Telecines use the edge of the film to keep the film stable.
Typical telecine gate
However, motion picture cameras do not use the edge of the film while exposing. Instead, they use the film perforations. The spacing between the edge of the film and the perforations is not well controlled by film manufacturers. The SMPTE 93-2005 specification for 35mm film dimensions allows for a margin of ±2 mils between the perforations and the edge. This corresponds to an error of ±4 pixels at 2K resolution.
Mechanical Pin Registration
Film scanners like the Lasergraphics Director directly use the perforations of the film to align each frame separately, thus improving the alignment accuracy to approximately 1/5 of one 2K pixel. This represents an improvement in stability of over 20x compared to edge-guided telecines.
Mechanical pin registration is great for new film but bad for damaged or shrunken film.
Optical Pin Registration
Superior to mechanical pin registration and edge-guiding.
Can correct for damaged or missing sprocket holes and shrunken film.
