2
Depth and Scale Effects
Along with the limitations noted above, simulated 3D offers some valuable tradeoffs. In
addition to the opportunity to use most of the creative effects that are familiar to 2D
filmmaking (among them: color effects, lens distortions, and a wide depth-of-field), 3D
filmmakers gain the opportunity to exploit depth and scale effects that are unique to the
3D medium. By choosing a wide I/O or interocular—the horizontal displacement between
their lenses—3D filmmakers are able to lend depth to objects that lie beyond the
stereoscopic limit—objects that otherwise would appear to be flat. And, by significantly
widening or narrowing the I/O, filmmakers can diminish or exaggerate the apparent size
of objects—a consequence of the fact that the fixed interocular of the human eye—about
2.5” in the average adult—is the primary frame of reference that viewers use to evaluate
the depth and size of objects they see in the world. Scenes shot with a narrow I/O display
the effects
hypostereo
; scenes shot with a wide I/O display the effects of
hyperstereo
; and
scenes shot with an average I/O (scenes which most closely replicate the scale of objects
in the real world), are said to be in
orthostereo.
Strong degrees of convergence can also
introduce significant scale effects. Familiar objects that are placed by the filmmaker far
beyond the screen plane may seem gigantic relative to the size of the screen, while the
same objects, when placed far in front of the screen, may appear miniaturized
relative to
the size of the screen. In addition, by strongly converging their lenses, 3D filmmakers can
cause the spatial geometry of a scene to seem to warp, as if the space were bending
toward or away from the viewer. Effects such as these, though unwanted in productions
that aim to portray the world as it is, can be useful tools for a filmmaker producing a
surrealistic or fantasy film. With its unique capabilities, 3D is not merely a tool for adding
verisimilitude to a film—a way to persuade audiences that they are “there.” It is, instead, a
medium that offers filmmakers an opportunity to communicate with their audiences on a
richer, more compelling level than was afforded to them in 2D. When skillfully employed,
the artifacts that are peculiar to simulated 3D—instead of being clever gimmicks or
awkward limitations—can be powerful tools that creative filmmakers can use to tell
effective stories.
Interocular and Convergence
In addition to the variables to consider when capturing
images in 2D (among them, focal length, focus, aperture,
shutter angle, imager size, dynamic range, codec, colorspace,
and frame rate) two important new options are available to
3D filmmakers: 1) the ability to adjust the
interocular
, or
displacement, between their lenses and 2) the ability to
adjust the convergence of the lenses. The term
convergence
,
as it applies in 3D imaging, refers to the rotation
,
or toeing-
in, of the lenses of the two cameras in the 3D rig. The
location of the point of convergence—the point where the
optical axes of the lenses cross—determines the separation at
the screen plane between the two components of the
stereoscopic image pair
. This separation, which is termed
parallax
, determines the apparent location in space of the
3D objects in the film relative to the screen plane.
Fig.2 – I/O and Convergence
