American Cinematographer - July 1983 - 103

subject distance in flat
photography just as it is in 3-D. In

approach to exploit the elongation ef¬

to be

fect associated with variable

aters, it is important to understand

order to frame

vergence

function of

a close-up with a wide
angle lens the camera must be closer
to the face than the eyes would nor¬
mally be when ones attention is con¬

centrated

on a

face. As

a

result

a

face

rendered

by a wide angle closeup
appears somewhat distorted.
The same thing applies to 3-D since
as

it is the distance from the

camera

combined with the interaxial which
creates the

depth relationships. As an
object recedes into the distance, the
depth cues created by a given inter¬
axial rapidly become less significant.
With

a

21/2" interaxial and

a

con¬

photography but in a more

controlled way.
The choice of

one

theoretical ap¬

proach over the other is largely a mat¬
ter of aesthetics. It depends on what
kind of 3-D effect

one

wants to create

in the theater. It also

depends on what
equipment one has available for pho¬
tography. Given the widespread pref¬
erence for the more compact single
camera systems from a production
point of view and the fact that double
camera footage must be optically

compatible with current the¬

the variable convergence
3-D photography.
If

the other hand

aproach to

prefers the
by variable interaxial
photography and the budget permits
the use of a double camera rig, there
is a fairly simple formula which will
probably suffice to determine an ap¬
propriate interaxial for a given stereo
on

one

look achieved

window distance:
Stereo Window
Interaxial

=

(in inches)

converted into the over-under format

Distance in feet

x

Depth to Width
Ratio @ Window

2.5 inches
Viewer to Screen
Distance in feet ■

35mm

lens the screen parallax created by an
object farther than 60 feet away is
generally pretty negligible in terms of
any 3-D effect in the theater. As the

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interaxial is reduced this distance is
also reduced, which is

why an advo¬

cate of variable convergence will say
that reducing interaxial in full scale

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however; this reduction in apparent

depth of the scene may seem natural.
This is obviously a subjective judg¬
ment that can only be made based on
viewing comparative tests.
Varying the interaxial distance of¬
fers the advantage of never forcing a
viewer's eyes to diverge and never
forcing them to converge abnormally
(assuming the interaxial is set prop¬
erly for the scene), but it complicates
the requirements for the camera rig.
Most single camera 3-D systems are
built around a fixed interaxial (al¬
though Stereoscope offers an inter¬
axial range from %" to 8".) Vlahos
estimates that the interaxial could get
as small as Vs" to Va" for closeups on a
feature production.
It is also possible to take a hybrid
approach in which both the con¬
vergence and the interaxial are varied
during photography. Generally when
varying the interaxial it is customary
to let the lens axes remain parallel. If
the system permits adjustment of
convergence

without the introduction

of

keystoning, then variable interaxial
photography could yield stereo pairs
which converge

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AMERICAN CINEMATOGRAPHER, JULY, 1983

101
American Cinematographer - July 1983

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