Dealing With Dust

[Mitchell Lewis]

I'm having problems figuring out where the spots on my video are coming from. It's obviously dirt/dust, but I don't see any on the Nikon lenses (it could be inside the lenses, because I bought them used). I'm guessing it's on the Letus achromatic lens as I haven't checked that in a while. Or maybe the EX3 lens.

When you see spots on your video, where do you find it's located? What parts do you look at first for cleaning?

[Rob Collins]

So far I haven't had any dust inside anything. I just check all exposed surfaces with a flashlight when assembling. I'm dreading the day I have to open up the Letus. Let us know.

[Mitchell Lewis]

Since I've posted this, I've seen some other comments from the Vimeo Letus Lounge. Sounds like the dirt I'm seeing is on the achromatic lens. I need to check that throughly before my next shoot. I'll let everyone know if this ends up being the solution.

But yeah, I'm crossing my fingers I don't have to take the thing apart. :)

[Mitchell Lewis]

I cleaned the achromatic lens today. Really easy to do. It just unscrews and you can easily (carefully!) clean both sides. But unfortunately, that didn't cure my problem. It seems to be a little better, but I'm starting to worry the dirt/dust is on the mirrors or something else inside the adaptor.

Anyone taken an Ultimate apart before to clean it? Am I crazy to try it? It looks to be fairly well sealed, I'm not sure how dust/dirt got in there. I think it came this way from the factory. I'm glad it's not too bad. See if you can see it in this video:

On A Good Day on Vimeo

Maybe it's not that noticeable.

[Bob Hart]

Mitchell.


Maybe post four frame grabs for us to study and troubleshoot from. Make the images this way.

Establish correct relay focus on the groundglass.


Take the Nikon/Canon or whatever lens off the front of the Letus.


Frame Grab 1.

With groundglass motor not running, point the adaptor at a point source of light only bright enough to shoot an image of the groundglass with the camcorder lens iris wide-open without burning out the image. Switching in the ND filters is okay - I want to see correctly exposed groundglass texture. The point source of light should be about ten feet (3M) from the Letus so that shadows cast from crap within onto the groundglass will stay sharp.


Frame Grab 2.

Same as Grab 1 except turn on the groundglass motor.


Frame Grab 3.

Close the iris of the camcorder to about f11. Brighten the point source of light but try to keep it as similar in size and position as for the first two grabs.


Frame Grab 4.


Take the Letus off your camera.

Fit a lens on front of your Letus. Set focus to infinity and aperture (iris) wide open. Make sure the backfocus is correct. Light the Letus from behind (the camera end) so that light passes forward through it. Make sure your light source is not hot or too close and damaging to the adaptor.

Position the camera lens to lens almost touching to look back though the Letus through the lens you have put on the front. Zoom in with your camera until the frame shadow on the groundglass is full framed in your camera viewfinder. Your own camcorder lens will also have to be set for infinity focus.

You may find that you cannot zoom in far enough to get a reasonable frame. Change the lens on front of the Letus for a wider or narrower one until you get as close to a full frame of the groundglass from front-on as you can. If this is correctly set up you should be able to see the groundglass texture sharply.

You can also use this method to set your Letus backfocus is absolutely pin sharp, provided your prime lens on front has accurate infinity focus and camcorder backfocus is also absolutely spot on. This is how an autocollimator works.


Furthur tests - no frame grab required unless you find something interesting.

To test for crap on the consensor, pull focus on your camera lens with the iris fairly closed like f11 or f16, you may see the blemish become distinct if it is on the condenser. The camera lens focus may not have enough range to do this successfully.

A furthur more accurate but fiddly test you can do to see if there is crap on any of the prism faces or the condensor is to remove the achromat, fasten it or hold it carefully to the front of your camera or simply use your own eye with the achromat held up as a magnifier.

Look through the adaptor with the camera onto the backlit groundglass screen with the motor off. Find sharp focus on the groundglass.

Then slowly move the camera (or your eyeball) and achromat together physically backwards slowly from the Letus. If there is crap on any of the prism surfaces or condensor lens, you will see it pop into sharp focus as you pull back.

The faces I describe next are transmissive faces (light passes through them).

First will be condensor front face, condensor rear face, compound prism front face, first compound prism bonded joint (should be invisible), second compound prism bonded joint, (should be invisible), compound prism rear face.

Now I refer to reflective faces which bounce light like a mirror.

