I wanted to keep it simple but still achieve very fine adjustment of the focus. I elected to use a worm gear drive, as opposed to the servo based belt drive versions I have seen described on the web. Some stepper-motor systems are described elsewhere on the web but then the complexity of the project goes through the roof, although some of the features can be very appealing.

The major drawback of a simple worm gear drive is that there is no option for manual adjustment (without disconnecting the worm drive.) The advantage is extremely fine adjustment - one turn of the servo output is only about 0.025 of a turn of the focus "knob" with my gear ratios (about 1/4 the rate of a belt drive) .

Speed adjustment of the servo is via an adjustable voltage regulator - about 1.5V to 12V. At 1.5V the servo turns at less than 0.5 revolutions per second.

The servo is mounted on an aluminium plate fixed using the focus shaft retaining plate screws (careful use of small spacers behind the plate to ensure the correct tension on the "shaft spring" when the mounting screws are fully tightened is essential).

I will be adapting my manual control box (Up and Down buttons plus Vout adj') for use remotely sometime, or perhaps I'll dig my old "Picstart" kit out and build a stepper (I do like the idea of preset positions.)