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Old November 30th, 2007, 12:00 AM   #1
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Letus HD100 Lens Help Needed

Hey all. Just got my Letus HD100 today. the adapter got hold up by NZ Customs for about one week and had taken to exam!!! (they prbly think that's some kind of parts for laser cannon or something) Anyway. I am excited!. tried my canon 50mm f1.4. Looked great!! So I headed up to a secondhand camera shop and bought 2 ZOOM lens! 21mm ~ 35mm and a 35~175mm. And only found out that I can't use almost half of the focal length!
for the 21mm~35MM one i can not focus on subject 2meters away from the lens when its at 21mm. For the 35~175 one I can't focus on anything when on the 35mm end.
I guess I am so stupid buying zoom lens. Shall I all get prime lens instead? any chance using zoom lens on the Letus HD? What lens shall I get? Guys please help.
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Old November 30th, 2007, 12:13 AM   #2
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Aaron.

Zooms are not so good on any groundglass appliance as fixed prime lenses. Some still-camera zooms do not ever hold accurate focus through the zoom range, though the modern ones are fairly good.

The good news is this can be fixed to some degree. Adjustment of backfocus is needed.

For Nikons you need the front edge of the mount to be about 6.5mm forward of the front face of the Letus35 housing. I don't know the flange to focal plane of the Canon lens mount you are using.

Whatever it turns out to be, subtract from that number in millimetres, 40mm and you will get the distance from front of Letus35 housing to front of Canon mount to aim for. (example for Nikon is "flange to focal plane Nikon" = 46.5mm. Subtract 40mm = 6.5mm. )There will be some very fine variation but this will get you in the ballpark.

If you want to scribe a focal plane mark on the Letus35 body, measure 40mm back from the front face of the Letus35 and make the mark or use a piece of masking tape if you don't want to scratch the finish.

The bad news about the good news is that the fix can sometimes be a little awkard to achieve, especially with heavy long lenses which hang well forward of the mount and try to pull it out.

With a long heavy zoom, extra support under the lens is recommended, usually taking the form of a bridge across rods and a cushion under the lens if it is one without a support bracket.

Have a look at the "Clayton's sticky" thread for Letus XL which heads the subject list in this section. The same processes apply to the HD100 family, except for the actual mounting to the camcorder lens mount. The Canon simply twists in and locks with a click. The JVC offers up into the mount and an outer ring on the camera mount must be turned clockwise.

In particular look at the YouTube clip linked at the bottom of the string of posts - this one

http://www.youtube.com/watch?v=yugfv4ZVXgg



This illustrates what you may need to do with spacer washers if the lensmount refuses to sit straight when you move it forward in the Letus body.

Last edited by Bob Hart; November 30th, 2007 at 12:32 AM. Reason: error
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Old November 30th, 2007, 05:11 AM   #3
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Bob
Thanks for your help, you answer has cleared up load of things for me. I have a read through the post from Clayton, and got a brief idea now. I also looked up the flange to focal plane for Canon FD lens, it's 42mm.
So that basically means I have to move the lens alone with the mount forward 2mm. Or can I just go in to the GG and move it backward 2mm and then re-adjust the focus of the relay lens? Would that work?
Anyway, thanks for your help, I will give it a go tomorrow plus i need to clear up the GG a bit, I guess the NZ customs must have gotten some dust in it, it's not no clear at the moment.
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Old November 30th, 2007, 07:54 AM   #4
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Aaron.

You pretty much have it right.

If you want to shim the mount from inside, the piece of PVC pipe which forms the sleeve I added for spacing the Nikon mount should probably be not used and only thin paper or cardboard shims used to pack out to the correct position. Weeties packets are about 0.5mm wall thickness. Sheetmetal shims would be better but too hard to cut true without getting bends in them which make them a poor fit and too springy to get a good square fit of the mount.

The PVC tube was the next water pipe diameter wider than the Letus mount outer diameter. I cut a section out of it so it could be squeezed in like a piston ring to fit snugly. Use a piece of PVC water pipe if the combined thickness of the cardboard shims becomes more than about 3mm as they will be inclined to crush and the mount work loose.

