Using the Letus35A, I am noticing, only in bright light situations, that when I pan the camera seems to be focusing on the imperfection in the Macro lens or the SLR lens. The streaks do not move when I pan, so therefore they are a part of the Letus. Has anyone else had this problem? I have cleaned the lens and the SLR with a lens cleaner, but it seems to be there. Again, bright light situations (sky or point at a light) with the SLR (50mm 1.8 Canon) stepped down to 2.8 - anyone else experience this? It seems to make sense because the DV is focusing on the GG, but also on the Macro lens...but it still sucks and I don't want it.

You've found a fixed pattern artifact.

On highly contrasted highly textured and darker backgrounds, you won't see it. Across brightly lit out-of-focus or plain larger areas like walls, sky and vehicles passing through the shot, you will see it.


There are two reported alleged causes :-

Internal reflections from within the flip enclosure, apparently treated by painting the unfinished interior walls with flatblack paint.

Unique elliptoid movement of the groundglass and insufficient motion excursion of the groundglass. The combination causes groundglass texture to remain just visible as a very soft faint freckle artifact. Except for significant redesign of the groundglass movement, this will not go away.


There have been some reported tweaks to minimise the artifact.

Two such are :-

Increase the motion of the groundglass with a stronger motor and heavier eccentric weight. (This has only been partially successful to date. It is not recommended that the average tinkerer attempt this remedy as it is destructive on the original parts.)

Turn the support pillars in their rubber supports. (There are apparently sweet spots to be found where the groundglass movement becomes closer to circular and cures the problem to a significant degree.)

When storing your Letus35 it is recommended you store it face-down so that the flexible components do not deform over time due to gravity which will re-introduce a more linear component to the elliptoid motion.

Do a search here for older posts by Phil Bloom and "film of grain" and you will get more of the story.

Thanks Bob, very nice insight. I also wondered about the Macro lens (the one that comes off). It looks like it is a sandwich set of lenses and I can see imperfections within it, but alas, there is no way to fix those.
I don't think it is the GG because the streaks don't vibrate, they are almost like smudges on glass - similar if you use Windex on a window and the streaks it leaves afterwards. Interestingly enough, they are always up and down (vertical). I will try to tape up or black out the inside and see what happens, but I don't have the FLIP module, so I am not sure if this is the issue or not.
When I watched Ben Winter's film, he shot outside and I did not notice this problem and he shot with the Letus. I wonder if it is different depending on the SLR, I will have to try that.

With the non-flip model. I dont think the interior of the groundglass enclosure will be the problem.

The groundglass motion definitely is.

If you want to know for sure, set your shutter on its highest setting. Set up a plain white strongly lit target directly level in front of your camera, piece of printer paper is fine. You'll need enough light to enable stopdown of your SLR lens to f2.8 which is where your problem emerges ut also to tighter apertures to see just where the tighter threshold aperture setting for appearance of the artifact lies. so you may have to put a 500w worklight nearby to illuminate the paper.

You should see the fixed pattern as smudges which are oriented vertically with a slight tilt to the left. If there is a small spot of dust on the groudnglass, you may see this appear as two orbiting spots each 180degrees or halfway around the orbit from each other.

Now set up the same target on the floor. Light it identically and with same camera and SLR lens aperture settings, aim your camera at it, facing directly downward. This puts the groundglass horizontal the the ground and removes gravity as a factor.

You may now see the smudges assume a different pattern, possibly less distinct and maybe they will even disappear. This is because with gravity no longer altering the excursion pattern, the groundglass motion can become more circular.

If there is a spot of dust on the groundglass, you may see the orbiting pattern become more circular.

I have theorised in my own mind about using springs or a rubber band from the lower edge of the groundglass panel up to the rear of the front end piece which supports the base of the pillars.

This would impart positive retaining force upon the pillars and hopefully counter gravity force on the excursion and make it more circular. Over time however, it might cause distortion of the grommets in the groundglass panel and move it forward off the focal plane, so I am not sure this would be a viable fix.

