Is there a single torque amount that can be used at which all focus rings on all lenses can be moved that will allow a stepper motor to move the focus ring to adjust focus of all lenses without risking damage to any lens?

If one is designing an external focusing rig to use with cinematic lenses that have geared focus and zoom rings, is there a certain amount of torque that will properly move all lenses without damaging any of them?


No. Well, yes, maybe. 0 ft lb of torque should be safe on any lens, but that's not really very useful. The existence of any non-zero value that is safe for all lenses across all manufacturers would imply that there is a minimum specification that all manufacturers are required to follow - there isn't.

If you have a specific subset of all lenses you're interested in, and you can wrangle the "safe torque" specs for those lenses from the respective manufacturers, you could simply apply torque that never exceeds the lowest value across that subset, but you'd have to potentially re-adjust every time you added a new lens to the collection. In the limit as that subset approaches "all lenses", you could come up with a minimum value, but AFAIK, nobody has ever done all that research and published it.

  • The body of the question indicates that the value of the torque must be sufficient to move the focus and zoom rings... – Michael C Dec 27 '17 at 15:23
  • @MichaelClark ... which is why I said that zero torque isn't useful, although it is guaranteed to be safe for all lenses, and went on to explain how you might find a useful minimum... – twalberg Dec 27 '17 at 15:30

A few suggestions at how to arrive at a solution:

A non-experimental lens will be designed to safely withstand the torque that, to an average human user, would not feel like seriously forcing it into any of the end stops. Some kind of ergonomics or medical professional can probably tell you what that kind of torque is.

Also, in most cases, a lens constructor must expect someone to twist or untwist a lens to/from the bayonet mount while holding it by the focus, zoom or aperture ring forced into an end stop. The torque needed to operate a given bayonet mount is probably specified somewhere, or could be measured with a torque wrench on samples...

Also, it might be beneficial to monitor your stepper motor more closely than just with an encoder.... monitoring the coil currents would very quickly tell you when you are actually hitting an end stop....

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