New (?) Oscilating Ground Glass Idea

[Martin Diruf]

Hello,

By reading through the articles I found a lot of concepts concerning an oscilating ground glass. Some good ideas (i.e. piezo-elements) turned out to be improper. In the end a fine mechanical device seemed to make the race. Graphic link: de.geocities.com/raiorz/vibro_old/vibro1.jpg by Rai Orz. It seems that building this assembly demands special tools working with an accuracy of less then 0.01 mm.

Has anybody yet tried something like this instead?

www.blackmary.de/bitmaps/oscilating_idea.jpg

The motor is installed directly onto the groundglas with the out-of-balance-weight axis in 90 degrees, so that the circular movement doesn't affect the level of focus.

The vibrations are uncoupled from the frame by some kind of rubber membrane.

The idea seems quite simple so - looking at the number of active members of the DVinfo community - something shouldn't work (Vibration? Noise?).

[Les Dit]

Martin,
The main issue is that even a slight vibration or tilt in the axis perpendicular to the ground glass results on the image shifting. That will appear as a vibrating or unsteady image being seen by the camera.
I built such a design approach with the GG being held in place by several flexure 'bearings' made of thin spring steel that I hoped would be stiff enough in the 'sensitive' axis. Unfortunately the motor/weight combo made enough force in the undesired axis to ruin the steadiness of the image. Even 0.001" of tilt motion in the Z axis ruins the image stability!

-Les

[Martin Diruf]

Thanks Les,

but that means, that every attempt to realize a "spinning-disc-adapter" is useless, isn't it? Aslong as you don't have the accuracy of let's say i.e. a hard disk drive (not very likely with the tools i've at home) you will always get a little movement along the z-axis.

How did agus get rid of this problem?

Cheers,
Martin

[Frank Ladner]

Martin,

I myself have given up on the oscillating designs because I lack the tools to build the device with the precision required.

However, I have now gone back to the spinning adapter. Why? Because I have the parts needed, and the precision is already in place. (Compact CD motor)

Even if you use a regular DC motor from a computer fan, you can design it as precisely as possible, and let the natural spinning motion keep the image surface in alignment.

I think the image quality would be equal with both designs, but the difference is in the size. With a spinning adapter, you would need a housing with a width of around 4.25 inches or more (depending on the motor diameter and projected image size). With an oscillating adapter, you can possibly get away with fitting the whole thing inside a 2 inch space. (This is with the 12-bearing version. The 9-bearing counterpart would form a triangle around the image surface, requiring more space.)

I've achieved nice results with some translucent cooking sheets (purchased in a 3-pack from Wal-Mart) cut into circles a little smaller than a CD. The device doesn't look nice, but the image is what counts.

Well, I will say that the aesthetics of the device / housing itself goes a long way too, because you do want the actors and crew to take you seriously. I'd be kindof ashamed to break out my masking-tape-reinforced adapter. LOL!

[Les Dit]

Making one with a spinning disk is a lot easier, because the motion is less 'violent' . Most of the CD versions of this still failed however, because it's still not that easy to keep the disk flat while it spins. Did Agus ever post a full res. high bit rate video that was artifact free? I'm not sure about that.

<<<-- Originally posted by Martin Diruf : Thanks Les,

but that means, that every attempt to realize a "spinning-disc-adapter" is useless, isn't it? Aslong as you don't have the accuracy of let's say i.e. a hard disk drive (not very likely with the tools i've at home) you will always get a little movement along the z-axis.

How did agus get rid of this problem?

Cheers,
Martin -->>>

[Karel Bata]

This has probably been covered in other threads, but looking at the image you posted: isn't there a point in the middle of the ground glass that is virtually stationary?

I'd always imagined the imaging area to be some place off the centre of rotation...

[Martin Diruf]

Hello Karel,

The concept of the image I posted is different to the "spinning CD type"-adapter. The ground glass doesn't turn around on an axis through the middle, instead the screen moves along in a little circle as a whole. Difficult to explain, but if you use a sponge to scrub a plate, you'll do the same movement. So because every Point of the screen does the same motion parallel, there is no stationary point.

Cheers,
Martin

[Karel Bata]

Ah, my bad (as they say). I didn't spot that. Now I see.

That's a neat way of doing it.