Gear ring on a lens for mechanical iris control via follow focus. Standard on cine lenses.
Technical Details
Standard iris gears feature a pitch of 0.8mm with a diameter between 54-95mm, depending on the lens type. The gearing is usually done in the 32-pitch system (32 teeth per inch of circumference) with an engagement depth of 0.25-0.4mm. In follow focus systems, an intermediate gear with gear ratios of 1:2 to 1:4 transmits the rotational movement to the gear ring. High-quality versions use hardened steel with a surface hardness of 58-62 HRC to minimize wear during frequent remote control.
History & Development
The first iris gears appeared in 1923 on Bell & Howell cinema lenses for the new generation of remote-controlled cameras. Arriflex introduced the 0.8mm pitch system, still used today, in 1937, which was ratified by the SMPTE as standard TE-30 in 1952. Zeiss developed the first electronically coupled iris control using stepper motors on the gear ring in 1976. Modern digital systems like Preston or Ambient have been using brushless servo motors with 0.001-stop resolution since 2003.
Practical Use in Film
In "Blade Runner 2049" (2017), DoP Roger Deakins used motorized gear ring systems for precise depth-of-field ramps during complex camera movements. The iris pulls were performed via the Preston MDR4 system with programmed curves from f/1.4 to f/5.6 within 2.5 seconds. Documentary filmmakers appreciate manual gear ring operation for responsive exposure adjustments without motor noise. In Steadicam shots, wireless gear ring controls allow for aperture changes without direct lens access.
Comparison & Alternatives
In contrast to electronic fly-by-wire systems, the mechanical iris gear offers tactile feedback and functions without a power supply. Video lenses often use servo zoom grips instead of separate gear rings but only achieve ±0.1 stop accuracy. Modern cine lenses integrate encoders directly into the gear ring for metadata recording and automatic lens mapping. At extreme temperatures below -20°C, electronic systems fail, while mechanical iris gears with special grease can function down to -40°C.