Guskers35 and inverted rail system...thanks guys!

[Ben Winter]

I'll agree. Sometimes we get people who rant and rave about their new homebuilt adapter that "just needs a few tweaks." Then the sample footage has the most vignetting you've ever seen, aberration up the wazoo and a horribly wrong focusing screen distance.

On the other end of the spectrum, there's yours, which looks really nice and clean. Thumbs up.

[Wayne Kinney]

Ben's right, the diffussion seems to be the only issue. The image is sharp, colours are good and i see no flickering from the spinning GG, the image looks solid.

How did you go about making your acrylic disks and centring them on the motor (what type of motor)?

[Dennis Wood]

Thanks gents...your feedback is very encouraging. I've been tossing ideas back and forth with François Poitras over at pana3ccduser on most of the adapter components so he deserves props on this too. It's great to collaborate on a project like this both from the buying/ordering side and the problem solving side.

The GG I have in there is not the optical acrylic and is 2mm thick which I'm thinking is not ideal. I picked up few samples from a local plastics specialist. The motor is from an audio CD player, and interfaced using a 5mm prop shaft adapter. In one of the vibrating GG threads one of the posters (can't remember who) mentioned checking runout by spinning up the GG and examining the reflection of light on the unground side. This GG shows some wobble in the reflection...so I know it's not perfect. It got pretty hot during the cutting process, so it's likely warped a bit.

I'm cutting the discs pretty simply by placing the acrylic stock on a flat surface, and drilling a 5mm hole through the material and into my plywood work surface. I take the drill bit out of the drill and insert the shank into the hole, creating a spin point. I'm lazy, so I've been using a dremel clamped to an overhanging board as the cutter. The acrylic is thus rotated in place, centered by the drill bit pin, past the dremel cutter, resulting in a perfect circle. I have a router that would work a lot better, but I'm being lazy. Wayne, your drill press rotating glass cutting method got me thinking in the right direction. The optical grade acrylic blanks we have coming were pre-cut by the vendor to a .1mm accuracy so I likely won't cut my own too often.

For blasting, I'm using a very fine (100 grid I think) glass media. The acrylic disc is affixed to a disc sander surface with two sided tape. I leave the protective sheet on the non-blasted side to make sure it is not damaged. By rotating the disc at about 1000rpm while blasting, my thinking is that a consistent surface is far easier to achieve. I've been playing a bit with gun pressure and distance from the media to achieve a good finish.

I believe Bob Hart posted a blurb on pitted surfaces being better than sanded surfaces for diffusion, but I can only find one reference to a journal abstract dealing with this topic. I'd be tickled if someone had data on this.

I've read a few posts on which side the GG surface should be facing. Mine is facing the 35mm lens. Is there any theory on which is technically better?

[Ben Winter]

There are arguments for both approaches, but I don't think it makes much of a difference either way.

[Wayne Kinney]

Dennis,
Thanks for sharing your cutting method there, very similar to the SG35 build process.

The SG35's ground side of the disk faces towards the camcorder, this is to minimise internal reflection.

[Francois Poitras]

Dennis, since we are testing GGs with different thicknesses, I would think it is best for now to put the ground side towards the camcorder and to set the back focus/infinity like that. This way, when you replace a 1mm GG with a 1.5mm one, the lens flange-GG distance should in theory stay the same (provided the collet holding the GG does not move).

[Bob Hart]

Dennis.

Ben Gurvich has my original pressed disk but I went over it again with a finer finish, so there were effectivly two grades of finish on it. In his short film he made with his AGUS version, the outdoors scene where the father answered the door was very sharp.

2mm may be a bit thick because any internal reflection may be more evident as a fringing artifact, but that is speculation on my part. I understand a similar issue called halation happens with film which is why they have the anti-halation (anti-reflection) black surface on the back of it.

I have been trying to dress split DVD+R disks and spacer disks out of DVD+R packs, which are from actual disk production. They take some starting but can be split. Once they are split, they are very thin and run true if the RPM is high enough. They tend to drift off in violent camera moves if motor speed is high

When dressing them in abrasives for glass, they seem to have some surface hardening or some treatment which resists the abrasion and kills the abrasive brew off pretty quick. You have to carefully search for the side which has the guide tracks on it and dress this surface.

It took me as long to dress a DVD+R as it did an actual glass disk.

I got a very nice result from it but damaged the disk trying to re-mount it. I have not successfully made one since.

Your blasting method might be the answer. Have you considered mixing abrasive media. Big pieces of softer media to get inertia to press very much smaller particles against the plastic.

You would need to have this arrangement inside a cabinet and some protection for the motor which will last a very short time indeed once the grit starts to fly.

[Dennis Wood]

Thanks for ringing in on this Bob. I've read and appreciated a lot of your posts in this forum section! I'm curious if you have any research on pitted vs sanded diffusion surfaces? Fortunately we have 1mm and 1.5mm precut optical acrylic discs coming...

