P3D depth calculation

[rmt <rmt@xxxxxxxxxxxxxxx>]

Tom Deering has created some attractive tables to calculate allowable
depth at various distances using Realist-format cameras.  He writes: 

While the exposure and composition were perfect, it was hard to view. 
This slide had too much depth.  I've designed a simple tool to help me
avoid
this situation in the future.  You can see it at:

http://www.deering.org/curves.html

These interesting tables seem to restate graphically the Charles Piper
Theorum (as I call it) that permissable depth for a Realist-format
camera stereogram may be calculated simply using the standard depth of
field scale on the camera, using f/5.6 in the case of the Realist.  I
have compared several combinations using the Kodak Pocket Photoguide
Depth-of-Field Computer, and the correlation appears to hold for all
possible combinations.  The 1 in 30 rule (or 1 in 25 for hand viewer and
1 in 50 to ensure projectability as Piper has it) does not assume, as
Deering states, that maximum distance is infinity.  Rather, it is the
minimum distance from the closest object to avoid what Ferwerda calls
"close-up misery".  This "rule of thumb" is not part of a depth
calculation, but rather a good starting point for closeups.  Obviously,
a C/U of a coin or other object with slight depth could tolerate a
smaller figure.

In Piper's work he shows how to calculate permissable depth for
non-standard baselines, such as slide-bar work (which is certainly
easier with an SLR than with a Realist on a slide-bar), but the latter
will have ease-of-mounting advantages when used with a mounting system
such as Emde.  Hence the ability to focus closer than is tolerable for a
stereogram with a 70mm baseline.

The point is, that in the field, if you confine depth to within the f5.6
DOF scale on the camera, no calculations are necessary.  I don't want to
quote Piper at length as his superb text is still in print and for sale,
but as the Super Duplex has been discussed recently, let me add that he
calculates f/12 as the parameters to use for this camera.
I await the flames...
Dick Twichell


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