helical

Definition: Mounting for lens or optical component that moves up and down a central axis when rotated: consists of corresponding or mating spiral threads cut into the mount and the barrel of the lens so that as one is fixed, the other is driven along the axis. * The rate of translation along the central axis depends on the steepness of the thread.
The more, the smoother
Multi-start helicals -- with several spirals running parallel to each other -- give the smoothest and most durable movements for focusing (the kind we know and love in e.g. Zeiss and Leica lenses) but are extremely taxing to machine. Unfortunately, they also require high torque to turn, which makes them useless for auto-focus lenses and their feeble motors, quite apart from very high cost.

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