LetusXL Edge of GG slightly visible at bottom of frame. at DVinfo.net

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Old October 29th, 2006, 11:27 PM   #1
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LetusXL Edge of GG slightly visible at bottom of frame.

Most people probably wouldn't have a problem with it, but I see a flickering haze at the bottom of my frame. Top is clear as a bell. There is some play in the different stages I've attempted to correct for but too much and I'll have focusing across the GG plane problems which I think I am anyway. The GG module is a very tight fit into the housing so I don't think there's any way to phsically move the GG down slightly. Adjustment through the optical path seems to be my available solution.

I discovered this problem by looking at live footage in my NLE and rotating the LetusXL around the relay iris/lens. Mine is not as press fit tight as I would like it to be, but I can compensate for that I think. Anyway, in the bottom right and left corners I can see the edge of the GG when I rotate it around. Once it's parallel and in the 'correct' position, it's like the whole edge is just slightly out of frame but when the motor is running it sneaks up in there. It's quite distracting.

So 2 questions.
1. Does it appear that I have a focusing problem, and how can I correct it? The middle is in focus, but outside that it loses it. This shot is almost vertical at my ceiling tiles so it shouldn't be that much difference in distance. Shooting a 50mm 1.4 Canon FD. I shoot wide open on the lens and step down the relay as needed.

2. Any ideas on eliminating the fuzzy blackness at the bottom of my frame?

check the video out to see. http://www.ehrichs.net/Letus.zip
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Old October 30th, 2006, 06:31 AM   #2
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Looks like zooming in a bit would solve the problem.
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Old October 30th, 2006, 06:49 AM   #3
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Jacob.

Is your device a flip model?

My following comments refer to the flip model for XL only.

If you can rotate the rear tube itself around the flip module, I would encourage you not to continue to do so. There is only about a 4mm shoulder there. The tube is retained onto the flip module by an interference fit and adhesive bond.

If you can move the tube, then you have a bond failure and the appliance from rear of flip enclosure forward could fall off.

You should not use the appliance until this is made good. You risk the appliance itself and any SLR lens you have mounted on the front if you do.

The rear tube has to be re-bonded to the back of the flip enclosure or very small holes drilled and screws put in or even better, both.

This looseness may account for why you are picking up a bottom edge and getting a travelling relay focus across the GG. Is the bottom edge is "as viewed" via the camcorder viewfinder?

If it is, then the appliance has probably bent downwards at that now moveable joint which will have the camcorder actually looking at the top edge of the groundglass. It takes little movement off the centre axis to do this.

This will also have the camcorder looking at the upper half or so of the image projected onto the groundlgass by the SLR lens on front rather than the very centre of the projected image. This might account for the quality of the image in the portion which raises your concern.

I would not recommend using the Letus XL Flip in the same manner as the Canon lens it takes the place of. For the sake of re-assurance and easing the ability to adjust the backfocus (rear lens) some sort of bridgeplate and rods support (Cavision makes one) is desirable.
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Old October 30th, 2006, 08:37 AM   #4
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Thanks for the advice so far, it is much appreciated. Currently I have the original LetusXL which does flip the image but with the XL zooming is not an option because it mounts directly to the camera.

It is supported by a Bogen 3420 lens support because of the loose press fit interface between the flip section and the relay lens/iris. You're right, that 4mm just wasn't enough. I found that out before I had the support and the weight of my lens out front caused it to separate. Scared me to death but all was well.

Quote:
Originally Posted by Bob Hart
Is the bottom edge is "as viewed" via the camcorder viewfinder?
The bottom edge problem is not visible on the Canon XL viewfinder because of some sort of overscan, but is visible in the NLE. It's outside the standard action safe range but most of my deliverables are shown on projectors that display the full 720x480 with no loss due to overscan.

Quote:
Originally Posted by Bob Hart
For the sake of re-assurance and easing the ability to adjust the back focus (rear lens) some sort of bridgeplate and rods support (Cavision makes one) is desirable.
Does this apply to the LetusXL? I am unsure of any way to adjust the back focus as the appliance mounts directly to the camera body with no intermediate original lens.
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Old October 30th, 2006, 09:42 AM   #5
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I don't know the non-flip version of the Letus XL.

By back focus, I should perhaps have used the words "relay focus".

On the flip version, between the flip module and the Canon XL mount, there is a lens which is mounted at the front end into a tube and on the rear of this lens, the Canon XL mount has been fitted.

This relay lens, in the two examples I have seen has been a Minolta 50mm f2 lens. On the Flip version, the front of this lens is enclosed by a plastic adaptor piece. This plastic piece, fits into a short tube which attached to the back of the flip module.

There are two screws which fasten in at the sides at the rear end of the tube into this plastic adaptor piece. It is possible for the adhesive bond at this joint to fail and the screws to then take weight in shear which eventually stretches the plastic and the screw junction just a little but enough.

This allows the front of the whole appliance to sag downwards very slightly, not enough to see direct by eye looking at the joints.

On your non-flip version, if there is a similar relay lens arrangement, then any sag and movement of the GG off the centre axis may be happening both here and at the XL lens mount to camcorder junction.

If this is the arrangement, slacken off the screws which go in on radius from the sides of the tube into the blue plastic adaptor on front of the relay lens. Try the joint to see if it moves. If it does, then you may be able to restore the centre axis by re-aligning then securing this joint.

To stop this joint becoming loose again, it might pay to put a third screw in on top after you have got the centre axis alignment restored. Don't try this unless you have good miniature engineering skills.

Where the Canon mount goes onto the back of the relay lens, it apparently has been fastened with small screws which are awkward to get at. I have not studied it myself as neither of the two Letus flips I dealt with had an off-axis problem.

There has been reported incidence of these screws having to be re-tightened after initial use when things have been stressed for the first time.

I would have thought that your Bogen support should have taken care of any off-axis sag of the Letus assembly.

It is probably more appropriate to consult Quyen on your difficulty as to follow my comments might bring you into furthur problems.
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