Motion control — or motion control photography — is a means of achieving visual effects. It was pioneered in part by Industrial Light & Magic for Star Wars. It has been used only twice on Doctor Who. On both occasions it has used cameras that were direct descendants of the original Dykstraflex cameras invented by John Dykstra and others for A New Hope.
As used on Doctor Who, motion control involves a camera mounted on a robotic arm. The arm can be computer-programmed to move in precisely the same way, as often as the director wishes. The camera can be programmed to focus in a certain way at a certain time or to achieve a certain exposure on a precise frame. This means that whatever is the subject of a "pass" of the camera will have the same position relative to the background elements in the shot as any other pass. It will also have the exact same focus, speed and exposure on every single frame. The "passes" can be composited together, creating the seamless illusion that the subjects occupy the same space.
Motion control was used to capture the opening model shot seen in The Mysterious Planet and for a scene with gangers and humans in The Rebel Flesh. Both shots are discussed in detail in their respective documentaries: The Making of The Trial of a Time Lord: Part One - Mysterious Planet and Double Trouble.
With multiple cameras Edit
The general notion of motion control photography considers only a single camera programmed to repeat the same motion and actions several times. Because Doctor Who of the past heavily used CSO, there was a need to be able to control multiple cameras at the same time. Slaving one camera focused on a model to another camera being used to film actors in front of a blue screen, allowed for a semi-manual form of motion control called "Scene Sync". (DOC: The Scene Sync Story)