Dust on the reflective faces of the compound prism would normally not be observable unless it is stuck on by a bit of grease or glue. Fingerprints may show. These will show up at points between the observable transmissive faces as you pull back.

Anything on the prism faces is unlikely to cause any real problem unless it is big or is an oily mark like a fingerprint which will cast an indistinct broad blemish.

Something on the condensor may only show up as a blurred shadow or sharper depending on how closed your camera lens iris is set. It will have more effect on the image than something on the prisms.

If it is on the faces of the condensor, then it will pop into sharp focus as you pull back, front face will show first, then the rear face.


Hopefully this helps

[Mitchell Lewis]

Bob, you've obviously got a lot more time on your hands than I do! hehehehehe (that's a lot of work to figure where the dust is)

BTW, I don't have a Letus relay lens....but I want one. :)
But I'm going to make some time over the next 3-4 days and try to duplicate your tests. Stay tuned..... (thanks for the very informative reply....much appreciated)

[Bob Hart]

Mitchell.


It is quicker to do the tests than to write about it. Having built adaptors helps.

Time on hands?? The time writing would have probably been better spent doing something else which needed to be done.

However, there it is, mature-onset ADHD or dementia maybe? You call it - I'll agree.

Pay the favour forward some day. It makes the world a better place, or so I choose to delude myself.

[Mitchell Lewis]

Hahaha! Thanks Bob. (I think we share some of the same issues!)

But yes, I try to pay it forward when ever possible. Forums are great for that! :)

[Leonard Levy]

generally dirt is on the ground glass. You should be able to see it start moving in little small, circles and come to a stop with the motor. if your monitoring is good enough to see it.
This is the problem with vibrators instead of spinners.

[Dennis Murphy]

Little black specks were regularly sent from the Devil into my Letus Extreme. They stuck out like dog's balls on brightly lit scenes.

I found that they were mostly turning up inside the prism area. Compressed air, no matter how I applied it, would not shift them onto the dust trap.

Because I live in New Zealand, sending it back to the USA for a clean up was not an option... so out came the Allen keys and it was time for a game of 'Operation'.

My first foray into it's guts was a disaster... when the prism came out, I chipped a corner off it... my heart literally stopped and I had to punch myself in the chest before I lost consciousness. Thank god it didn't do enough damage whereby it showed in the frame when zoomed in to the appropriate depth.
Another thing was how damn hard it was to remove all traces of handling off the different glass surfaces (prism, achromat, condenser etc). I literally have spent hours cleaning and polishing glass. Cleaning lenses is an art that I am getting better at, but still far from being a grand master.

Upon re-construction, the same little black specks re-emerged after a couple of months. I concluded correctly/incorrectly that they were paint specks.

IT WAS TIME TO GET EXTREME WITH THE EXTREME!

It completely stripped it down. Removed the glass that is hot glued in, unsoldered the motor so the wires could be removed from the body etc so that I was just left with the metal shell.
A couple of coats of paint stripper later, it was back to black alluminium.
I wiped a coating of axel grease inside for a dust trap (which might not have been a good choice of sticky stuff) and re-assembled.

I haven't had black specks inside the prism section since, but still have to give the GG area a clean out every so often (which is not that big of a deal).

All I can say is that after literally days of work on my Extreme, I have now developed a neurosis around dust. If I see even a speck on my computer monitor, I go into a cold sweat and curl up into the fetal position, rocking back and forth whilst talking in a childs voice.

I hate dust.

[Rob Collins]

That freaks me out. I'm going to make my Extreme like the boy in the bubble. Or sell it and my EX1 for a 5D Mark II.

[Mitchell Lewis]

Originally Posted by Leonard Levy

generally dirt is on the ground glass. You should be able to see it start moving in little small, circles and come to a stop with the motor. if your monitoring is good enough to see it.
This is the problem with vibrators instead of spinners.

Thanks Leonard. We're using a Ultimate so dust on the GG isn't as big of an issue.

I still haven't taken the time to do Bob's tests. But after cleaning the lenses with Ultra Clarifier (again!) on my last shoot I didn't notice any dust spots. Maybe it was just a coincidence.....dunno. I've got another shoot on Monday and then we're leaving for London until the end of the month. But then I plan to really figure this dust problem out.

[Mitchell Lewis]

Dennis: Wow! That's quite a story! Thanks for sharing. Hopefully the men in white coats don't show up next time you find dust on your footage. :)