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Alexa vibration


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#1 Stoddy

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Posted 08 April 2011 - 04:25 PM

Hi all,

Hell of a day today, Working with an Alexa and found I got, what at first, seemed to be camera shake when moving forwards and backwards on the rig. Tightened everything up but wouldn't go away. Added wedges, no change. Even had shake when camera was static! Camera fault I hear you cry, three Alexa cameras, all the same result.
Ended up with track and dolly.
WTF? What could possibly change when putting a self powering camera on a rig?

Would put large amounts of money on the fact that it's not the rig.

Anyone else experienced this? Is it an Alexa thing? Rig all tight, no loose monitors, batts etc, slow moves with bended knee and still it seems to shake.

Please help!!!!!
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#2 Ramon Engle

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Posted 08 April 2011 - 05:29 PM

You should use your real name.
Do you have the intermediate plate between the sliding base plate and the body? I think its called the BPM? I found that plate becomes loose with little use. Especially if you are operating handheld using the 19mm baseplate and gripping the matte box rods.

Ramon
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#3 Robert Starling SOC

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Posted 09 April 2011 - 12:26 AM

Real names only here please.

The camera body is built like a solid block. It's either the rig, mounting plate or accessories.

Robert
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#4 Stoddy

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Posted 09 April 2011 - 01:32 AM

Apologies, I thought nick names counted. My real name is Richard Stoddard and I'm based in the UK. I'll update my profile.
Thanks for the replies, I'm convinced it's not the rig but I'll tripple check base plates etc today.
Even if loose, a static camera and camera shake?
Wish me luck.

Richard Stoddard. GBCT
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#5 Sean Jensen

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Posted 09 April 2011 - 08:32 AM

Hi Stoddy

I've been having this problem on the show I'm doing. It's never been so bad that the shot was unusable but it is really frustrating. It's not a constant vibration but an occasional, short (1-2 second) rapid vibration. I'm using a PRO2 sled with TB-6 monitor, VZ Gimbal, PRO arm and Vest and a Preston with DM-1 and DM-2 motors. I've switched to a different DM-1 on focus, changed the springs in my arm and tightened and checked everything. The DP on my last show (also Alexa) said he was noticing small vibrations at times but not to worry about it. I have no idea what is causing this. I have seen it in running shots, static shots and "normal" shots done on Steadicam. These are the only times in the 17 years I've been doing Steadicam that I've seen this. Both on Alexa.

I really want to get to the bottom of it.

Sean
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#6 Stoddy

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Posted 09 April 2011 - 10:23 AM

Hi Sean,

We've been busy today testing the B camera with the A focus system(Preston) and thought we'd solved the problem unwittingly, only to abort another steadi shot after it appeared again. Wed even walked the shot before hand without actors yet on the take, there it was again. The latest is that we have possibly found a glitch in the Preston brain which shakes the lens when it occurs. My theory is that the
glitch only occurs when shooting as we focus through eight feet or so and because 'on shot' we are really focussing slowly, this is when it shows up. It doesnt show on the dolly because the camera is secured to a more solid mounting. I'll up date if wr find more out.

Best,

Rich.
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#7 Lawrence Karman

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Posted 09 April 2011 - 11:22 AM

[quote name='Stoddy' timestamp='1302362614' post='64799']
Hi Sean,

We've been busy today testing the B camera with the A focus system(Preston) and thought we'd solved the problem unwittingly, only to abort another steadi shot after it appeared again. Wed even walked the shot before hand without actors yet on the take, there it was again. The latest is that we have possibly found a glitch in the Preston brain which shakes the lens when it occurs. My theory is that the
glitch only occurs when shooting as we focus through eight feet or so and because 'on shot' we are really focussing slowly, this is when it shows up. It doesnt show on the dolly because the camera is secured to a more solid mounting. I'll up date if wr find more out.

