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Old November 27th, 2005, 09:16 PM   #1
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Ground glass ... Sanding or media blasting?

The search function brings up 0 hits on sand or media blasting to create a GG, so I have some questions I'm hoping you folks can help with. I'm building a spinning GG 35mm adapter.

I've used my sand blasting equipment to create "diffuse" glass for some custom staircase lighing fixtures. Can anyone comment on wet sanding vs media blasting to create a GG? If blasting is an option, what media is preferable?

I have 1000, 1500 and 2000 grit wet paper so it's one of these grits, or media blasting. BTW, most automotive finishing suppliers stock these wet paper grits as they are commonly used to colour sand new paint jobs. From what I've read, 1000 is good, and 1500 is too clear for sanding GG. Is this correct?

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Old November 27th, 2005, 09:57 PM   #2
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I hate to mention it, because I want to be the first and get all the glory, but has anyone tried using glass etching acid? It is the stuff sold in craft stores for adding frosted images on glass. I would think that it could be used, it's effects increasing over time exposed. It could be tried on some cheap glass UV filters. I assume that it could be used for a short interval then re-applied if the etching was insufficient. Using this method, I would think that the effect could be controlled precisely and would cause no "scratching".
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Old November 28th, 2005, 12:59 AM   #3
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Acid etching is really hard to control. It's hard to evenly etch the glass and the texture is not useable for GG. Thanks.

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Old November 28th, 2005, 01:33 AM   #4
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Sorry to interrupt, but........

Are you supposed to sand/grind both sides of the plastic/glass or just one side of it?.... and if only one side, which way should it face? (macro lens side or 35mm lens side?) Thanks.
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Old November 28th, 2005, 04:54 AM   #5
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Acid etching is indeed very difficult to get even, it just comes out patchy.

You should only grind 1 side of the glass/plastic, it facing the ground side towards camcorder prevents reflections.
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Old November 28th, 2005, 07:22 AM   #6
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Okay, acid etch is out. How about stage/film diffusion gels? They seem to have very fine grain and there are several strengths.

I find this thread to be very informative.
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Old November 28th, 2005, 09:50 AM   #7
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Ok, so etching is out...but the original question was wet sanding vs media blasting. Perhaps I could rephrase by asking what folks have had the best success with?
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Old November 28th, 2005, 10:09 AM   #8
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How thick does the glass/plastic have to be? I found these clear acrylic cirlcles that could be sanded on one side and are about 3.17 mm thick. I think they are used as analog dial covers for machinery or something. I'm not sure of the machinability of acrylic (how easy it is to drill) but maybe someone on here knows better.
http://www.usplastic.com/catalog/pro...uct%5Fid=21189
maybe you could use one of these to replace the clear "CD dummies" that have been used to improve quality a little bit. Just trying to help. Also, If you are someone looking for materials to construct supports etc. look around this site to find rods and other materials that may help.
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Old November 28th, 2005, 11:10 AM   #9
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Thanks Steve. We're pretty much OK on the materials list, achromats, DCX lens etc. From what I've researched, thinner is better, and only one side is ground..the one towards the 35mm lens.

My big question now is how to best construct a circular ground glass. I haven't tried cutting the circular glass yet, nor have I attempted the diamond drilling for mounting holes.

I like the idea of media blasting the surface as it's just easier. So either no one has tried it, it plain doesn't work but the experimenter hasn't read this post, or it's a secret :-)

Ultimately perhaps it doesn't matter too much. The idea is to scatter light...I guess the fine point is how much light (meaning how coarse is the surface) to scatter. It looks like the M35 folks have dealt with this by providing two ground glass elements, one for low and one for good light.
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Old November 28th, 2005, 01:00 PM   #10
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Stage diffussion gels are an interesting idea. Since they're made to optically diffuse you'd think this would be an ideal use for them. Who knows--my school has a bunch, I'd certainly be willing to try this out.
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Old November 28th, 2005, 01:52 PM   #11
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Lee filters offer small booklets with filter samples for free. Get the diffusion edition, it goes together with the cinematographers edition. These small pieces are large enough to do ground glass tests.

I have tried some, but most of them have either lines in them, or don't diffuse enough. One was quite good, but this was only a very quick test I did while trying something else. It's also nice that they come in colors.

These samples are certainly good enough for viewfinders. They can also be handy as in-camera filters, should you ever shoot with a Bolex camera.

http://www.leefilters.com/
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Old November 28th, 2005, 09:16 PM   #12
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Maybe I missed someone mentioning this, but just in case....has anyone tried asking an Optician? The tech that is supposed to be an expert on optical glass. maybe they could suggest ideas or solutions.
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