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Robert Brahms

How much torque will a fixed lens-camera horizon accept in worsted case without serious damage?

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The wireless lens control system used here comes with a very strong default torque, causing a strong pressure from top down on a robust lens when hitting the end point of the lens focus spin, if the calibration has failed or has been accidently changed or reset. I watched with eagle eye how the whole camera body horizon including the lens dipped down on the front side coming minimally closer to the rods, mounted on a shoulder rig and slipped into a VCT plate on a Sachtler fluid head and tripod. I was shocked about this situation, asking myself if there is any hidden damage on the body or the PL-mount or the lens now possible. Since this is an expensive robust cine lens, I do not worry about the lens focus ring that much. But exactly this sturdiness has caused the body-lens axis to come down based on the lens control motorized cogwheel trying to spin ahead. And even serious professional digital cinema bodies come in various levels of robustness and I think/worry about which week point could have been stressed here (and I hope for the baseplate / shoulder-plate connection, not for the bottom of the body or even worse, the PL mount lens connection).

I deeply hope that this had no effect on the PL-mount, or the mount-sensor construction, nor on the metal body of the professional digital cinema camera itself and that this small movement is maybe acceptable(?) in terms of baseplate flexibility or the whole rig. Thus my question since I never had such scenario to compare to before. I would love to hear from some long year experienced DPs or Ass. that this minimal dipping is not such a dangerous thing like I think it is, at the moment. Maybe a dip of one or two millimeters, on a lever length of 200-300mm (body + lens) all together. But you can see it when you concentrate your eye on the horizon of the body-lens level in comparison to the rods below and the background behind. The rods stay horizontal while the body-lens axis comes down forced by the pressure of the lens control cogwheel on the lens.

Additionally, I wonder if the motorized lens control system cogwheel was always that noisy before, or if it has been damaged by this little event from before?

Can anyone share experiences with such situations? I doubt that this is a rare case since such systems often "forget" their calibration end points. All I wonder is, why the torque was so strong that it was able to dip/squeeze the body-lens axis down closer to the rods a little bit. (Torque was 50%).

I carefully tried to check the flexibility of the baseplate shoulder-plate connection, but I am not able to reproduce this dipping, so the torque must have been quite strong to create this dipping.

I did a sketch of the whole thing to illustrate it better.

illustration_of_cam_dip_down_by_motorized_lens_control.jpg

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i would not worry about a 1-2mm dip

it happens quite frequently with strong motors

preston dm1x can move a lens at either end point. it almost never happens on primes, but mostly on zooms

optimos for example  (28-76 or 45-120) have their focus ring furthest away from the mount, so a strong motor will push them

you can turn down the torque, use your hands to keep the motor and lens steady on calibration, or add lens support bracket if possible

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I believe, if you are using a preston, that it's possible to change the calibration torque with an adjustment inside the MDR module.  Check with Preston, if this applies to your model about how to do this.

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8 hours ago, Bruce Greene said:

I believe, if you are using a preston, that it's possible to change the calibration torque with an adjustment inside the MDR module.  Check with Preston, if this applies to your model about how to do this.

Yes the original Preston MDR had torque adjustment pots inside:

282251815_mdr1torque.jpg.f0acadbd110b2f7f6b636feb1c3fcef5.jpg

 

The MDR 2 has external switches with 3 settings:

896385201_mdrtorque.jpg.aabe35e80e50f9dbe80b1053e54d3bd7.jpg

And the MDR 3 has buttons that electronically adjust torque with coloured LED indicators:

130914487_mdr3torque.jpg.444916ef059780e9b2a8b546c6349af0.jpg

 

I wish it were a bit simpler.. I fix a lot of lenses due to damage caused by over-torqued motors these days. I suspect there are assistants who just keep the motors at max torque to cover all contingencies so they don't have to spend time adjusting the torque for the odd stiff lens. I can understand the motivation on sets where there is pressure to be time-efficient, but it really does cause a lot of issues. Especially on lenses that are over 10 years old.

If a zoom is supported then the motor cannot flex it down (as both motor and lens are supported by the same rails). For shorter lenses a PL mount should be strong enough to take the force of a strong motor, but I would have concerns for EF or other flimsy stills mounts. 

If the whole camera is flexing down there may be issues with how the camera is mounted (ie play in the snap plate or bridge plate etc), but a mm or two isn't a big deal. As long as it only happens during calibration and not during a take..

As Kyrryll mentioned, turning down the torque, using a lens support or holding the lens during calibration are all good ways to help avoid issues.

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Thanks for all the helpful replies! Very much appreciated. I just take my job very serious and tend to bother more than required but do it in the right time (not while shooting, but when I write my reports). But I think it is better than the opposite. The available torque adjustment of this unit here (I do not want to promote brands) has a minimum of 30% adjustable but still too powerful in case of lost calibration for end to end point in my opinion. From now I tend to care about turning on and off while keeping the wheels off as long as possible. But I think you are right, that this is maybe exceptable rig squeezing, if not too often and not too much.

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