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Old May 27th, 2008, 12:55 PM   #1
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Anybody else experienced this?

This is a new one. I have my Canon XH-A1 and Letus hooked up on the Letus Rod Support System, on a tripod, with a 28mm lens or 50mm lens (my shortest lenses). Put the tripod on a solid surface (wood, hard dirt or tile). Turn the ground glass on. Pointing straight ahead, and the image becomes shaky. Anything in focus looks like it's had an earthquake effect put on it. Tilt up or down, the problem is gone. Point back straight ahead, and it's back.

This only happens with the shortest lenses I have, and on hard surfaces, which has led some to think it is some kind of a resonance issue. If I move to carpet, or put the tripod on a dolly, the problem is gone. Is this just an issue with having a vibrating ground glass?
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Old May 28th, 2008, 08:45 AM   #2
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Hmm sounds like almost the opposite of the problem I recently had, which was solved by switching the steadyshot off (like it should have been off in the first place) - just in case there's another Homer -style D'oh! moment round the corner.....
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Old May 28th, 2008, 08:52 AM   #3
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Do you actually feel your tripod vibrating a bit? Do you have rubber feet on your tripod? Try that and see if it helps.
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Old May 28th, 2008, 12:31 PM   #4
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I hadn't considered checking to see if the image stabilization was on. I turned it off a while ago, but I lent the camera out for someone else's shoot, so it might have somehow got turned back on...

Also, my tripod does have rubber feet, and I am still having the issue. I can feel the vibrations in the tripod. So, I guess I just need to put like towels or something under my tripod feet?
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Old May 28th, 2008, 06:24 PM   #5
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A quick and dirty experiment. - Maybe try adjusting the tripod centre support forward on the rails so that the combined centre of mass is neutral for that lens fitment on the tripod centre or even rearwards of the tripod centre.

Check the position and security of the pillar and bridgepiece directly under the front of the Letus body.

If you loosen and then re-tighten the vertically sliding adjustment whilst applying a very slight downwards preload on the camera-Letus combination ( I mean one gentle pinky finger pressure only) it might be just enough to tone out any resonances.

A rearwards position should alter the leverage any vibration has upon the tripod structure or the rails themselves or change any resonant frequency occurring in the rails or the total assembly including tripod if occurring.

Also check to see if the clamping screws on the various bridgepieces need snugging down a little more or the wedgeplate screw lever under the right of camera is locking before it contacts the camera body if this is happening.

Let us know of the outcome.

Last edited by Bob Hart; May 28th, 2008 at 06:25 PM. Reason: error
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Old May 29th, 2008, 06:36 AM   #6
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Furthur to above, another source of your problem could be the lens itself if it is a plastic bodied autofocus lens, especially if it is one of those which focusses by a knurled narrow manual focus ring at the very front.

The focus mechanism of some cheaper lenses can have very loose clearances to enable plastic on plastic wearing surfaces and no grease lubrication in order that the very small motors can turn the focus mechanisms.

Their image will jump when manually focussed and they might be vulnerable to the tiny vibration of the Letus.

For the front-end focus style, a simple elastic band strapped over the moving part of the lens and the ends of the rods should keep it quiet.

If it is a retrofocus type and the internal moving group is a loose enough fit to move the image, you are pretty much out of luck. These lenses however do not seem to jump their image about when focussing so I think this would be a rare problem.

Last edited by Bob Hart; May 29th, 2008 at 06:42 AM. Reason: error
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Old May 29th, 2008, 10:40 AM   #7
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I have yet to try to move the camera around on the tripod plate, but I do know that my lenses are the older Canon FDs, not automatic. I'm not sure if they are plastic or not, but these lenses seem pretty heavy and solid (not plastic-y) to me.
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Old May 29th, 2008, 06:35 PM   #8
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The FD lenses should be solid enough.

There is another thing you might try. The FD mount is secured in front of the Letus by grub screws. Loosen those, rotate the mount itself in the body and then resecure the screws. That may change some behaviour if it is happening in the lens, ie., a loose element.

Check also the security of the four grub screws at the junction of the tube which enclosese the groundglass and the flip enclosure.

My imagining however is that the problem may be at the junction between the camcorder itself and the Letus.

There is enough compliance in plastic bodied camcorders at the lens to body area and at the filter mount itself to allow significant angular deviation of the optical centre. I experienced this with a Sony PD150 and home-made AGUS35.

Applying a very slight preload on the joint between the camera and Letus is a dirty attempt to reduce this movement.

A quick way of checking for this as a cause without messing with the screws on the framework would be to wrap several elastic bands over the combination and under the rods at the camera-Letus junction which should cause enough dampening to affect the artifact.

