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Thread: Ground glass troubles

  1. #1

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    Ground glass troubles

    I just tried grinding my own GG the other day and have a question for those who may have gone this route before. I used the 5 and 3 micron Aluminum Oxide "dust" from Willmann-Bell and went at it for quite a while - I think my hands will never feel the same agin! I spent about an hour and a half on it in total.

    The glass looks extremely smooth and is more fine than any GG I have used before. However, when I compare it with my other GG's (a B&J and whatever is on my Gundlach), it isn't as completely opaque as the others. Of course, it is also finer and smoother. Now, when the camera is focused, it doesn't seem to make a difference, but when it is defocused, there is a hot spot in the center and I can see an outline of the lens aperture.

    My question is simply this: should a finely ground glass be totally opaque or will will it be more translucent than one which is roughly ground by comparison? Do I need to go at longer with the 5 micron dust perhaps, or is this as opaque as it's gonna get?

  2. #2
    Robert A. Zeichner's Avatar
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    Re: Ground glass troubles

    One big advantage of being able to see the aperture is that you will have no problem avoiding vignetting even if the corners of the gg are not clipped. With my barn door shade, I can adjust the doors in seconds without having to take my head out from under the focusing cloth. Works great.

  3. #3

    Re: Ground glass troubles

    I have made my own gg using the same technique. And yes, the glass will be a little clearer than one that had used coarser powder. After all, using no powder at all will produce a clear piece of glass. The aim of using such fine powder is to get a brighter image ie letting more light through. As you said, once focused you get a bright image.
    I followed Dick Dokas's instructions without realizing they were for a 8x10 screen. After talking with him I realized that probably 30-45 minutes is all the total time needed for a 4x5 screen. Which size were you grinding?

  4. #4

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    Re: Ground glass troubles

    5 and 3 micron grit size is pretty fine ( in excess of 1000 grit). The ground surface you get will be very fine with minimal light scattering resulting in a significant hot spot especially when viewed at 8X10 with a wide angle lens. The tradeoff is that the finer the grind the more detailed the image. I would typically use 600 to 320 grit size (about 16 to 30 micron avg. size) in silicon carbide. This will grind faster; a 4X5 in about 10 min. But the degree of scattering and hence the angle of view is a matter of personnel preference when considering a global view of the screen. When employing a loupe one would tend to like a finer grind for better resolution of the image.

    Nate Potter, Austin TX.

  5. #5
    wfwhitaker
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    Re: Ground glass troubles

    3 to 5 micron is awfully small stuff for a ground glass. It's going to work only very slowly, as you noted. In the past when I made a ground glass, I used 500 grit and it worked fine. Others may argue. If you want to end up with a glass as fine as you did, it actually may be more expeditious to start with a coarser grade and work up. YMMV.

  6. #6

    Re: Ground glass troubles

    Check out the Dokas link;
    http://www.dokasphotos.com/techniques/ground_glass/

    Also use your light-meter to see how much brighter your screen is compared to others.

  7. #7

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    Re: Ground glass troubles

    Thanks so much for your input! The GG is 5 x 5 inches - for a wetplate back that I am making for a B&J 4x5 field camera. It does make sense that the glass won't be quite as opaque, it's just a matter of knowing when enough is enough! It was a PITA, but it is very bright and being able to make my own, any size that I want, and have it ready when I need it without waiting for the UPS guy makes it worthwhile.

  8. #8

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    Re: Ground glass troubles

    If you keep on using finer grit, you will end up with a clear, smooth sheet of glass again.

    Maybe stop with a bit larger grit...

  9. #9

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    Re: Ground glass troubles

    Yeah Paul,
    I always just use the medium Alum grit size, .0002. I never go finer, there seems no point in polishing it back to clear! Even for an 8x10 it only takes me about an hour.

    I just use about a 1/4 tablespoon of grit, and a tablespoon of water. I use a square grinding glass about 3x3 inches....if that helps.

  10. #10

    Join Date
    Mar 2008
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    Re: Ground glass troubles

    Hi Paul,

    I used the same abrasives from W&B you mention. For 5u I only polished long enough to make the surface all consistent haze. For 3u I polished maybe 20 to 30 minutes.

    I replaced and old ArcaSwiss model B ? or A? screen an noticed the focus was more precise then original screen (using a 6x loupe). I did not check light output deltas. There is no indication of center hot spot or anything like that.

    Perhaps you went way to long with the 3u. You could rough surface again with 5u and then the 3u for shorter time.


    Tim

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