Wayne and Francois, I've been mulling over the GG issue, and having the GG towards the camera makes good sense. The extra air-polymer interface after diffusion cannot be a good thing. Logically, diffused rays will be exiting the diffusion surface at all kinds of angles..which is why the fresnel works so well to immediately redirect them. If you have an air-polymer interface after that with some highly incident rays, there's bound to be some diffraction occuring. Add in a slightly wobbling GG and you're likely guaranteed ghosting.

Wayne, my research for (and subsequent design) of a 16:9 hood for my .66 WA lens tells me that your suggestion to put a 24x36 frame immediately adjacent to the GG is a good thing. There's no optical use for the extraneous areas of light in the circle surrounding the frame. It hadn't occurred to me, but it seems pretty obvious now that you've pointed it out. A film SLR shutter pretty much does the same thing.

The scene I taped is easily reproducible, so rest assured I'll be reposting a clip with the rigid 1mm disc and fine tuned diffusion surface. Your feedback here gentlemen is a very positive catalyst for thought...

[Bob Hart]

Dennis.

After I did the pressed version and did a quick finer dressing in my glass machine on that later, I did not perservere with plastic as I had by then started working on glass.

Using any paper backed abrasive sheet on plastic makes a pattern of scratches. Dressing using a grit in a fluid suspension tends to make pits as the cutting action of the grit is more of a rolling action across the two surfaces rather than a dragging action.

I found it very hard in fluid suspension to get a good pit finish on plastic. There was always a scratch there somewhere. Then they started making this newer plastic which seems to be harder to dress using suspended grit.

The pressed method was simply using a 600 grade silicon carbide wet and dry paper, placing the CD-R spacer disk on a smooth clean semi soft surface like paper, then placing the wet and dry face down on it.

Then I would move a round tipped stylus shaped stick or a small alternator bearing over the wet and dry with lots of pressure on it and making sure the wet and dry did no slide on the CD-R and scratch it. The wet and dry had to be lifted off and reset again and the process repeated.

You got out of this a texture which was finer than that the actual dispersion of pieces of grit on the paper.

I tried finer paper but was unable to get good penetration into the CD-R with the method I used. Other methods might be more successful. After starting with the glass I moved on in that direction.

[Dennis Wood]

I did some work today on a 1 mm GG. The good news is that I've refined the "blast" procedure and am very happy with the GG surface. I did the tests you suggested Wayne...the discs are diffusing very well. With the adapter about 2 inches from a red LED (sans 35mm lens), GG off, the entire 24x36 frame is red when viewed through the cam's side of the adapter. I also did some work on the prop shaft adapter so that it spins true with the disc on it.

So in subsequent tests, I was quite disappointed to see what you had described as ghosting..until I tried the test with the GS400 alone. The same artifacts were present! I've seen this before on this cam...but didn't think to try just the cam until tonight. I believe what we're seeing here is plain old DV colour blooming. I found that as I diffused the light (the shot used a 500 watt halo..no diffusion) the artifacts were reduced. In fact, the adapter footage had less blooming than the straight cam. Grabs here:

Bare GS400

Adapter attached GS400

GG


You can right click and save the image to zoom in.

[Wayne Kinney]

Dennis,
Cool. Could you please setup a shot with an object as close to the lens as possible and the background as far away from the object as possible, then pull focus from the close object to the background. It just does not look like your getting shallow DOF, but its hard to judge from these grabs. What SLR lens are you using? Keep it up man!

[Dennis Wood]

Ha, I know the DOF camp has been disappointed with my grabs. Once I get outside (still snowing!) I'll do some more DOF type shots. I was more concerned with image quality and critical focus with these little grabs. The few DOF shots I've done in my editing nook are typical of what folks are seeing with these adapters. I have a few Minolta MD lenses...an F/1.4 50mm Rokkor, F/2.0 45mm Rokkor, and an F/4.0 28-130 zoom. These grabs all used the F1.4 50mm. It has a min. focus distance of about 1.5 feet....so prepare for some seriously shallow DOF.

The upside to all my anal retentive fiddling is that I've got the GG plane very close to the 43.5mm flange to GG distance so that my lens focus marks are quite accurate now. Given the difficulty focussing in some situations, I know I'll be using them. I bet if I had $.05 for every time someone came back to the editing suite with out of focus adapter footage, I wouldn't be making my own adapter ....

To be truthfull, the first time I fired all this up, I was pretty disappointed! Designing the rod system to flip the cam and finding out the LCD will never display the image correctly, initial issues with the achromat too close, the blooming etc. Zooming down the barrel to that little light at the end of the tunnel, you would never guess these things can actually generate a decent image! Whenever I get discouraged, I pull out a few of my SLR DOF shots and then I feel better....

[Wayne Kinney]

Dennis,

OK, would be great to see the pull focus shots and shallow DOF, i think your adapter is a winner!!!

[Dennis Wood]

Finally, here's the clip demonstrating some DOF. You may need to click the "launch in external player link" if you're using firefox...and it's a wmv, not an avi. It's indoors, at night, F/1.4 50mm. The new 1mm GG needs a bit more work as I think it's a bit light on the diffusion side...but I think it's almost done.

[Leo Mandy]

Wow, that is a nice grainless image!