3D Production and Post
Barry Clark
03-26-10
Real World 3D
When a viewer’s eyes focus on a real object, they automatically converge on the object.
From the separate perspectives seen by the two eyes, the viewer’s brain fuses a coherent
3D image of the object. All of the objects that the viewer sees in 3D occupy a cone that is
bounded by the edges of the overlapping fields of focus and convergence of the viewer’s
eyes. Everything outside of this cone is seen by the viewer in 2D. As the viewer’s eyes
focus on progressively more distant objects, the zone of convergence shifts with the zone
of focus and the cone shrinks in width until an
outer limit of distance is reached—a distance of
100-200 yards in the average adult—beyond
which the viewer can no longer distinguish the
perspectives seen by the left and
right eyes.
Everything that is located further away from the
viewer seems to lie on a flat, 2D plane. To judge
the relative position in space of objects that lie
beyond this
stereoscopic limit,
a viewer must rely
on
monoscopic depth cues
, including
motion
cues
(nearby objects seem to shift position more
rapidly than distant objects),
atmospheric
cues
(the hue of objects shifts toward blue as they move
into the distance), and
occlusion
cues (near
objects obscure the view of more distant objects).
Fig.1 – Real World 3D
Simulated 3D
The experience of viewing a 3D film is significantly different from the way a viewer sees
3D in the real world. The most obvious differences between real world 3D and the
simulated 3D that is viewed on a screen are a consequence of the fixed depth-of-field and
the fixed point-of-view of the lenses that capture the images. As a result of these
constraints, viewers watching simulated 3D can no longer alter their point-of-view simply
by shifting the position of their heads, as they can in the real world. And when turning
their attention from one object of interest to another, they can no longer simply refocus
their eyes, as they can in the real world. In a 3D film, the point-of-view and the focus are
invariables established on the set. In addition, when looking at a 3D object displayed on a
screen, a viewer’s eyes must focus on the screen while, at the same time, they converge on
a point in space that may be located
beyond
the screen,
on
the screen, or
in front of
the
screen. As a result of this process—which differs from the way a viewer sees the world—
the viewer has the sensation that the 3D object is located either in the space beyond the
screen, on the screen plane, or in front of the screen. A 3D object that appears to be
located on the screen plane is relatively easy for a viewer to watch. But, over time, a
viewer may experience eyestrain from the effort involved in fusing coherent 3D images of
objects that reside far beyond or far in front of the screen.
