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Arri 16s question re: fluctuations in speed / focus - why?


Luke Fowler
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Hi there 

my first post on here. Hoping someone can help me.

I purchased two ARRI S cameras and accompanying variable speed motors. The previous owner used the camera with an 8v battery but the charger was broken so I (perhaps mistakenly discarded them). I’ve been shooting on this camera for a few projects now using 12v RC battery packs as recommended to me.  I’ve mostly been shooting 400ft magazines with torque motors that say they have been modded for 12v. I burnt out a few arri fixed speed motors using 12v (I was again mistakenly told they were 12v motors) but these variable speed ones have been going ok for a few years now. On Recent shoots though I’ve noticed fluctuations in speed and visual distortions when I get the footage back.  I can only describe this fluctuation as like someone rapidly pulling focus at the start of the shot (only sometimes my carefully set focus is pulled out by the force of the fluctuation). I can only assume that this is caused by a surge in voltage that is too much much for the camera and is momentarily pushing the film out of the gate.  I thought that tightening the torque motor between shots would help reduce this but it’s not solving the issue alone.
 

Does anyone know how I can address and solve this issue?

Is it my batteries / motor or is the arri in need of a service?

I was looking to purchase a 12v Tobin motor and a more reliable 12v battery (any suggestions?)  - but I want to ascertain that this is indeed the solution before throwing money at it irrationally.

thanks kindly for any help or advice,

all the best

Luke 

Glasgow, Scotland 

Edited by Luke Fowler
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Out-of-focus images have nothing to do with the drive. You can crank an Arriflex 16 ST with a gear insert that was once produced or with a pressurised air unit, focus shifts should not happen, least with an electric motor.

If a magazine motor pulls too strongly, you should still not notice anything focus-wise. The first thing to take place in that case is damage to the perforation. You will hear a tearing noise at the sprocket(s) when lid is off.

You may have the film loops too tight, firstly the upper one. Try to find a size where the film literally drops into the gate like a waterfall. The lower loop just mustn’t touch the body bottom during a cycle.

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As Simon said, the motor voltage/speed should not affect focus. There is a fixed loop through the gate, that can accept various speeds. If the footage is going out of focus then the film is moving away from the gate somehow. Usually this happens if the loop is too tight. Have a look with some dummy film, check your loop size, pressure plate is locking in ok, sprocket guides, no torn perfs etc. 

Does it only happen at the start of every shot? 

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That’s an odd one, the picture isn’t going out of focus, just doing a sort of zoom in and out, just after each shot starts.

I wonder if it’s not the scanner reacting to the light flash at each cut. Can’t think of a way the camera could do this. 

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Posted (edited)

That effect looks like on YouTube the "auto stabilization" effect is turned on. 

Notice there is warping of the image from left to right. This is the "opposite" side of the gate to the pulldown system. So either it's pressure plate float, or it's the scan. 

What throws me off is the fact the frame line steps outside of the framing of the scan.

EDIT: I'd love to see an underscan. 

Edited by Tyler Purcell
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I don't think the scan is bad. So long as the lens aperture is tight, movement of the film off the gate will not affect focus as critically as when the lens iris is opened wide. 

What I am seeing is consistent with the shape memory of the upper fold of the loop going through the gate and momentarily lifting the pressure plate as if there is a weak spring.

The signature double-clatter of some CP16 cameras is the shape memory of the film passing through the gate and momentarily lifting the pressure plate. In the CP16,  The film becomes deformed when parked around narrow rollers in the upstream film path within the camera.

In the ARRI style cameras, the upstream and outgoing film paths are within the magazine itself. However, as with the CP16, I expect maybe 15-20 frames of film would pass through before the artifact occurred. Your issue is occurring after about four frames which may eliminate shape memory from narrow rollers as a cause. 

The visual fault in your film is also consistent with the pressure plate being hung away from full contact against the back of the film due to accumulated debris under its limit stop.

This might also occur if the magazine is prevented from seating fully home in the camera by worn latching, debris or physical damage having bent something. Is the magazine when latched to the camera snug or a little loose?

