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Old October 22nd, 2008, 02:39 AM   #1
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Letus EX Ground Glass dirt, smudge, ect (example posted)

I've been having a terrible time with someone which appears to be on the ground glass of my LEX. Any ideas on how to remove it? See for yourself:


Many thanks,
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Old October 22nd, 2008, 08:34 AM   #2
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I see something (not someone) on right of the viewable image coming and going and it seems to be of sharper texture than the groundglass texture itself. The display of the clip on my computer is a huge size, about a quarter of frame and I cannot access a control set to zoom it smaller or larger.

The rest of the image area seems consistent for a groundglass being operating with lens on front of the Letus being at or very close to f5.6 and/or shutter of 150th sec or higher. The freckle effect is broader grains in the groundglass texture being diffused by the movement but not entirely eliminated. I may be wrong here as I am only guessing.

Have you tried cleaning the groundglass?

Were you switching the motor on and off again when recording the download clip?

The sharper blemish comes and goes, either from the groundglass being switched on and off or a shutter speed higher than about 1/100th sec being selected. If the freerunning groundglass motor rpm is hunting back and forth through the camcorder frame rate or running very close to it, this may cause momentary freezes of the groundglass motion as the shutter syncs up with the slower moving ends of the elliptoid excursion of the Letus groundglass.

If not, what was your shutter speed?

How far did you have the camcorder zoomed in?

What is the iris setting of your lens on front?

What was your camcorder iris setting?

If you have attempted a clean, what did you use? (ie., cotton wool, cotton tips, lens pen, tissue wrapped on a biro end, cleaning solutions, like water or wood alcohol?) The mark may be fine debris from a tissue or cotton tip.

It is unfortunately also consistent with a scoring of the groundglass texture from holding a piece of tissue or cotton wool against the groundglass with hardchromed tweezers which have penetrated the material and touched the groundglass whilst the motor has been switched on to aid the cleaning process or without the motor running, though those are generally longer single marks.

A ballpoint end with a tissue wrapped around it will do the same thing if it breaks through. It is not so much a scratch but a fine point backpolishing of the groundglass texture.

The groundglass texture is quite an effective abrasive surface and will wear through a thin layer of tissue to a hard object fairly quickly.

If the mark is a very fine piece of fibre debris from a cotton tip or tissue, a fine art brush may shift it. Wash the art brush with detergent and dry it off before using it as there are oils in the hairs which will come off on the groundglass. Tape around the chrome ferrule on the art brush as this will scratch the groundglass finish if it touches.

If you have been cleaning and this mark is eluding your efforts, there is also just a chance this blemish is actually a shadow from a piece of debris on the front smooth face of the groundglass.

Last edited by Bob Hart; October 22nd, 2008 at 09:02 AM. Reason: added text and errors
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Old October 23rd, 2008, 11:31 AM   #3
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Thanks for your thoughts Bob - I also enjoyed your other post about "Roiding" the LEX.
I haven't attempted to open the unit for cleaning but as far as I know, this unit shipped with that blemish (as everything I've shot, when framed against a solid color - usually sky - large black dots and the blemish that you see on the QuickTime have been apparent).

Shutter was 1/24th, the zoom was 20 on the HVX-200, the iris on the lens was open.
I guess I'm weary of opening this thing up. Does Letus provide a cleaning service for a fee?

Thanks again for your thoughts.
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Old October 23rd, 2008, 12:05 PM   #4
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The Letus is less intimidating to open up than you might think, but you do need care and a steady hand when drawing the tube forward over the groundglass as the clearance is only about 2mm or so. Unlike the older versions, the groundglass and wiring is now attached to the flip enclosure, not the front housing, so fear and anxiety of breaking something is reduced as you no longer have to fret about things pulling up short on very thin fragile wires.

You only need to take the tube off to get at the groundglass. Don't go inside the flip enclosure though. The brothers recommend against this. That is a whole different ballgame and fraught with all sorts of unfortunate possibilities unless the skillset is well advanced.

