7
Convergence Considerations
As noted above, excessive convergence can produce unwanted warping effects (including
keystoning effects, which result from vertical disparities at the edge of the right and left
images), as well as scaling effects—a consequence of placing familiar objects far beyond or
far in front of the screen plane. In addition, images that are strongly converged (images
with high parallax values) may display more ghosting when displayed on a screen—a
consequence of the unavoidable “leakage” of light between the two lenses of the 3D
viewing glasses. However, converging the cameras has the mitigating effect of centering
the screen image on the optical axes of the lenses, where any radial distortion in the
lenses is likely to be at a minimum. In addition, by converging the cameras, a filmmaker
may be able to include a larger area of the set within the scene, increasing the width of the
3D zone in the space closest to the camera. While filmmakers are free to converge their
lenses wherever they wish, audience tests have established a
comfort zone
within which
parallax values are considered to be within an acceptable range. The depth of this zone is
inversely proportional to the width of the viewing screen: a movie screen that is 30’ wide
can afford a comfort zone that is only 10% of the depth of the comfort zone that is
considered appropriate for a TV screen that is 3’ wide. Factoring in the average audience
viewing distance from a 30’ movie screen, this comfort zone is generally considered to
extend from about 30’ in front of the screen (where extreme effects may intrude into the
viewer’s space) to about 30’ beyond the screen (where the furthest 3D objects reside in
screen space). Stated another way, tests have shown that negative parallax should extend
into the viewer’s space a distance that is no more than 50% of the distance between the
viewer and the screen. And the extreme limit of positive parallax (beyond the screen)
should be a comparable distance. A larger negative parallax value may force viewers to
cross their eyes to an uncomfortable degree, and a larger positive parallax value may force
viewers to diverge their eyes to an uncomfortable degree—the so-called
wall-eye
effect.
Because the depth of the comfort zone is dependent upon screen size, 3D films designed
for distribution to multiple platforms need to be treated to convergence adjustment
passes that tailor their parallax values to the size of the screens on which they will most
likely be displayed.
The range of parallax values employed in a film is termed the film’s
depth budget
.
Typically, a filmmaker making an action or suspense film may choose to employ a larger
depth budget than would be appropriate for a conventional drama, which may instead
confine much of the on-screen action to depths that are close to the screen plane. In the
case of a production designed for a 30’ screen, the separation of the right and left images
(the parallax) may range from a negative extreme of -10” to a positive extreme of +2.5”,
for an overall depth budget of 12.5” (3% of the width of the screen), while for a
comparable production designed for display on a 3’ TV monitor, the appropriate depth
budget might be 2” (6% of the width of the screen). Because of this scale-dependency, a
production whose convergence is appropriate for a theater screen may lose much of its
apparent depth when viewed on a small screen, while a production whose convergence is
considered to be appropriate for a small screen may force the viewers to diverge their eyes
to an uncomfortable degree when they watch the same production on a large screen.
While efforts are underway to standardize 3D specifications to conform to the needs of
the various projection and display systems in the marketplace (each of which uses light
and the color spectrum in a distinctive way), the prospect of standardizing specifications
to meet the needs of the vast range of possible
display geometries
– from IMAX screens
to personal computers – remains a distant hope.