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First follow focus - bumpy operation. Help!

Focus Follow focus Focus ring Focus gear Gear Cine lens

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#1 John_Burton

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Posted 12 February 2017 - 06:26 PM

I finally own a decent follow focus. It's a used Zacuto Z-focus. Works great, smooth turn, zero play.

 

I'm using it together with a Sigma 18-35 f1.8, with a this ultra cheap focus ring: https://www.amazon.c...ewer focus ring

 

The Sigma lens itself has a buttery smooth focus turn, and like I said, the Z-focus is basically silk when it's just spinning by itself.

 

But the moment the Z-focus is actually put to use turning the Sigma lens, it takes a little more effort to turn than I feel it should, and there are some subtle vibrations in the movement. Adds up to a less-than-stellar focusing experience.

 

I had a similar problem when I first got the Sigma and the Z-focus. I'd bought a gear ring that was specially fitted to the Sigma, and it proved totally unusable--vibrations galore.

 

Is this a normal problem with cheap, crappy focus rings? I can't find anyone else who describes experiencing this issue quite like I have. Help!


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#2 Satsuki Murashige

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Posted 12 February 2017 - 10:28 PM

It's probably a combination of all three elements. I would suspect most of it has to do with the lens itself. Autofocus still photo lenses just were never intended to be used in this way. Also, still photo lens mounts are much less robust than something like an Arri PL mount, so there may be a little play occuring there.

The Sigma 18-35 focus ring is a bit stiffer than a typical autofocus lens, if I remember correctly. Also it's a pretty wide focal length, so that would tend to minimize most errors of this type.

If you're experiencing lumps in the focus travel, it's possible that your baseplate and rods are at a slight angle and not perpendicular with the lens. If this is the case then your lens gear would vary in distance from the follow focus gear, causing the gears to bind when they get too close.

Also, focus gears made of harder material will have less play and be smoother, assuming that everything else is lining up. It's possible that the gear itself may be slipping on the lens. The ones that work best usually attach with set screws from multiple points, rather than with tension.

Last thing, it could be that your tripod, baseplate, or other parts of your rig are adding some extra play into your system. Ideally, everything should be as rigid as possible. Best of luck, hope you figure it out!
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#3 Dom Jaeger

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Posted 13 February 2017 - 01:03 AM

I'm not familiar with that follow focus, but presumably the drive gear supplied is the correct pitch for cine gears? Should be 0.8 module. If the pitch of either ff or lens gear is different it will vibrate badly.
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#4 Kyryll Sobolev

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Posted 14 February 2017 - 08:56 PM

what you describe is normal for that set up

 

the follow focus will take some more effort to turn in comparison to it spinning freely

sometimes you can mitigate that by backing the gear slightly further away from the lens, so that it does not "dig into" the lens (even preston motors sometimes will not turn if you jam the gears into the lens barrel).

 

subtle vibrations, or bumps, will be normal with the cheap belt-like attachable gear rings.

on EF lenses i've seen these set-ups physically move the lens so much that that the picture "jumps" as well

 

I finally own a decent follow focus. It's a used Zacuto Z-focus. Works great, smooth turn, zero play.

 

I'm using it together with a Sigma 18-35 f1.8, with a this ultra cheap focus ring: https://www.amazon.c...ewer focus ring

 

The Sigma lens itself has a buttery smooth focus turn, and like I said, the Z-focus is basically silk when it's just spinning by itself.

 

But the moment the Z-focus is actually put to use turning the Sigma lens, it takes a little more effort to turn than I feel it should, and there are some subtle vibrations in the movement. Adds up to a less-than-stellar focusing experience.

 

I had a similar problem when I first got the Sigma and the Z-focus. I'd bought a gear ring that was specially fitted to the Sigma, and it proved totally unusable--vibrations galore.

 

Is this a normal problem with cheap, crappy focus rings? I can't find anyone else who describes experiencing this issue quite like I have. Help!


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#5 Patrick Fee

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Posted 17 February 2017 - 11:24 AM

I agree with with Kyryll.  I used to work for Zacuto and I know their follow focus will pull in toward the lens gear when you tighten it down.  Try to leave like a half mm or so and watch the gap between the gears close when you tighten everything down.  

The strap type of gear could be a problem too if its cheap and bumpy.  Also, the pitch (distance between the peaks) will change slightly depending on what diameter the lens barrel is and could lead to some engagement issues.


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#6 JD Hartman

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Posted 18 February 2017 - 07:08 AM

Gear pitch does not change with diametrical pitch or pitch diameter.  If the pitch changes than the gear is cut incorrectly.

 

Gear mesh can be roughly set by rolling one of more strips of printer paper (.003 in.)  between the spur gears until you achieve smooth motion


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#7 Patrick Fee

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Posted 18 February 2017 - 09:10 AM

Gear pitch does not change with diametrical pitch or pitch diameter.  If the pitch changes than the gear is cut incorrectly.


In theory it shouldn't but since he's talking about a strap with gear teeth on it instead of a continuously geared surface it does. The teeth shape stays the same but the valleys between them is different depending on the diameter of the lens it's wrapped around. It may not be a whole lot but consider the difference between laying the strap completely flat vs wrapping it around something the size of a quarter. The distance between the teeth stretches out the smaller you go.
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#8 JD Hartman

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Posted 18 February 2017 - 10:06 AM

It's made as a rack and the pitch of a rack is the same as the pitch of a gear.  Gears mesh at the pitch circle not the tops of the teeth.  Pitch error would diminish the closer you get to the rood diameter.   OP's problems seems to stem from not setting adequate clearance.

The product linked to in the first post is just a crap design. 


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