From American Cinematographer, Speed and Sharpness by David E. Williams (Sept. 1998)
... While not a replacement for a good first A.C., Preston Cinema Systems' unique Light Ranger follow-focus device has proven itself invaluable under certain conditions. This tool can automatically focus the lens through the use of an operator-controlled infrared laser and a geometric triangulation system, which determines the distance between a subject and the camera focal plane and drives a focus motor. The unit is normally mounted on its own tripod, separated from the camera by a convenient distance. An encoder head provides pan and tilt angle data so that parallax effects may be canceled out. In cases where parallax effects aren't significant, the unit may be mounted on a conventional head.
"I'd never heard of the Light Ranger before making this film. But everybody started talking about it once we began doing some tests."
- Conrad Hall, ASC on Without Limits
Both cinematographer and director Robert Towne decided to utilize extremely long lenses and high frame rates to shoot key portions of the film's many running sequences. This would allow them to compress the space between central character Steve Prefontaine and his competitors, and also separate individual athletes from the field while analyzing their movements and expressions in slow motion.
"There are shots done with 800mm lenses where you can see an entire 220-yard or longer run as Pre comes directly toward the camera, perfectly in focus as we tilt from his face to his shoes and back. Without this device, it would have been a nightmare for us to get those shots the traditional way, with follow focus marks. Also, there are only so many times that performers can repeat action like that, so doing fewer takes was a benefit."
- Conrad Hall, ASC
The Light Ranger offers four modes of operation:
- Manual: In which the focus is controlled by a manual-control knob and the focus setting is displayed on a digital readout.
- Automatic: Where the lens is automatically adjusted to the measured subject distance in the readout.
- Offset: In which the focus is still automatically adjusted but offset by a distance manually set in the readout.
- Split: Which allows focus 'pulls' to be manually controlled by the operator.
Despite the Light Ranger's capabilities, Hall notes that the device can have operational problems under specific shooting situations, such as while trying to follow focus on someone driving a car ( a situation in which the windshield or other glass surfaces may deflect the distance-measuring laser and create false readings). Also, having the camera and Light Ranger at differing elevations can affect accuracy unless such variations are accounted for.