The sandwich set of lenses you describe I am a little unclear about. What camcorder have you installed it to?

Go here if you want to see what a modified Letus flip for XL does.

http://www.youtube.com/watch?v=-O-Dl7oZTZs

and here

http://www.youtube.com/watch?v=SB1IN8HTM10

if you want to see the innards at work. This is not representative of Quyen's build work as I have substituted a much larger motor and eccenrtric mass.

This was only a partially successful mod as the motion remained elliptoid but became a random pattern. The YouTube grade clip doesn't show it but the 1/400 sec shutter on the first sky shot brought up fixed pattern artifacts which looked like asterixes or stars.

Thanks Bob, I will check that and report back.

Ok, I pulled the little stoppers out a tad, measured up all of them and tried again. Streaks are still there, and yes they are up and down, slightly tilted to the left. Must be the oscillation problem. Is there a way of using a bigger battery pack or something that won't damage the unit? I guess I need it to move faster.
Also, if I could attach a counter weight to the motor, but it doesn't look like with the vibrating motor, there is a place for it...Bob?

Lastly, what is the best way to clean the GG?

David.

I recommend you don't pull the little stoppers back. Leave them fully home in the front, otherwise this will move the focal plane back or skew it, which does not make for good images with wide lenses.

What other owners have done is turn the little white pillars each a little bit, one at a time. Test and observe with each small adjustment.

A simple and easy strobe light source is your television set or CRT computer monitor. LCD monitors don't work so well for this.

Hold the motor/groundglass/battery pack assembly very close to your monitor screen. The groudglass must be in its normal vertical orientation. Run the motor and observe the motion pattern which will become apparently slower and visible.

It may be helpful to put a very small half-pinhead sized spot of white-out (Tippex) or quickdrying paint on each corner of the frame of the groundglass panel.

It is not helpful to change the motor speed or power. Bridging the battery pack back to a single 1.5v cell did not help either.

You may have observed a small dob of glue on the eccentric weight. This is not a spill but intended to slow the motor speed slightly with air frction for better groundglass movement. Too much speed and the movement is inferior.

More eccentric mass does not work. The motor is very small. It will not start with the extra mass needed to increase the groundglass excursion because of gravity in the normal operating orientation.

This is why I experimented with the heavier CD tracker motor. For all its extra grunt, the larger motor and much heavier eccentric mass yielded a disproportionately small increase in the excursion to about 1.5mm versus the 0.75mm or so of the damaged Letus35 I have inherited.

More speed with this motor also yielded an inferior motion. I run the larger motor on 1.5v and could probably afford to run it slower as Quyen has done with the smaller motor he uses.

The GG is best cleaned with a dry lens brush. If you use an artists brush, clean it first in alcohol and dry it.

Art brushes sometimes have added oils in the hairs to keep them sharp pointed for sale display. This comes off on your optics and it harder to clean off.

You could use a lens pen on the shiny side of the GG but not on the groundglass side as it will leave a mark.

Excellent, I will try turning the white sticks and see what happens. Also, I think maybe some alcohol on the GG will help too.

Don't touch the GG with alcohol or apply any rubbing action to it until you find out from Quyen whether it is a piece of glass or acrylic. If it is acrylic, one little gouge or a polished spot and it will be ruined.

Ok, but others must have cleaned theirs up at this point - what are other people using?

All care taken but no responsibility for unforeseen consequences accepted.

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

Is that your handy work Bob? Nice job Quyen on the working, it looks very complicated and nice job taking it apart as well!

As for the white legs, they are glued to the GG holder - so this is making it problematic to turn it.
Quyen emailed me and said water with a cotton cloth for cleaning, but I do notice the GG is definitely streaked to begin with...maybe just dirty?
Ok managed some adjustments and it seems a little better. I took off the glue, re-measured the FL and pulled out the stoppers somewhat because it seemed to help more then just turning the white sticks. I think maybe Daniel with his Leaf-spring design would be nice for a mod of the letus. Even thinking about it, I am sure I could carve the GG holder with some holes around the GG to make it more flexible, but that might not happen because it is very delicate..

