The Advanced Art of Stop-Motion Animation: Digital Cinematography: Part 2
In stop-motion, a rack focus must obviously be done frame by frame, so often it will be the animators themselves who do this, along with animating their subjects. The same principle applies of knowing where the start and end points are on the focus dial, and moving them incrementally between frames. For stop-motion, of course, it is important to avoid touching the camera itself because this can create unwanted bumps and jitters in the shot when played back at speed. On big-budget stop-motion productions, shifts in focus are often programmed by a motion-control system, where a computer programs the start-end points of the focus and moves them infinitesimally between each frame, along with all other camera moves. Before motion control was an option, or even today for those who cannot afford a motion-control system, a rack focus can only be achieved by touching the lens. This is still possible without touching the camera, but it must be done very carefully. According to stop-motion director of photography Pete Kozachik (full interview in Chapter 5: Interview with Pete Kozachik, ASC):
[A] solution that many people employ is to attach a stick (such as a chop stick) to the lens with hot glue. This provides a more accurate lever arm with more control, and doubles as a pointer to line up with calibrated marks on cardboard or tape around the lens. Another thing that helps is to include a slight pre-load from a rubber band so the lens can’t flop around, and it helps to move in one direction only.
This functionality of hand-animating a rack focus has been taken a step further by Brett Foxwell, a mechanical engineer, machinist, and stop-motion animator originally from Chicago. Brett devised a special focus-pulling mechanism (Figures 4.13 and 4.14) that he describes here:
For the rack focus, I attach a stiff metal strip sticking radially out from the camera's focus ring. Then a lead screw or a micrometer is mounted in a position such that its travel pushes the metal strip and turns the focus ring. The lead screw has a ball attached to the tip, and a spring pulls the strip onto the ball. The lead screw typically has an inch of travel for the typical rack focus shot. The rig is attached to the camera base, so a firm mounting and a very light touch on the lead screw helps. The focus ring is still a problematic aspect and is likely to be bumped during the animation, so I have had to re-start some shots or just live with a slight shift midway through the shot. Higher-quality lenses have sturdier focus rings, and the tension spring helps with this problem.
Figure 4.15 shows a series of frames that display the stages of a rack focus shot from Brett’s independent stop-motion short film Fabricated, a creation myth of the life-forms present on Earth after the age of man. His film is still in progress and has been in production for about 6 years.