.2mm on-film deviation
effectively so as to maximise the stereo effect within the view or at
least not decrease the stereo base when it is unnecessary."

Dr. T's assertion that the 1/30th rule will not violate the 1.2mm OFD
requirement is correct; your implicit assertion that the Bercovitz
formula gives "optimum" *depth* is also correct (for the empirical 1.2mm
OFD).  Both will give viewable pictures; whether the maximization of
depth is desirable in a particular composition is a subjective
determination.

If your goal is to maximize depth in every shot, then one should use the
Bercovitz formula instead of the 1/30th rule, but that's about as far as
one can go in stating any great truths from this debate.

     -Greg W. (gjw@xxxxxxxxxx)



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