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Thread: cover glass

  1. #1

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    cover glass

    A while back I asked about whether people thought adding a top-glass to my 35mm glassless negative carrier (for the Omega B66) would improve negative flatness by reducing popping. One of the intriguing suggestions was to use a piece of glass from a slide mount and place it over the negative in the carrier. I had some very old but unused/unopened Gepe slide mounts lying around. The box indicated they were anti-newton also. So after very carefully cleaning the glass I tried it. Indeed, sharpness was noticeably improved on an 8x10" enlargement. However the improved sharpness was of little use since other artifacts showed up, like imperfections in the glass, a slight texture showing on the print, and even tiny scratches. When I inspected these pieces of slide glass carefully, even before cleaning, I noticed they are really not of sufficient quality for this use. There are small surface imperfections on every piece of glass I tried, including tiny scratches, gouges etc. And the anti-newton texture is in-focus enough to show up in the print. My guess is when used as initially intended (to mount slides for projection), these flaws are probably not noticeable.

    Does anyone have any suggestions as to how I might proceed from here? Is there some type of higher quality slide mount glass available? Or some kind of alternative? I'm not sure how the Newton Ring issue can be dealt with either, but I feel like I should continue a little more before abandoning this top-glass idea, since the improvement in negative flatness was substantial.

  2. #2
    Ian Grant's Avatar
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    It depends on your negative carrier. I've always disliked a top glass and haven't used one since the mid 70's. I've not had an issue with negatives popping out of focus with any of my enlargers, but the glassless top masks hold 35mm & 120 negative flat. Sometimes for critical large prints I've taped a negative as well.

    Ian

  3. #3

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    Enlargers can vary in how much heat is transmitted to the negative. It might be that larger, more professionally-oriented machines filter excess heat more efficiently than enlargers intended for amateur use and will keep the negatives flatter as a result.

    Dichoric enlargers have built-in IR and UV filters and tend to keep negatives cool. Condenser enlargers can have heat-absorbing glass installed above the condensers to remove excess heat before it gets to the negative and it might also be practical to install lower-wattage bulbs to help limit the heat.

    I have glass carriers for several enlargers. By examining the projected image through a 20x Thomas Instruments Scoponet I can see the grain sharply defined when projected through a simple glassless carrier.

    When I use the glass sandwich carrier the grain is resolved, but it isnít quite as distinct as it was in the glassless carrier. Iíve experimented projecting the image for varying lengths of time while observing the grain with the negative in a glassless carrier. The image of the grain of 35mm negatives in my Omega B66 with dichro head stays sharp for at least 6-7 seven seconds with most negatives. Some negatives stay flat much longer or show no popping for any reasonable printing time.

    But with denser negatives that require longer than average printing times the focus of the grain fades as the negative heats causing the negative to belly up out of the plane of focus. This is more a problem with B&W than color negatives on this enlarger.

    It seems that overly-dense B&W negatives absorb heat and pop quicker than average negatives. When this happens I divide the exposures into 6-second or shorter doses with 10-second cooling periods in between so that the negative never gets hot enough to warp out of focus. In this way I can make the sharpest prints with no intervening glass. Itís inconvenient, but it works.

    For negatives that require printing in, dodging, or vignetting, it may be necessary to work with the negative in a glass carrier to keep it flat during long single exposures to allow the manipulations.

    Flat slide glass intended for glass negative carriers is available from

    Focal Point
    3129 Canterbury St.
    Deltona, FL 32738
    Phone 386-860-3918 Fax 386-951-4098
    email mike@fpointinc.com

    http://www.fpointinc.com/

    FP will cut glass to any dimensions you specify.



 

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