Follow focus

A gear ring clamps around the lens barrel and the wheel drives it, so the focus pull comes from a larger, slower movement than grabbing the barrel directly allows. Marked discs set hard stops at two distances, and travel between them is identical on every take. That is what makes a planned rack focus reliable rather than lucky. Wireless versions put the wheel in a second operator's hands, or on a motor driven from a hand unit. Focus peaking only shows you where focus currently sits, and autofocus makes the decision for you. A follow focus does neither.