P3D Re: double depth

[aifxtony@xxxxxxxxxxxxx (Tony Alderson)]

A couple of people have asked for an explanation of "double depth."  Since
I'm the idiot that brought it up, I suppose I should respond.

Basically, the idea is to move the stereo window forward, to accomodate a
slide with excessive parallax, yet still keep the infinity separation
constant.

Let's say we have shot a Realist format stereo with a range from 3.5 feet
to infinity, and the near object is cut by the frame of the film.
Therefore, our near object has to be placed behind the stereo window.  If
we mount to the window in a standard mask,  and then converge the frames on
the screen during projection, our infinity points will be too far apart.
(In this case, by a factor of two.)  If we reconverge our projector so the
infinity points are at about 60mm, the stereo window will be floating off
the screen, and the image will be more tolerable to view.

But, as we've discussed before on P3D, projector adjustments are strain on
the audience and are best avoided.  So we modify the mask to bring the
stereo window forward at the mounting stage, so we don't have to readjust
the projector.  And, in fact, projector adjustments undercut the whole
point of double depth.

One way to modify the mask is to tape off the outer edges of the apertures,
but it is very difficult to match the masking, and this also crops the
image.  A better way to move the window is to cut a thin strip out of the
center of the mask, then tape the mask back together again.  One should
measure the deviation on the chips to determine how big a strip to cut
(about a millimeter maximum), and care must be exercised to keep everything
square.  A quality metal straightedge is helpful.  You should use a
mounting gauge to check infinity separation.

Needless to say, this is a lot of work.  Double depth is a technique for
lunatics that want to save a slide that breaks too many rules.  It's true
that it's better to control your depth range, but sometimes you want to
take stereo where you can't control the situation.  I have used this for
party and crowd pictures, where I couldn't help having someone too close
with a too distant background.

An easier solution is to set such slides aside for hand viewing only.  And
show them only to friends and family that will forgive your transgressions.


See Ferwerda in "The World of 3-D" for more info.

Tony Alderson
aifxtony@xxxxxxxxxxxxx



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