You might get a slight improvement in the motion if you wrap a small stationery band of about 1/16" thickness (thin rubber band) about 1 and a half inches to 2 inches long in a figure of eight pattern around the two pillars on the motor edge of the GG carrier.

Double-wrap around the pillars so that the band goes from about 1/3rd frontwards on one pillar to the other pillar right against the grommet on the groundglass carrier. Looking from beneath or overhead, you end up with two equal triangles of rubber band.

Downside is this imparts a side load on the pillars therefore they may deform with time. The effect is like an offset fixed countermass. It transfers some of the movement towards the third central pillar. How this happens is anyone's guess.

If you don't want to disturb the pillars to get the rubber band fitted, make a double-ended hook out of a thick staple and connect the two loop ends of the rubber band onto that.

With the bigger motor, the freckled fixed pattern is still there at f5.6 to f11 but the linear patch has become smaller and moved to the lower corners in the image, upper corners of the GG panel. How it will translate with the smaller standard motor I have no idea.

I tested with the Letus partially dismantled and offered up to a FX1 with 4+ achromatic dioptre. With a wider image off the groundglass, the 864 horizontal TV lines block on the Lemac chart shows clearly with a 35mm f1.4 lens on f5.6 and holds down to f2.

It is not a fair test however as everything was handheld and it might have been closer framed on the chart as the FX1 does not have an allscan display setting to check for this.

There is a moire pattern on the 1920 horizontal TV lines block but all this really means is that the CCDs are seeing every second line because the FX1 cannot resolve more than 1440 horizontal TV lines even if it is having a good day.

Bob,

Do you have a picture of this setup? It sounds good, but I am not entirely sure if I get it...
I did try putting an elastic loosely between the two white sticks on the bottom - and attached them together with a stable-like pliable metal piece. It seems to work ok, but I need further testing.

Basically what you have done is what I did.

I forgot to mention, I ran the band under the motor. This has the effect of lifting the groundglass carrier. For less leverage, position the band right against the groundglass carrier grommets. For more leverage, move the band forward on the pillars.

Where the wrinkle comes in is that I have mounted the larger motor in centre of the groundglass carrier where Quyen has the vibrator motor in a smaller hole on one side. Leverage by the rubber band will not be central therefore not directly against gravity.

To fix this you might need to glue a dummy motor in the hole on the opposite side to run the band under to balance things out.

One of those little dessicator capsules out of vitamin bottles might be a good start, or just a scrap of cereal packet cardboard formed into a single roll and glued.

It might be prudent to put a bit more stiff glue on the frontside of the groundglass panel to support the motor can directly. The main mechanical support for the standard motor otherwise is the rear bearing/brush holder/solder tabs assembly which is an insertion fit into the rear of the can and retained by folded tabs.

I have put all my stuff away for the coming working week but will get round to making an image as soon as I can.

The principle Quyen has used is elegently simple. I don't know if he arrived at it by deliberate design or serendipitous means, however to work properly, centres of resistive mass and resistive countermass must balance either side of a radial centreline leading from the input of forces from the eccentric mass on the motor centre.

Conceptually but an incorrect description :-

Imagine the crankshaft, connecting rod and piston travel in the cylinder of a recip auto engine.

In the Letus, centre of the eccentric mass becomes the crankshaft main bearing centre, the centre of the motor itself becomes the centre of the lower conrod bearing, the radial centreline of all masses relative to the eccentric mass becomes the path of the centre of the wrist pin in the piston.

At any point along the conrod there is a cyclic motion. At the crank end it is truly circular or orbital. At the wrist pin end the motion is purely linear along the cylinder centreline. At points between, the motion is elliptoid, a circle strtched along the centreline.

Toward the wrist pin end, the ellipse becomes very narrow. Towards the bearing end, the ellipse becomes more round. Beyond the conrod bearing centre where the cap bolt heads are, the elliptoid motion exists again but here, it is now a circle stretched across the centreline not long it.

