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What is Ground Glass?

ground glass film analog cinematography aspect ratio gate scan

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#1 Jacob Mitchell

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Posted 02 June 2017 - 12:22 PM

Hey all!

 

Just finished the ASC Manual and there are a few spots that speak about "Ground Glass". From what I've collected, it seems to be a piece of glass that works between the gate and the reflex and often includes different aspect ratio guides? Can anyone give me a clearer definition? 

 

Thanks,

Jake


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#2 Stuart Brereton

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Posted 02 June 2017 - 12:28 PM

A ground glass is a piece of frosted glass in an optical viewfinder that sits at 45 degrees to the mirror shutter. You'll find them in film cameras, and reflex stills cameras, where they are sometimes called viewing screens. They are interchangeable, and come with various different markings


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#3 David Mullen ASC

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Posted 02 June 2017 - 01:54 PM

The fine frosted surface of the glass allows the light projected from the lens and bounced off of the mirror to be seen as an image, like a rear projection screen. That screen can have frame lines drawn on it.
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#4 Simon Wyss

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Posted 03 June 2017 - 07:33 AM

Frosted glass can be made by several quite different methods. Accordingly you have differing qualities, literally, of surface.

 

One method is the obvious, glass is ground. That is a lapping process with particles of a substance harder than the glass, a lapping liquid and a lapping plate, generally of softer material than the particles and the glass. A follow-up of ever finer grit affords a fne enough roughness. Common with ground glasses for view cameras. Executed by a trained and knowledgeable technician, rather fine matte surfaces can be had.

 

Another method is pounding, small particles being blasted onto the glass piece by pressurized air. Rather difficult to do but very fast. In use with elongated massives

 

Thirdly we have the chemical way, etching. Hydrofluoric acid ( ! ) bites the glass, very fine dimples of more or less equal size are produced. Common with what we know from movie cameras. Again: Danger !

 

Last and not least, LASER burns. Helps predominantly refine ground glasses by melting down ravines. Pulsed LASER can bring unique structures, even directional. See also Laserbrighten


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#5 Robert Hart

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Posted 03 June 2017 - 09:28 AM

There is another system, which was discussed on the dvinfo.net forums when groundglass based 35mm film emulation adaptors were being made. The lens image from the 35mm format lenses was normally cast onto a groundglass screen in similar fashion to a camera viewfinder and that projected image was itself aquired by a small chip video camera.

Many builders preferred the traditional "ground" glass, which was spun, oscillated or vibrated to eiliminate by motion blur, the texture of the finish.

Others employed another method which was a thin layer of a special blend of beeswax and paraffin wax which when it set from molten state, coalesced into a very fine crystalline finish. It was so fine as to barely show its texture in an image taken from it. To control the uniform thickness of the layer it was in its molten state, wicked up in between two warmed thin planar sheets of optical glass and allowed to cool slowly and set in place.

In large format plate stills view cameras, screens made by this method were called Boss screens after the inventor/manufacturer. They yielded superior light transmission and sharpness of image in view cameras for focusing prior to inserting the sensitised plate or film back for exposure. In hot conditions, the Boss screen can be adversely affected. So far as I am aware, the Boss screen was not adopted into widespread use in 35mm format stills camera viewfinders.

I attempted to make a spinning disk with a wax layer between two disks but the variable density issue with the wax thickness I could not control with my backyard methods. The image was very sharp and clear but also the was a bad flicker artifact.

There you go, a bit of useless history you did not need to know.


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#6 Simon Wyss

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Posted 03 June 2017 - 10:59 AM

Not at all. I’m in landscapes of non-profit information by birth, it seems.

 

Wax is nice because cheap and easily removable. Redoable

 

Motion-picture cameras are designed, uhm, were designed after deliberations a little different from still photography’s. They must withstand bigger temperature changes in shorter time, should be demountable and remountable within an hour, and should not yield to transport shaking. Film movie cameras are relatively rugged. Stories have been told of Mitchell cameras blown from tripods downhills and put back to service after recollection, Eyemos dashed about, and so on. Prisms therefore are preferred to glass plates. Those who make prisms simply leave out the polishing on desired surfaces. Same with planoconvex lenses, a widely used form of the frosted element, cf. Bell & Howell.

 

Let’s never forget what beautiful films were shot without sophisticated viewfinder equipment. Once fully acquainted with his camera a cinematographer begins to rely on other components such as raw stock, lab and development, and lenses. Sure is it nice to be able to follow a character or whatever with a TTL reflex system but some decisions lying at deeper levels, say, do I move the camera at all, what focal length has which effect, and the like must be taken with every camera. I’m having an 8mm camera on the slab right now that has a big spring. By that spring it runs for almost a minute, about triple the length most 8mm cameras are capable of working. So an owner of such a model can plan for longer continuous shots. Would s/he need a reflex finder then, a ground glass or a video tap? I don’t think so.

 

My 200 unnecessary words


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#7 Jacob Mitchell

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Posted 21 July 2017 - 05:45 PM

Thanks for the knowledge everyone!! I love hearing the science and chemistry behind everything.
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