Multicam Technical Information FILM and DIGITAL SYSTEMS
Still Camera Arrays
Still Camera Arrays consists simply of many synchronized still cameras, and they have a history longer than that of the movie camera. One frame from each camera is spliced together, in sequence, to make a motion clip of the desired visual effect, such as frozen time.
The technical challenge comes in dealing with many cameras instead of one. Each camera must be aimed, loaded, have its exposure set, and then fired precisely at the correct moment. After the shoot, the individual images for each take must be sorted, and assembled, then stabilized and color corrected to account for the inevitable, subtle differences from camera to camera.
Furthermore, if the cameras are not properly synchronized, slight errors in shutter timing will introduce a small amount of movement into a frozen scene because each camera will not capture precisely the same moment in time. This error cannot be fixed in post without extensive reconstruction of the moving elements in the scene.
It is possible to achieve camera array effects using computer morphing/animation techniques, but with only two or three motion cameras running to provide keyframes, only a small change in point of view is possible, resulting in a somewhat flat image.
MULTICAM system
Reel EFX inc.'s MULTICAM successfully addresses all of the problems inherent in still camera arrays. The control computer communicates with each camera separately over a common communications line of standard computer network cable. The computer sends information to each camera on exposure and timing sequence, and sends the fire command. New profiles with different exposures or timing sequences are generated by the computer quickly on location. The computer also monitors each camera for any errors during the take.
The computer has a program for generating timing sequences. A timing sequence is a program which tells which camera to fire when, and at what shutter speed and f-stop. The simplest sequence would be to fire the cameras simultaneously at a constant shutter speed and f-stop. Other sequences with linear and exponential ramps in timing and exposure can be custom designed to achieve a variety of visual effects. New sequences can be generated in minutes during a shoot.
Since accurate timing is crucial, the cameras and computer are designed to provide highly accurate timing using an external hardware timing generator accurate to +0.0000005 seconds (1/2 microsecond). The unmodified cameras have a timing error of as much as +0.1 seconds.
Visual Effects
The difference between a still camera array and a movie camera is
that the speed of the action being photographed, and the speed at which the point of view
of the viewer moves through space are not related. This permits the action being
photographed to be slowed and even stopped while the point of view (camera) keeps moving
at a reasonable speed, constant or changing.
Following are brief descriptions of some of the visual effects which are possible with
MULTICAM, and how they are achieved.
Frozen time - when all of the cameras are fired simultaneously, time seems to freeze at
one moment. With shutter speeds up to 1/2000 sec. even fast moving subjects can be frozen.
The motion of the point of view through the shot can move at a constant speed, or
accelerate and decelerate.
Motion to frozen to motion - a mixture of motion and frozen time can be achieved by firing
some of the cameras at 24 or 30 fps, and the rest simultaneously. The result is action
which freezes, unfreezes, or both, all during a continuous dolly move.
Lead-in/lead-out of frozen time - a motion camera can be placed at the beginning, end, or
both sides of the MULTICAM track to provide an unlimited amount footage from a stationary
point of view which connects smoothly with the MULTICAM footage.
Moving in frozen time with multiple passes - MULTICAM can act like a motion control dolly
or crane, performing multiple passes with perfect accuracy. Moving elements, frozen
elements, and background plates can be shot separately and assembled in post.
Ramp into/out of frozen time - the cameras can be fired at any speed, so that instead of
going immediately from motion to frozen, the action can "ramp" into a freeze;
smoothly slowing and finally freezing at any rate, linear or exponential. The same holds
for unfreezing.
Super slow motion - Photosonics type speeds, up to 2000 fps, can be while the
"dolly" still moves at a reasonable speed through or around the scene.
Time lapse - frame rates down to hours or more per frame can be achieved, again while
maintaining a reasonable, smooth "dolly" speed. Streaking lights, stars, or even
blooming flowers can be captured alone or as elements for compositing.
Vary action speed with constant "dolly" speed - speeding up and slowing down the
action, a popular effect, can be achieved without any associated change in the dolly speed
or smoothness.
Physically impossible "dolly" shots - physically impossible speeds and
accelerations of the point of view can be achieved by varying the spacing between the
cameras. The only limit is the length of the cables between the cameras. You can travel
thousands of miles per hour, and get there from a standstill in a second.
Shutter speed and f-stop ramps - because each camera has its own exposure settings, a ramp
from slow to fast shutter speeds could bring a motion blurred object slowly and smoothly
into crisp focus. The same holds true for f-stops, permitting a smooth, controllable depth
of field zoom.
Multiple times around circular track on same take - if the cameras are mounted on a
circular track so that the first camera is next to the last camera, the point of view can
go more than once around the circle at speeds up to 96 fps, with portions of frozen time
in the sequence.
Keyframes for computer morphs - any number of extremely accurate keyframes can be
generated from different points of view on a scene for use in animation or morphing.
Precision timing - using optical, proximity, or contact switches in concert with
programmable delays, extremely precise timing of several events can be attained. Any event
which can be electrically triggered can be controlled, including pyrotechnic charges.
The possibilities are endless…
