Help with DIY Remote Follow Focus

[Gomer]

Hi all,
I just joined this site after finding it on Google as I was looking for follow focus ideas. I made my own mechanical follow focus with miter gears and a pulley/belt set-up, but am dying to try and make my own wired remote follow focus. I need help with what to buy. I want a servo motor that turns 360 degrees and a controller with a potentiometer. How strong does the servo have to be to turn the focus ring on a prime telephoto? If using a servo that turns 360 degrees, will the potentiometer still work? I will be using a pulley and MXL belt to turn the focus ring. Please help me with part numbers and where to buy. The whole contraption is going to be wired, not wireless. I would like to put the battery pack, controller and potentiometer all in one box and just run the connecting cable up to the servo.

thanks for the help,

Lucio

 

[Richard Crowley]

"I want a servo motor that turns 360 degrees" "I will be using a pulley and MXL belt to turn the focus ring."

How much rotation you need depends on other factors not known (or at least not stated here). How many degrees does the focus ring need to travel for full range? What will be the ratio of the teeth/diameter of the focus ring and the servo pulley? Have you already worked out that exactly 360 degrees of servo motion will make a full range focus control?

Note that most solutions we see use gears and not belts. Probably because it is a more exact control and not subject to belt slipping. Zacuto sells plastic gear strips ("Zip Gear") that can be fastened around any lens control barrel. http://store.zacuto.com/zip-gear-replacement.html

"How strong does the servo have to be to turn the focus ring on a prime telephoto?"

That depends entirely on your lens. Some focus rings are quite easy to turn, and others much stiffer. But even smaller servos are pretty strong.

"If using a servo that turns 360 degrees, will the potentiometer still work?"

That depends on the servo. There may be servos that can handle exactly 360 degrees with feedback. But most continuously-rotating servos have the potentiometer disabled to make continuous motion possible. There are special "winch" servos which have more than 360 degree motion, but still have the position feedback pot in place.

"Please help me with part numbers and where to buy."

I have found that http://www.servocity.com/ has a great variety of servos, gears, etc. I am using many parts from their website for both pan/tilt and for lens control servos. I am using the Zacuto "Zip Gear" strips and gears that directly fasten to a servo output shaft. I am waiting for parts to arrive, but I believe that the Zacuto Zip Gear is 32-pitch.

According to this thread http://www.dvxuser.com/V6/showthread.php?t=215905 using "auto moulding tape" works quite well to secure the Zacuto plastic gear strip to the barrel of the lens. It seems much cleaner than that expensive clamp from Zacuto. OTOH, we don't know what sort of life span to expect from the double-stick tape. Although you rarely see it fail on auto bodies, and using it on a lens seems much less severe.