"The operator was looking through a 'pistol scope,' which had a little red laser dot in the center of the eyepiece," explains Edlund. "He was following Rickman's face as the actor fell away from camera. At the same time, the encoder was sending data back to a computer, which then crunched the information instantaneously and sent it back to the motor on the focus ring on the lens."
"Basically," continues Edlund, "we set the camera up and moved the lens to Position One and would record that point, then moved it a few more inches and did a logarithmic curve of test spots. Then the electronics guys could spline that and turn that into a smooth follow-focus program that would hold the focus on Alan automatically. So it was like a little science, and it required pretty astute engineers -- mechanical and electronic engineers -- to figure out how to do this."
Edlund went into the shoot with Rickman planning to capture everything in a single take. He didn't expect the actor to want to repeat the backwards falling stunt more than once. They got the shot, but Edlund says McTiernan talked Rickman into filming another, just in case. (Ultimately, the first take is the one that appeared the movie.)