Best,

Rich.
[/quote

Are your Preston motors mounted to rods that attach directly to the camera or off your steadicam dovetail plate? And does this vibration show up on playback? Are you hardwired to the village or transmitting? Recording to SXS cards to external deck?
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#8 Sean Jensen

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Posted 09 April 2011 - 02:15 PM

Lawrence and Stoddy

When I first had the problem I was using my old Preston (MDR1 and Hand Unit 1) which I've had for 13 years and now I'm using my new MDR2 and Hand Unit 3. We've done full speed rehearsals and takes with no shake and then for no apparent reason it appears in one take and not the next. The motor is mounted to the rod on my dovetail plate and not directly to the camera. There is flex in my rod because the lens is stiff and attaching to the camera is a great idea. I'll try that and get back to you.

We are transmitting using The Boxx and recording onto SXS cards. I've never seen the vibration in my monitor, only in playback.

Stoddy, are you using the Preston on the dolly too?

Sean
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#9 Stoddy

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Posted 09 April 2011 - 05:36 PM

Hey,
Motors mounted to camera, footage viewed from camera and HD continuity playback, on board cards. Current conclusion is faulty wireless focus brain or hand set which makes motor shudder in particular positions.

Rich.
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#10 Stoddy

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Posted 09 April 2011 - 05:37 PM

Hi, yes, Preston used on the dolly too.

Rich.
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#11 Sean Jensen

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Posted 10 April 2011 - 07:24 AM

Stoddy

If it is a faulty Preston then I have two of them. One that is 13 years old and the other brand new. Not sure about that one. Like I stated before, I've only had this problem with the Alexa. What sled, monitor and arm are you using? Are the vibrations you are seeing short like the ones I described?

Sean
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#12 Lawrence Karman

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Posted 10 April 2011 - 10:48 AM

It's hard to see how the Preston would induce vibration to the image unless a rapid change in focus might cause the lens to "breathe" showing up as a rapid change in image size, but if that were the case you would see a focus shift. Did you? Not to mention you would probably hear it happening. Wondering if there might be some kind of RF hit to the recording module? Have you sent the footage to ARRI?
I know this might sound obvious but the first time it happened to me I was not aware of it:the top stage on the PRO can come loose after a while and the threaded ring on the post must be checked occasionally to be sure it is tight using the special spanner provided by PRO.This would manifest itself in subtle camera shake if loose.
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#13 thomas-english

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Posted 10 April 2011 - 11:14 AM

Have you tried using the Sony V-Lock plate system instead of the BPA-1? I use this if I know I am doing low mode running shots as I see vibrations then.

The point with the Alexa is you have a tiny camera footprint at completely the wrong position for CG. I am betting its your topstage thats vibrating.
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#14 Sean Jensen

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Posted 10 April 2011 - 11:29 AM

Lawrence and Thomas

The top stage was the first thing I checked. Everything is rock solid. I can't move my top stage 1mm with the camera on it or off it. It's really perplexing. I will have the footage sent to Arri and see if they can figure it out. This is not like any shake I've seen before. I wish I could post the shots so you could see exactly what I'm talking about. Sometimes it is the most obvious solution that solves the problem so please keep suggesting things. I feel like I've explored almost everything. No focus shift is visible and the lenses we are using are the Angenieux Optimos. Great lenses for Steadicam.

Sean
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#15 thomas-english

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Posted 10 April 2011 - 02:25 PM

If you could move your camera 1mm you would have more to worry about than a little shaking. I don't think that is a test nor am I saying that your top-stage is faulty. What I am saying is that certain frequncies manage to set up a vibration across your topstage and plate due to the shear amount of leverage that Alexa is having on the dovetail plate. You have around 10kg whose CG is over 6 inches away from its pivot point (BPA1) all twisting that dovetail upwards and on top of that it is doing so outside of the topstage's fixings since you have to move the camera so far forward when you have a 26v battery onboard.

I would put money that doing the following will completely cure it.

Use an XCS Dovetail plate
Use the Sony V-Lock adaptor and the long Sony'esque baseplate that locks into the v-lock thingys and into the nodule at the back. These come out with every Alexa kit.

These will both act to reinforce your topstage. I suspect that just using the Sony parts will do it. I do both and am vibration free and I've been doing a lot of running shots recently.
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