Can someone tell me the purpose of GG?

bleutuna

bleutuna

Member
When building a 35mm adapter, what's the purpose of the Ground Glass? What do people mean when they write they've used WAX rather than GG? Furthermore, what's the difference (technically and quality wise) regarding moving and static?

IF there's a FAQ on this, please, just link me to it :)
 
There's probably a few FAQ's but I don't have any direct links. I'll try to explain this in as simple a manner as possible.

35mm lenses "project" an image to a certain distance behind the lens. For most lenses this is from about an inch to two inches behind the lens. In a 35mm camera, this distance is from the lens to the film. In a 35mm adapter, this is the distance from the lens to the ground glass. Since you are not recording the image onto film as you would in a 35mm camera, you need something for the image to be "projected" on. The ground glass is the medium that the image is projected on. If there is nothing for the lens to "project" onto, the camcorder has nothing to record. So, basically you have it set up like this: 35mm lens -> GG -> Camcorder. Without that GG your camcorder can't "see" anything to record. Well, nothing normal at least. Hopefully this helps to explain things.

You can use wax as a "ground glass" although technically it's not a ground glass. Basically any medium you are using as a "screen" for the image to be projected on is usually referred to as a ground glass. The purpose of using wax is that you have better diffusion and less (some say NO) grain. It's tricky to get right, but it's one of the elusive "perfect" screens if it can be done right.
 
Thank you, that now makes sense :) And now it makes a lot more sense as to why people just don't buy a few adaptors and connect them together ;)
 
A ground glass is a focusing screen, nothing more. The ideal solution would be an aerial image where the taking lens has a specific flange back focus and the relay lens is focused to that point in space, with nothing in between. That is how most optical printers and film chain transfer projectors work. No reason it shouldn't work for a DOF adapter. I would suggest that someone try making one with a removeable focus screen that is used for set-up (doesn't matter how grainy). Then you shoot an aerial image, no light loss, no grain, no spinner artifacts, I'm suprised no one has thought of this already.

I'm not patient enough for precision building so I haven't tried one yet but hey.

Also if you put a second lens, a cheap 50mm with close-up diopters, or a macro lens inside the tube that would flip the image. I don't know how much light you would lose, but hey happy tinkering.
 
dop16mm said:
The ideal solution would be an aerial image where the taking lens has a specific flange back focus and the relay lens is focused to that point in space, with nothing in between. That is how most optical printers and film chain transfer projectors work. No reason it shouldn't work for a DOF adapter. I would suggest that someone try making one with a removeable focus screen that is used for set-up (doesn't matter how grainy). Then you shoot an aerial image, no light loss, no grain, no spinner artifacts, I'm suprised no one has thought of this already.
Click to expand...

Wait wait wait. I just came to this conclusion too, independent of this thread and I am taking steps to test this. However, I thought that it had been mentioned that this wouldn't work. But if this is how optical printers work then why wouldn't this work for an adapter? Can anyone tell me why this wouldn't work? I agree that the idea of no light loss and no grain are incredibly compelling goals.

Edit: Darn. I just found this example video by Wayne Kinney that shows why this wouldn't work. Obviously you can see the image through the camcorder, but apparently this cancels out the depth of field given by the SLR lens. Just thought I'd put this up for others wondering about the same thing. Darn!!!!
 
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how about using a bettie intiscreen? They use these for the viewfinder in an medium format camera it actually adds a couple of stops brightness to an image.
 
Well, that's been suggested and done, with very good results. The thing here is that there is still grain. Any sort of focusing screen or ground glass will have grain. A lens would not. That's why I was so intrigued with the possibility of it. I've got some other ideas (and parts) coming in soon that I will be trying out. Hopefully with some good results. I'm also very excited to see the results of the Linhof screen that Edweirdo has ordered.
 
If you focus on the aerial image (no screen or ground glass etc) you will not achieve a DOF any different than the camera without the adapter given the same field of view.

The whole idea is to allow the 35mm lens to form a larger image with it's shallower DOF and photograph that image.

