[photo-3d] Depth ranges (was Re: The Stereoscopic Society Annual Competitrion)

[Paul Talbot <list_post@xxxxxxxxx>]

I wrote:

> My guess: only a very small stereo base is needed when there is
> something that close to the camera and the scene extends a long
> way.

Now that some time for background cogitation has elapsed, let me
try to expound.  Again, DrDave originally asked:

| This is what appears to be a macro image of agaric mushrooms, but
| what I don't understand  is how he was able to get an infinty
| background in without too much deviation?

I'm sure Abram can explain it better, but I like the suggestion he
has made in the past to visualize an available 3D image "space."
(Please excuse my paraphrasing errors.)  When we create 3D images,
we should try to fit the original scene's total depth range within
the recommended maximum 3D image space (usually expressed as 1.2mm
MAOFD*, or as "2 degrees.")  A scene that ranges from about 7 ft to
infinity, recorded with a normal stereobase, fits nicely within (and
just fills up) the maximum 3D image space.  Still using a normal stereo
base, we can also "fill up" the maximum 3D image space by shooting
a scene that extends from (roughly) 4 feet to 10 feet.  The same is
true for a scene that extends from about 2.5 feet to 4 feet.

* MAOFD = Maximum allowable on-film deviation

(Note, for "simplicity," I am leaving out all discussion of FL of
viewers and cameras.  Assume a normal Realist 5P system for purposes
of these examples.)

Some people promote the use of the MAOFD formulas, because they
are a way to ensure that the maximum 3D image space is always
completely filled up.  That means that if our scene only ranges
from, say, 30 feet to infinity, we should increase the stereo
base.  And in fact a hyper stereo base is probably better suited
to such a scene.  But suppose we change the 4 to 10 feet example
above and say the scene only extends from 6 feet to 9 feet.  If
we use a normal stereo base, we will not fully utilize the maximum
3D image space.  If we are so inclined, we might decide to use the
MAOFD formula to calculate the larger stereo base required to fill
up that 3D space (but depending on the subject, it might look rather
strange when viewed stereoscopically).

Now many of us are accustomed to hearing that we should not record
a scene that extends, say, from 4 feet to infinity, because it will
"have too much depth," "exceed the MAOFD," "be impossible to mount,"
or "be difficult to view."  But note that we *can* create an easily
viewable scene with this over-large depth range--we just can't shoot
it normally with our Realists! So how do we do it?  Simply the reverse
of what we did with a hyper base!  We saw how we could use a larger
stereo base to expand a 30ft to infinity scene to fill the 3D space.
Now we just select a reduced stereo base to compress the 4ft to
infinity scene so we can squeeze it into the recommended maximum 3D
image space.

So, already knowing that the image was in fact made successfully,
http://www.stereoscopicsociety.org.uk/Pages/Slides_A.htm
I believe the formula would tell us that there is in fact a
stereo base that would allow the mushroom scene to be recorded
without creating an excessive amount of deviation on the film.
What is that base?  Well, if the distant trees are at "infinity"
as you initially supposed, and the mushrooms are at, say, 6 inches,
the simplifed version of the formula (aka the 1/30 rule of thumb)
tells us that a stereo base of 0.2 inches (6"/30) should do the
trick.

If the distant trees are not at infinity, the full version of the
MAOFD formula can be used to calculate the maximum stereo base that
will allow the scene to be squeezed to fit within the 1.2mm OFD
guideline.  (That number will be larger than 0.2 inches.)

Paul Talbot, who hopes he has earned a passing grade from Abram!  ;-)