You already have the toolkit in the allen keys which come to secure the camcorder adaptor rings and lens mount. The allen screws in two pairs either side of the tube at the junction with the flip enclosure are the ones which hold the tube on. If you get the courage and take it off, just make sure that the screwhole for the pillar support is on the lower side when you re-assemble it.

If you have a small tabletop vise, it might be re-assuring to secure the flip enclosure with it, otherwise have your partner or a friend hold the flip enclosure firmly against a table surface while you extract the tube forward. Make sure you protect the paint finish with some pieces of paper or cereal packet or gaffer tape on the jaw surfaces of the vise.

You just need to make sure you are in a clean, dust free environment. If you are prone to psoriasis or dandruff, damp your hair or smear some ointment to stop the fallout.

I don't know there is much to be done about the general blemishes but they may also be particles of something sticking to the groundglass. The mark, in absence of any flaw or scratch/point polish of the groundglass texture looks like a bit of fluff which might have come off onto the groundglass front or back during transport.

If the blemish is fluff and not throwing off during operation of the motor, chances are it has something sticky or greasy in it which means you may have to use a good lens cleaning solution or detergent to shift what it leaves behind when you pick it off.

Even the mighty P+s Mini35 is not immune as I had to remove a fine flake of threadlock or varnish of some kind which had been dislodged during shipping onto the field lens near the groundglass. I understand the Mini35 is assembled in a purposed airfiltered clean-room but you can't guard against everything fate has to throw at you.

The P+S technik people were quite alarmed to learn we had let the entrails of their beauty see the light of day on set in amongst the dust from cast and crew galloping about. But we got away with it.

I don't know whether the Le brothers have access to the same sort of clean room resources. You would need to enquire with them if they will service it for you.

As for your lens on front of the Letus, what is the widest aperture (iris) lowest number it opens to?

I am getting that sort of blemish/artifact only when shooting against the sky downsun with the f6.3 long end of a f4-f6.3 zoom which is in the hostile zone tighter than f5.6. You can see it here :-


The Mini35-400 behaves the same.

Other footage I have shot does not show it, especially in controlled artificial lighting with good primes.

Last edited by Bob Hart; October 23rd, 2008 at 12:19 PM. Reason: errors
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Old October 23rd, 2008, 11:31 PM   #5
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Yeah- this is all great info. I'm using the Zeiss Planar 50mm (Nikon) and it is open at its widest - f1.4.

I've definitely seen these types of artifacts you are talking about while stopping down the 35mm lens. Yet, not even with my Letus XL did I have this type of distortion and dirt on the GG.

I'll take your advice about opening up the unit and let you know how I do. Thanks again.
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Old October 24th, 2008, 09:09 AM   #6
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If you are getting worse artifacts than with the original Letus XL, something seems to be not quite right, especially when you are using a Zeiss prime.

In the older direct relay XL model and JVC model, artifacts were a definite issue and the practical limit for the iris setting when filming a plain object or background seemed to fall closer to f3.8 with the older models I got my hands on. This artifact came to be described as "film of grain".

This was because of the individual "grains" of surface texture in larger scale relative to the smaller image area scanned off the groundglass which I understand was also of a coarser texture compared to the Extreme.

Other models based on the same groundglass and elliptoid movement performed better but still had the issue to a lesser extent. They were able to aquire a larger image off the groundglass due to those cameras being able to use in-built lens zoom to fine tune the image area to its widest available. The individual "grains" were scaled smaller proportionately to the image area and became less apparent. With the XL and JVC direct relay versions you were stuck with the 22mm or so wide image area.

Do you have the .pdf "User Guide" manual for the Extreme which shows owner-service instructions? At the time I received one it was not in general distribution as it was still being proofread.

You mention a distortion. Could you make a jpg image from a frame grab of something like a chessboard or test pattern for us to examine here.

Last edited by Bob Hart; October 24th, 2008 at 09:33 AM. Reason: added text
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