Re: [photo-3d] Digest Number 94, to Boris Starosta

[Olivier Cahen <o_cahen@xxxxxxxxxxxxxxxx>]

	You should have heard of the "thirtieth rule", i.e. that the stereo
base width should not exceed the distance from the camera to the
foreground. This rule already appeared in the book "The Stereosope", by
Sir David Brewster (1856). It is very useful in macrostereo, if you want
an image without stretching, at least at the foreground: but if your
focus, or the lens-to-film (focal length plus extension) is too long,
you would get a "frustum effect", in which objects lying too far beyond
the foreground are seen too far, too large and much stretched.
	These effects are not too annoying while you watch to your pictures in
a stereoscope, but much more in projection. You must put a limit to the
maximum deviation on the film, most authors set this limit to 1.2 or 1.5
mm. This is approximately the thirtieth of the focal length of your
stereoscope. For printed images such as anaglyphs, the limit can be set
to the thirtieth of the viewing distance.
	The value of the maximum deviation is easy to calculate: the stereo
base, multiplied by the lens-to-film distance, multiplied by the
difference between the reciprocals of the foreground and background
distances (all measured with the same units).
	For normal views taken with a stereo camera, the thirtieth rule is
convenient; it means that nothing must be seen closer than 6 to 8 ft
from the camera.
	A few years ago, a calculator to find the limits of stereo base, as a
function of lens-to-film, foreground and background distances, was
designed by Gehrard Herbig (Germany), with the assumption that the limit
of ofd is 1.2 mm.
	I hope that this is an answer to all your questions.
	Olivier Cahen, Editor of the "Bulletin du Stereo-Club Français".



> Message: 15
>    Date: Tue, 23 May 2000 20:36:30 -0400 (EDT)
>    From: boris@xxxxxxxxxxxx
> Subject: ofd calculator
> 
> Hi everyone!
> 
> There was a time last year when the discussion turned extensively to
> calculating on film deviation, and what maxima are permissible, etc.  I
> largely ignored those discussions, because I knew they were not for me: I
> had always shot subjects of about the same size, relatively nearby; and for
> me shooting with something close to a "normal" interaxial spacing (i.e.
> close to 2.5 inches) with a "normal lens" (close to 50mm f.l. for 135 film)
> was something of a religion!  I shot all my slides the same way, they
> mostly looked good in a viewer (and I suppose projected), and that's all I
> cared about.
> 
> But you know how it is.  Times change, science and life present new
> discoveries and opportunities, or else curiosity and boredom drive men to
> explore uncharted waters.  If they are not gobbled up by deep sea monsters,
> religion must keep up, evolve, or be cast aside useless.
> 
> Most of the last year was spent experimenting with anaglyphs - suddenly on
> film deviation, or I should say on _print_ deviation, became an important
> issue.  Now I find myself shooting for dual (or even triple!) purposes: for
> slide dupes (for viewers), for anaglyph prints (some large, some small)...
> and who knows what formats the future may bring.
> 
> Now I ask the members of this list, or perhaps _the_ member, who
> presented/proposed some kind of on film deviation calculator - was it ever
> made?  This would be a device, preferably portable without too many vacuum
> tubes, that would correlate stereobase, near point and far point distance,
> and lens f.l. with on film deviation.
> 
> I seem to recall a spreadsheet mentioned, but I don't know anything about
> them, except all those collumns and rows make my head spin.  Maybe someone
> has a routine I can program into my HP 15C, buried in some drawer around
> here, a relic of

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