You may find the bayonet lugs of the Nikon mount section which will be the back face of the mouint, if your mount is the reversable style, are smaller diameter than the shank of the mount and the PVC spacer or cardboard shims may ride over one or more of the lugs but butt up against the third, which will make the lens mount sit crooked if you press it back firmly in its space against the shims before tightening the thumbscrew. You may have to glue another piece of PVC pipe inside the first one to get enough wall thickness to butt against all of the Nikon lugs.

Don't mess with the position of the groundglass. You may hurt it or worse, never get the thing to sit exactly parallel to the focal plane. Be very gentle when you clean bits of dust off.

(That business you see with the tweezers turning the pillars is a demo of a trick to fine tune the shape of the groundglass movement which is not truly circular, but elliptoid. (Rugby ball or Aussie Rules or American Rules football shape, instead of Irish Rules or Soccer ball which is round.)

If the unit is stored unused for a long time in warm conditions, horizontal to the ground and not vertical with the lens mount end pointed to ground, the groundglass motion might very rarely become more linear which will aggravate artifacts.

This also may occur if the unit has a lot of hours up from being left switched on and flattening batteries, again in a horizontal position.

Twisting all pillars 45 degrees in their holes can sometimes quickly restore the elliptoid shape of the movement.

Otherwise twist pillars as follows --- pillar 1 = 45 degrees, pillar 2 = 135 degrees, pillar 3 = 225 degrees for starters and then fine tune from there with each pillar in turn if the motion remains almost linear.

Matchmark all pillars with a felt tip pen and whiteout spots on the groundglass carrier so you can return to "go" and try again if you get your positions lost.

When the pillars are twisted, they must move within the grommets on the groundglass carrier. The grommets themselves should not be turned with the pillars.

You can check the shape of the ellipse by switching on and holding the LETUS35 up close in front of a CRT television and looking at the groundglass with the TV shining on it, either from behind, or shining in through the front. About 0.7mm long by about 0.3mm wide is about as good as it gets and the shape of the ellipse is normally canted anticlockwise about 20 degrees from vertical when viewing from behind.

Stay away from messing with the pillars unless things get really really bad.).

Don't pull the base rubbers out of their holes to pull the groundglass panel back to where you can more easily get at its front surface. That won't fix anything because you will find the motor power wires pull up tight and you will have a hard go getting the base rubbers back in their holes all the way in.

Last edited by Bob Hart; November 30th, 2007 at 08:47 AM. Reason: added text
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Old December 2nd, 2007, 04:24 PM   #5
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Hey Bob, Thanks for everything. I managed to get the thing work now. with proper back focus set. I am quite happy with the letus image, only that sometimes the GG grains are still quite visible.
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Old December 2nd, 2007, 05:36 PM   #6
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With lenses apertures tighter than f3.5 (a higher number than f3.5), you will likely see a soft fixed pattern artifact against a bright blue sky with bright clouds, light bright overcast or large bright areas like bright vehicles moving past. This artifact has been variously called also "film of grain" and "soft freckles". It is a product of the small travel and shape of the Letus movement which means there are two points of slower movement at each end of the football shape.

Try to set up the composition of your shot to keep your backgrounds darker. Outdoors light from behind the camerman is best, early to mid-morning, mid-arvo to evening. Indoors controlled lighting is less problematic as long as you have enough of it.

Maintain shutter speed at 1/50sec (25P 50). If you handy carry the camera/tripod combination by hooking the tripod head over your shoulder and letting the legs hang, you may find your ear or collar resets the shutter wheel without you knowing.

Last edited by Bob Hart; December 2nd, 2007 at 05:44 PM. Reason: added text
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Old December 5th, 2007, 01:47 AM   #7
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NZ Customs arrrgh!

Hi Aaron,

cograts on your new purchase, if you get a chance it would be great to see a pic of your rig. Let me know how you get on with it and what your thoughts are...I am thinking of getting one myself.

cheers
Sam
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