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Diagnosing Jitteriness


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#1 Terry Mester

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Posted 12 July 2008 - 05:49 AM

Diagnosing Jitteriness

Conventional wisdom has dictated that the cause of jitteriness in the Super 16mm Camera is related to the fact that it only uses one Sprocket Hole for Film advance. However, conventional wisdom is not always correct. I have determined a cause of jitteriness in the Super 8mm format that is not related to its use of only one Sprocket Hole. The main problem of jitteriness is caused by a tight Clutch for the Take-up Reel. In a Movie Camera (and Projector), the Take-up Reel regularly runs out of slack Film as it reels in the used Film coming down from the Film Gate. Every time the Take-up Reel runs out of slack Film, the Clutch on the Camera turning the Reel has to slip until more Film is pushed down by the Film Gate's Sprocket Arm / Claw. However, if the Clutch slippage is too tight, the Film will be tugged by the Take-up Reel -- thus pulling the Film in the Film Gate and causing a jittery picture! This problem would affect 16, 35 & 65mm Cameras just as it affects Super8. The Take-up Reel is only supposed to wind up the Film as it enters the Take-up Chamber. It is only the Sprocket Arm which is supposed to move the Film through the Film Gate -- one Frame at a time. In addition to jitteriness, a tight Clutch will also put a strain on the Camera's Gears and Motor causing it to slow down -- thus screwing up the Film timing! This strain on the Camera's Gears can and will cause damage.

It is very easy to test the slippage strength of the Camera's Clutch. You test it with the Camera ON. Just stop the Clutch with your finger as it turns, and note how much force is required to stop it from turning. It should slip with only a little bit more force than is required to turn the Take-up Reel. In order to appreciate the small force required to turn the Take-up Reel, use your finger to turn it from the center -- not the outer edge which is easier due to leverage. You will quickly and easily determine how little force is required to turn the Reel before the Camera's Clutch needs to slip. It's a small amount of force.

To verify if the Clutch is too tight and is tugging on the Film in the Film Gate, just load a couple of feet of old Film in the Camera from the Take-up Reel into the Film Gate. Cut out the Sprocket Holes from the Film in the Gate so that the Sprocket Arm cannot catch the Film. With the door open, turn on the Camera, and note the Film to see if it's getting pulled by the Take-up Reel. The Film in the Gate should not move whatsoever. If it's pulled down any amount, then the Clutch is too tight. The Thread Link below shows some pictures of a couple of Super8 Camera Clutches. Super16 Clutches would be similar in nature.
http://www.cinematog...showtopic=32243

In addition to not tugging on the Film, the Clutch when slipping should not in any way slow down the Camera's Gears. With the cover removed you can verify whether or not the Gears slow down when you stop the Clutch with your finger. Even if the Clutch doesn't tug on the Film, if it causes the Gears to slow down when slipping it is still too tight! This is important for maintaining the Camera's "Timing" in addition to preventing damage.

If you've suffered jitteriness in the Super16 format, please identify which Model and Year of Camera.
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#2 John Brawley

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Posted 13 July 2008 - 02:51 AM

Diagnosing Jitteriness

Conventional wisdom has dictated that the cause of jitteriness in the Super 16mm Camera is related to the fact that it only uses one Sprocket Hole for Film advance.

If you've suffered jitteriness in the Super16 format, please identify which Model and Year of Camera.



I think the main issue with your proposition is this first sentence is not really true. There are many causes of image instability and I would suggest that one set of sprocket holes is the least likely cause. Sprocket holes are mainly for transportation and film advancement, and sometimes for registration.

As already discussed on other formus here, there is an argument that registration pins are not even required, and may even cause some instability. You have correctly though identified one of the causes. Magazine feed and take up tension is usually set by a technician using special gauges and rigs to measure it. On Arri's they use a device that locks into the take up and feed core and pulls on a spring which then tells you the tension. On Aaton's belt driven magazines, they use a different type of gauge again, but there are specs that the magazine needs to conform to.

Lateral or side film support makes a huge difference, as seen on the more recent SR3 Advanced and 416. The cut from kodak and fuji can also affect the amount of weave.

One of the most common reasons is misloading or incorrect loop size and setting.

I would suggest that the reason for some much instability in super 8 cams, is that the pressure plate is only made of plastic and gets changed every time you put in a new roll. So any manufacturing tolerance variations will show up as instability...not to mention scratches ! When the pressure plate is disposable, what can you expect....

jb
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#3 Tim Carroll

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Posted 13 July 2008 - 09:10 AM

John and I may disagree strongly on issues of registration pins, but he is right on with the issues he listed above.

Terry, your observations about take-up tension may play some role in what you refer to as "jitteriness" in Super 8 cameras. What you are failing to realize is that the relationship of the take-up spindle (where the take-up reel mounts) and film gate in a Super 8 camera is entirely different than that same relationship on a 16mm camera or 35mm camera. It's been years since I tore apart a Super 8 camera, but from my recollection, there is nothing between the gate and the cartridge take-up with Super 8. In a 16mm or 35mm camera, between the gate and the take-up spindle, there are at least one set of film sprockets. This keeps any tension from the take-up spindle from being transmitted to the film in the gate area. The take-up spindle can tug against the final film sprocket and if the take-up tension or torque is set way too high I have even seen it rip some sprocket holes, but that sprocket keeps the tension from being transmitted to the film going through the gate area, partially by the size of the loop (excess film between the gate and the final sprocket) as John mentioned.

It is important to set the torque or tension on the take-up spindle, and as John mentioned we have gauges to do that. But in either a 16mm or 35mm camera, if the loop between the gate and the final sprocket is set properly, and the take-up spindle tension or torque is not so excessive that it rips the film through the final sprocket, take-up spindle tension will not effect picture stability.

Best,
-Tim
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