On the Letus free floating arrangement, simple inertia replaces the controlling cylinder and piston. At some point in the air outboard of the edge of the groundglass carrier there will be a notional linear movement if not a real one. In Quyen's current Letus evolution, the point of linear motion is too close to edge of the groundglass, hence the narrow ellipse and part of the fixed pattern artifact problem where the two slower moving ends of the elliptic movement become visible to the camcorder.

Add resistive mass to the point where the conrod caps bolts would be and the notional point of linear motion opposite is pushed furthur out from the groundglass and the groundglass motion will become closer to circular.

More power and eccentric mass will be needed as there is now more resistive mass to be moved. Weight could be reduced at the far side but ot much can be done about the resistive effects of the third pillar

This is an altogether far too simple analogy to make but it is the best I can do.

There is a whole host of other effects, tensile resistance in the rubber parts, the pilar themselves, bending moments in the groundglass carrier and the motor and all manner of resonances which can develop which can enhance or work against the desired motion.

An aerospace engineer would probably be able solve this issue in a heartbeat as the geometry has been around since the first recip steam engine.

The ultimate solution would be a steel ring ariund the groundglasson the carrier, a surrounding array of tiny electromagnetic coils and a chaser circuit, to select them on and off in a rotational sequence, but that is high science way beyond my kitchen table and backyard skillset and not what the Letus35 and other alternative projects are about.

In another thread, Ben Winter said :

If you come across a situation where you have too much grain, you can increase the number of batteries by replacing the battery back instead with one that is wired for 3 AAs, providing 4.5v. I did this and it helped immensely in heavy grain situations with the lens stopped down."

Is there a reason you opted not to do this Bob?

1. Quyen suggested don't do it.

2. There is a small dropping resistor on the motor to slow it.

3. There is an adhesive blob on eccentric to slow it.


Basically I trusted the designer on this one. The subject came up some time back and the observations as I recall were no improvement in the groundglass movement and concerns over possible shorter life of motor in continous duty at higher voltages.

9v was being talked about at the time. 4.5v is probably more realistic.

The motor's original function called for intermittant duty only.

Bob,

I got the same message from Quyen. I am thinking of upgrading the motor though, but I want one that is small and light enough to work, but that will give me the "axle" so I can put a counter weight on it - any suggestions?

There are two styles of simple DC tracker motors in junked CD/DVD players.

One is identical with the CD tray actuator motor and almost identical to the disk drive motor itself except that the shaft is much shorter. As these motors are in intermittant duty only in a CD player, they are almost always in good condition.

These are a little to large to be used in a Letus35 but are fine for a full CD-R disk sized spinning groundglass design where space or size is not an issue.

Another style, is a smaller motor with a plastic worm drive pressed onto the shaft. This is the style I used on the Letus35 mod I attempted. It got left on and forgotten long enough to flatten a battery set so longevity should not be a problem.

I think I have seen this motor style also used in 3.5" 1.44Mb floppy drives.

There is a newer brushless motor used in some DVD players. This requires a control circuit to power it. Small scale aircraft motors likewise cannot be simply powered up with a battery two wires and a switch.

The small tracker motor is a very difficult fit in the available space in the Letus35 and the motor shaft needs to be shortened a little by trimming with a small cut-off wheel on a Dremel.

When you do this, you also need to tape over the exposed area of the motor bearing to protect it from abrasive contamination from the cutting disk. It otherwise it won't last very long before becoming noisy.

I think you will find Quyen did a very thorough search for suitable motors and there may not be much else out there he has not already examined and rejected. That is not to say the search should not continue.

I have contemplated actually dismantling a DC motor, drilling out part of the laminated centres in the armature fields, on one field, filling the drilled metal with lead to deliberately set up an out of balance state in the motor and use the motor itself as the eccentric. There will be a performance hit but the motor itself will not be otherwise loaded and air resistance will be minimised.

As I do not have the camera this Letus35 works with, I have not had it on the front burner as such.