What's going on?

[David George Ellis]

I was on a job with an Aaton XTR PROD and Zeiss lenses. Everytime at a specific point on the 16mm (9" to 10"), the gear would grind off the focus ring and create that "Crap, I hate that sound," sound. Thinking the problem was caused by the torque of the motor, I tried to remedy the situation by running the motor off of two dovetail support rods bridged by a dogbone, as the matte-box rods coming off of the camera body will spin loose from continual engagement. Same result as the motor buzzed right off at 9".

 

Consequently, we took measurements, set a hyper-focal and shot. Things like that really question your resolve, but you can never tell the quality of an olive until it's pressed, and we all made the day. Wow, what an Oprah moment.

 

I'm pretty sure this has happened once in everyone's career. Maybe not. Was there something I may have missed in making a vain attempt to calibrate? I was told by the AC that the lenses breathe and were tight, but could that be the only prognosis as to why the motor would jump off at the same point in the travel of the lens?

 

Thanks for any advice,

 

David

[Joshua Harrison]

Some lenses as they go through the entire throw of the lens move in and out on the focus and possibly will cause the gear to fall off in the near focus of the throw. Since you were using a Bartech (and even with a preston) you don't have to calibrate the entire throw of the lens.

Having not been there I don't know, but I kind of doubt you were using the lens into 9 or 10 inches. Next time try just calibrating into 2 feet or so and see if your problem still occurs. Also depending on what kind of pitch and what motor system you have you may be able to pop on a wider gear (the M1 has a wider gear for the 32 pitch) that will allow you to move further. Anyhow hope that helps.

 

Joshua Harrison

[Mitch Gross]

These lenses can be known to bind on the close end of focus, ewspecially if you are using a clip-on mattebox. Uneven pressure on the front housing can bow an inner ring slightly out of round, and then the focus will not turn properly. Rotating the clip-on can halp with this sometimes. Did you try turning the focus ring by hand to see if the lens itself was suffering?

[Benjamin Treplin]

The weight of the clip-on mattebox it self can have great effect on the lens. You can test this by exerting light pressure with one finger to the front housing of the lens and turn the gear. Furthermore a possible explanation may is Standard and highspeed Zeiss lenses have set screws in the gear ring. They connect the focus gear to the worm drive. Some times on comes loose and makes it way up and fills one tooth gap of the gear. They are very small and and often covered with "dirt" or old lock-tide and hard to see.

Hope this makes sense.

Best

[David George Ellis]

Hey Guys,

 

Thanks for the response. We ended up using the support rods for the matte box and I went off my dovetail rods for the motor. But then again, we were so loaded up with filters cuz we were shooting tungsten film at an 8 to 22 stop, that it may have simulated the encumberance of a clip on. It looks like a combination of all your statements.

 

Josh,

You are correct in your assumption that we didn't go to the closest focus. And even if we did, I believe we had enough depth of field to go a little long to keep that sharp, going forward.

 

Mitch,

Yeah, I tried the foucus ring manually and it seemed to go through fine. That's what was confusing me. Everything felt solid with no play or flex, it's just at that 9" to 10" travel that made things go bump.

 

Ben,

I had no idea there was a screw to keep the lense in check like that. I doubt the AC knew either. Kids!! Sometimes they surprise you. Sometimes not. Looks like I gotta read my AC handbook in more haste.

 

Thank y'all again fellas. I feel better now b/c on set it became really uncool, but we just did the damn thing and hoped the hyper covered all distances necessary. At least I now have a contingency for the next time.

 

ONE,

 

David