Securing the camera to the baseplate, the Letus body to its pillar and making sure the baseplate is firm, the vertical supports are firm is where the solution may rest.

If you can slide any of the supports on the rods, they must be tightened until they do not move. There are grubscrews in the centres of the bridges between the which compress them onto the rods.

My guess is you would have found them when setting it up but doesn't hurt to mention them in case you missed them.

Last but not least, is to add some weight to the tripod centre by hanging a sandbag or something of similar weight.

That about exhausts my imaginings on what the source of your problem is.
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Old May 30th, 2008, 12:21 AM   #9
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Well, I took it off the camera, put it back on, took the lens out, screwed it back in hard, checked all the screws -- I forgot that I am missing one of the four grub screws at the junction of the tube, but it is not loose at all. I'll try to get my hands on a replacement grub screw and see if that helps.

Also, I've misplaced the original screw that tightens the Letus to the rods, so I had used another screw that fit, but is a bit longer (thus employing a washer to tighten it). That might be a problem.

I've got a shoot coming up this weekend and no time to go screw shopping, but, I have a temporary solution for the time being. On carpet, it's not a problem. Confirmed again tonight. On a hard surface, if I put rags underneath each tripod foot, the problem is gone. Should work for this weekend.

The grub screws holding the rod system in place are firmly screwed down, as I had found the lengths and heights I wanted months ago and tightened them down.
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Old May 30th, 2008, 01:49 AM   #10
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I am fairly new to all of this, could you explain the extra lens for the xh ai, I thought it was a fixed zoom lens on camera.
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Old May 30th, 2008, 07:33 AM   #11
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Brandon.

The missing screw on the groundglass/flip enclosure junction might allow a some flex at the joint but given your investigations I come down on the side of there being a natural resonance in the total combination including the tripod, most likely springiness in the tripod legs as the cause.

Adjusting the length of the tripod legs slightly shorter may or may not help. The Letus oscillating system is a free movement and not regulated or governed.

Therefore it is possible that a structural resonance period that remained fairly close to the rpm of the Letus motor might induce the Letus motor to run slightly faster or slower and sync up to the resonance period which might then become amplified.

One or more of the tripod feet might be sliding on a hard surface or if they are rubber, because of a narrower surface contact area on a hard surface may be flexing just enough to allow the vibration to amplify.

Your solution of putting down a piece of carpet might be the only solution. Maybe spreaders would also work.

As to why it is only evident with the wide lens I can only guess. The movement may be happening with all your lenses but on longer lenses or lenses which render backgrounds softly focussed, the camera may well not see any movement, either because of the diffuse background or because the apparent movement is large enough to become a motion blur within this background and invisible. Try a long lens focussed on a distant sharply defined object and see if this turns something up.

The wide lens might then be the only lens capable of resolving this movement and the scale of the movement might be so tiny as to be a single or a few pixel rows wide, seen sometimes alternatively by a single row and then two.

I speculate that this would make the movement more apparent than it may otherwise be. On an interlace camera this becomes apparent as what I call an interlace artifact in that every second line or even multiples of lines become emphasised in a sort of comb artifact. When such interlace footage is converted to progressive, it becomes manifest as a slight periodic vertical movement of the image, much slower than the actual vibrations which caused it.

I may be way off base and am really only guessing based on my experiences with an AGUS35 and a CD-R sized glass disk which was running out quite badly and creating a vibration much more severe than I have felt on the two Letus models I have had my hands on.

My guess is any artifact you see is most apparent where horizontal sharply contrasting edges are in your image.

Please convey to us the results of your furthur experiments. Hopefully this will include fitting the combination up to a heavier tripod if you can borrow one or set up on a demonstrator in a showroom. This small glitch aside, I am sure you are otherwise well pleased with the options a 35mm relay device can bring to your creative efforts.


Terry.

The Letus, Brevis, SGpro, Redrock M2 and P+S Technik MINI35 are devices which fit between a videocamera and 35mm still-image film camera or motion picture camera lenses to enable the video camera to see through those lenses as if it is a 35mm film camera.

Last edited by Bob Hart; May 30th, 2008 at 08:33 AM. Reason: error
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Old May 30th, 2008, 10:52 AM   #12
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Is your OIS enabled on the camera? It should be disabled...
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Old May 30th, 2008, 12:24 PM   #13
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It's disabled.
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Old June 2nd, 2008, 05:34 AM   #14
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Some tripod heads like the 501HDV from Manfrotto will vibrate with LEX when not locked down rendering moving the head useless during the shot.

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