Focus gear with continuous tooth pattern that enables smooth follow focus operation without gaps or interruptions during lens rotation.
Definition
A seamless focus gear is a continuous gear ring on a lens without mechanical interruption or joint, allowing for a 360-degree rotation without any jump points. The ring typically features 0.8mm pitch gearing and enables precise focus pulls via follow focus systems. The seamless construction prevents mechanical vibrations and jerks during continuous camera movements.
Technical Details
Seamless focus gears are milled from a single piece using CNC machining or produced via injection molding with subsequent processing. The standard pitch is 0.8mm with a 32° pressure angle, resulting in exactly 45 teeth per 360°. High-quality versions use hardened aluminum (7075-T6) or stainless steel with a surface hardness of 58-62 HRC. The inner diameter varies between 65mm and 114mm depending on the lens, while the ring width is typically 10-15mm.
History & Development
Zeiss developed the first seamless focus gears in 1975 for their Compact Prime CP.2 series, after welded rings failed during intensive shooting of "The Deer Hunter" (1978). ARRI adopted the concept in 1982 for their Standard Primes, establishing the 0.8mm pitch as an industry standard. Canon introduced electronic encoders into seamless rings in 1995 with the CN-E series, enabling the transmission of digital focus data.
Practical Use in Film
In "1917" (2019), seamless focus gears on modified Trinity Steadicams allowed for fluid focus transitions during the apparent one-shot sequences. For "Blade Runner 2049" (2017), DoP Roger Deakins exclusively used lenses with seamless rings to avoid image jumps during characteristic zoom pulls. Remote focus systems like the Preston FIZ require seamless gears for automated focus ramps.
Comparison & Alternatives
Welded focus gears cost 60-80% less but exhibit micro-jumps of 0.1-0.3mm during precise movements. Follow focus systems without mechanical coupling (like RED DSMC2 internal control) eliminate the problem but limit response speed to 200ms. Clip-on focus gears offer a retrofitting solution but increase lens weight by 180-350g and diameter by 15-25mm.