Bill Turner
Schneider Optics
Century Division
 
Which is what I have found the hard way. Since we have someone now who actually deals with optics professionally, let's ask a few questions. :D

Is there any possibility of creating a system which allows you to film an aerial image while retaining the depth of field properties? Is this even optically possible?
I am assuming that the answer is no as there have been many many people who have tackled this DOF problem and have chosen to use a groundglass or some other sort of imaging medium. So I am assuming it cannot be done strictly by optics. I just thought I'd ask.
 
Thank you Bill. It's good to have that option ruled out. Now we will continue in our pursuit of the "perfect" screen.
 
I'm not about to start an argument with Bill Turner, I'm no expert of any repute, and I've never built a dof adapter, but how is rephotographing a ground glass optically any different than an aerial Image?

Quote from " The Technique of Special Effects Cinematography; Forth Edition, By
Raymond Fielding. Chapter 9, Aerial Image Printing..."

"Principals of the Process. An aerial image is a real image which is cast into space, rather than on to a screen, ground glass, or piece of film. If, for example we set up and 8x10 inch view camera, and focus a scene upon its ground glass, we have caused a real image to be produced at the plane of the viewing glass, where suitably diffused, it becomes clearly visible. If, with the camera in the same position, we now remove the ground glass entirely, the real image becomes an aerial image. The Focused image still exists in the same position, but since it exists only in space, it cannot be seen with the eye. What is important from our point of view is that this invisible aerial image can be re-focused by a second by a second optical system so as to produce a real image at another position."

Sounds like what you are trying to achieve. All of the DOF/FOV characteristics are created by the original lens, the secondary lens simply copies that image. Now you need a focusing screen in order to calibrate everything, but once set up it should be able to be removed.

Now, it may not be possible to focus a video lens precisely enough to see just the aerial image and not the back of the taking lens, that may be why it has proved impractical.
 
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This is my thought on the matter. For the purposes of special effects this would relate to the use of optical printers (as far as I know) to create a composite image. The problem here is that you are simply projecting two pieces of footage in the same place and then re-photographing that with another camera, thus forming a single (composite) image. This works for special effects because the depth of field in the footage has already been created by the original camera that filmed the scene. The depth of field is already in the footage. For our purposes this doesn't work because we are trying to capture depth of field as it happens. Like Bill said, all it does is add another lens into the optical "chain" and doesn't transfer the depth of field properties along with it. Does that make sense? You can try it, but it doesn't work. :(
 
It doesn't work. It's been debated and tried over at dvinfo. You might, in theory be able to do something like this with a digitally controlled optical device, but a optical device like an adapter will simply "see" and aerial image with infinite focus.

The "perfect" screen as far as optical properties are concerned can only be one of two things: a crystal clear nano-particle suspended in an optical polymer or other hardened fluid; or an engineered optical acrylic surface that is, essentially, a mico-lens array with each individual "lens" optimized for light transmission (these lenses would not be distinguishable from one another by the naked eye and would instead look like "frost").

Both are very expensive options, which is why the "near grainless" G35 is a $1500 adapter, and not $300. The Brevis35 and my forthcoming Go35HD use the latter option :)
 
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isn't the spinning soley made for the fact that it emulates film stock?

I have a question: Does motion picture cameras ground glass spin as well?

Does the RED camera? Or does it just have a regular ground glass like an SLR would without the spin? I'm not even sure if I know what I'm talking about...

Why doesn't interchangeable lens like XL-1 or Fujinon have the same DOF capability as a 35mm lens would or as RED would?

sorry for bringing up an old thread- but i had the same questions in mind. hoping to get some new answers as well...
 
dvpixl said:
Does the RED camera? Or does it just have a regular ground glass like an SLR would without the spin? I'm not even sure if I know what I'm talking about...
Click to expand...

Nope, the red-camera simply has an oversized image sensor, and bigger sensor means shallower Dof.

As for 35mm filmcameras. They don't have a ground glass in front of the film, I believe. I'm not an expert, though.
 
The GG spins because when you record the image on a GG, whatever defects that are on its surface also shows up in your footage (grain, dust). Spinning or vibrating the screen blurs these imperfection together and helps eliminate grain and other artifacts.
 
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