My test footage was shot on a Canon HF 10 and I was happy with the results. However I tried on the newer HF S10 with a larger sensor and larger lens and the GG was moving. Do you think that the shutter speed makes a difference? I would of thought that the F-stop would be the deciding factor. I would think to shoot with F1.8 would be better than F8.

Actually thinking about the shutter speed afterwards makes more sense. The high shutter speed would freeze the motion on the GG.

Thanks for the practical tip.

I am using a vibrating motor holder see post.

Ee-S screen and usually no shutter faster than 60mostly because of light loss.
I found a problem with the motor speed changing during the hour in use so a cut
with a fresh battery after a cut on used battery the glass was noticeable at shutter
faster than 180.

An active speed control would be nice to control this but there
is not much room to place a speed sensor. I was thinking about a mic
inside the tube and a phase locked loop circuit but never had the time
to build one.

Maybe if the camera has some form of video output you could use a sync separator to feed the PLL?

Bob.

Hi Bob,

With the shutter speed and speed of the vibrations of the pager motor (it slings a weight
around causing the movable part of the holder to vibrate) there is a sweet spot
where the image is clearer. The vibrations actually produce a high pitched audio tone
that could be picked up with a tiny mic inside the tubes. A PLL could be used to lock
the output voltage. When the battery is drained a bit the voltage drop causes the
motor's tone to drop as well. Raising the controlled output voltage would increase
the tone keeping it locked with the PLL. (until the battery is too far drained)

Something that looked at the video signal for increased noise would be far
too big (and complex) to fit in the tubes. But liked the idea.

Are there different types or grains of Fresnel lenses?

On my tube DOF the grain is lighter and is usable but on my vibrating box DOF with rods it seems to have a distinct pattern so I want to replace the Fresnel lens, any suggestions ?

Also is the distance between the camera lens and the HD acromat lens causing the grain issue?
I am using a rubber step up from a 58mm cam lens to 72mm to the HD acromat not a solid ring so my cam lens is further away from the acromat lens, could this cause the grain to be more prominent?

Thanks for the speed suggestion Bob, makes sense but I am not getting the noticeable pattern with the tube DOF.

I don't use these things (I think usually known as 35mm adapters) so I'm a bit puzzled by some of the terms used here. My understanding is that a 35mm format lens forms an image on a ground glass screen (which is generally vibrated or rotated to blur its granular pattern) and that this screen is videoed by a small format camera via a relay lens (serving the same optical function as a close focus supplementary lens). I guess it's optically possible to replace the GG and relay lens with a fresnel (http://en.wikipedia.org/wiki/Fresnel_lensfresnel lens[/link]) which would save weight and perhaps cost, but I would have thought at the loss of image quality. Is this simplified system that being discussed? Or do you perhaps not mean fresnel?

Not exactly my idea, I think the more expensive units use something like this technique. It makes sense in that "vibrating" motion has to reverse direction at some time. At that point the movement of the GG screen is minimal so that's best done while the shutter is closed.

Bob.

Apologies guys (my round , put it on my huge tab) I should have said Ground glass lens not Fresnel

I suspect my current GG lens in my box DOF is too grainy , I was wondering if there are different grains available, also glass and not plastic? If this would be better I don’t know.

I replaced my GG lens in my tube DOF once and used super glue which simply ran into the grain like a river and glued the lens to the tweezers,
Simply marvelous.

Thanks Serena for the clarification.
I thought fresnel was another name for GG (ground glass). Wiki tells me I have 2 fresnels in my car lamps.. cool :))

Serena,

The more expensive DOF adapters do use a glass that has been ground
(purposely scratched). Some kits make use of a plastic fresnel pattern focusing screen
(like the Canon Ee-S).

The method of mounting the ground glass and vibrating/spinning also makes a
difference in the image. The holder I have does not use stilts (rods) to separate
the glass holder from tube body. Plastic fingers formed by milling prevents
free movement in all directions.

For a paid shoot owning/renting a larger pickup camera is the best choice over
any DOF device.

Rory,
The acromat is to assist in getting the screen to be seen in full view by your camera.
Are you getting dark corners in your image?

No dark corners as I am zooming into the image off the glass and the focus is good so I assume that the distances between cam lens and GG must be ok as I am getting a good focus so it must be my GG which I should replace. Or is zooming into the GG going to worsen the GG patern?

Does the acromat also assist with getting A shallow DOF image?

I am only having this problem with this DOF

thanks

http://www.google.co.za/imgres?imgurl=http://www.dvcity.com/dof%2520adapter/dof-adapter-4.jpg&imgrefurl=http://dvcity.com/dve/product_info.php%3Fproducts_id%3D206&usg=__k_776wGFyqD70RCor9BpflHPPGc=&h=253&w=325&sz=37&hl=en&start=50&sig2=xXrSnkOgRf9eRH9D87mqXg&itbs=1&tbnid=DjLNCcpzWcZRjM:&tbnh=92&tbnw=118&prev=/images%3Fq%3D35mm%2Bdof%2Badapters%26start%3D40%26hl%3Den%26safe%3Dactive%26sa%3DN%26gbv%3D2%26ndsp%3D20%26tbs%3Disch:1&ei=V7UxTMqeF4SFOPzInLkB

My tube type DOF IS ok but the above model is supposed to give better results??

I was with Rory a week ago and we did get dark corners so we had to zoom in a bit. Although the GG was spinning we definitely had a pronounced visible pattern even after using a slower shutter speed. I think the GG is in question. I'm using a static GG in my setup with a 50mm 1.8 lens and I don't get such a pronounced pattern. Thats why I asked if one could add a 0.3% blur on post.

" Thats why I asked if one could add a 0.3% blur on post. "

One certainly can but of course it will blur everything.
You can try something like a Smart Smoother (free) or maybe a grain removal tool (expensive).

One other factor to keep in mind. The moving pattern from the GG can seriously stress a HDV or AVCHD encoder. Possibly to the point where it simply stops encoding the grain and you end up with a static pattern that changes every GOP.

There are various ways to make the screen. Microwax is one material people were experimenting with some time ago. Rather than "grinding" the glass I wonder if anyone has tried etching the glass?

Bob.

Bob that’s a good idea, my mind went back to when I studied fine art and we acid etched plates and glass. I was always amazed at how uniform
and perfect the etches were. Worth looking into and easy to do.

Some folks on the web have done their own and are getting good results, I ordered some stuff from the States and a local camera shop, if that doesn’t work that will be my next step.

Years ago I tried my hand at glass etching using hydrofluoric acid which eats into glass. Warning this stuff is very dangerous to work with. Wiki has some interesting notes on it.