In that event, the film will be finding its own place between the pressure plate and the gate face and floating in a clearance between the two. As the shape memory of the upper loop fold passes the the gate, the film will momentarily move closer to and away from the image plane until the fold passes and the film settles.

The pressure plate as far as I can observe is the smaller rectangle within the longer chrome guide path in the front of the magazine. 

There may be some other defect if the magazine has been dismantled for servicing and not reassembled correctly. Except for the initial bad frames after buttoning on, the performance seems satisfactory and may not have attracted the attention of the operator or been reported back to the operator by an editor.

My bet is that if you shoot in low light with the lens aperture wide-open, whilst your viewfinder image will be sharp, your film image may momentarily sharpen, soften and clarify a little but remain out of focus after the fold passes through.

Do you have another magazine to test with? I doubt that this issue will have anything to do with drive motor speed variations.  

Another possible cause may be that your loading is causing the upper loop fold to be a little tight maybe by one frame and the initial start-up before the magazine film transport responds may be momentarily pulling the upper loop fold snug.

If there is workspace within the magazine for the upper loop fold to be one frame deeper then that would be my next trick to try. 

As a defect, it may have always been accepted as an ideosyncracy of that particular camera. In the edit suite it might not have attracted enough attention to prompt reporting back to a camera department or the operator.

Edited by Robert Hart
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3 hours ago, Robert Hart said:

In the edit suite it might not have attracted enough attention to prompt reporting back to a camera department or the operator.

The first few feet would nearly always be slate anyway. It would hardly even show on a pic-sync or Steenbeck and would soon be on the floor. It's only nowadays you want to use every frame.

I'd suggest that the "fold" idea doesn't really account for the fault appearing in shots made a short time apart. Overnight, yes. But only on the first shot of the day.

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thanks for the input so far... still not sure if its in the film loop/pressure plate..  I tested this AM with very expired film - both magazine and 100ft load ... with 12v/8v batteries... can anyone see anything out of the ordinary?

Also - I noticed that the pressure plate has a bit of play when you push at it... it does not click in  or have a spring like the bolex...

BTW - its def. NOT the scan -- as it only appears sporadically - I just uploaded the footage where it is the worst case example.

Sometimes its not there at all.. which made me wonder if it was to do with the voltage/loop tension..

https://drive.google.com/file/d/1zKeOyiMQpv6RTp5hsuUJtJoz4e08Rymf/view?usp=sharing

https://drive.google.com/file/d/1DvTM_B4eHITE_SKf56OvpcbOCzvuU9FC/view?usp=sharing

https://drive.google.com/file/d/1arPj4xDW6LMzIkNMLUQCLhTVZIJE497p/view?usp=sharing

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OK! just spoke to Arri Service in London. 

Very nice chap. Told me that It could be build up of dirt at the sides of the film gate or the pressure of the springs inside the pressure plate gone weak. 

For future ref. he showed me how with two pieces of film inserted top and bottom - you can slide the pressure plate off and then softly softly pull the springs up so that they put more pressure on the plate. 

He said -see if that helps and if it needs it more - make sure your not pulling the spring in the same place as befiore- it should be like an S - if you pull it too much at the same place the spring can snap and your done for!

He also mentioned the tip of ramping up the variable speed motor rather than putting in a high voltage to begin with. 

I seem to remember my old mentor Peter Hutton doing that but sadly he's not around to ask anymore.

Does this sound familiar to anyone with an Arri S/ST?

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QUOTE: "I'd suggest that the "fold" idea doesn't really account for the fault appearing in shots made a short time apart. Overnight, yes. But only on the first shot of the day."

My CP16 only required about ten minutes of rest to make it happen. I should clarify that I was using it in a hot climate.  

I think the original author of this thread is now on the right track with his most recent reply. 

Edited by Robert Hart
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On 10/5/2022 at 6:06 AM, Luke Fowler said:

Also - I noticed that the pressure plate has a bit of play when you push at it... it does not click in  or have a spring like the bolex...

The pressure plate is spring loaded like a Bolex, just you can't see it. 

When you say "doesn't click in", that's disturbing because when you close the pressure plate, it should lock in place. 

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