focus shift LPL 4500

Discussion in 'Darkroom Equipment' started by H. James Wolf, Oct 26, 2009.

  1. H. James Wolf

    H. James Wolf Member

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    I'm having problems with focus shift on an Omega/LPL 4500. I've tried two easels, two lenses, and checked for excessive heat on the negative with no result. The focus seems to creep between the time the lens is focused [stopped down is in focus at this point] and the time the exposure is made. Has anyone else had this problem and come up with a solution?
     
  2. Oren Grad

    Oren Grad Subscriber

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    One important step in figuring out where the problem lies is to try making some prints using a glass-sandwich negative carrier, if you're not already doing so.

    I did some printing over the weekend with my LPL 4500II - modest enlargements (4.5 x 6.75") from 35mm Tri-X negatives. As it happens, I did precisely this test - made some prints using both the LPL glassless 35mm carrier and also the glass carrier. Regardless of how careful I thought I was being with the glassless carrier, the prints from the glass carrier were distinctly sharper and cleaner looking, even to the naked eye.

    I suspect that it was a combination of the negative being flatter and also not flexing unpredictably under the on-and-off heat from the enlarger lamp and the varying intervals between when I focused and made the printing exposure.
     
  3. Ronald Moravec

    Ronald Moravec Member

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    The only things that can cause this are

    1 focus shift built into the enlarger lens as it is stopped down. Get a better lens.

    2 negative popping from heat. Heat absorbing glass helps a little. A Glass carrier is the cure.

    3 the focus control drifts. repair as necessary
     
  4. bdial

    bdial Subscriber

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    There is (or should be) a heat absorbing glass in place already, built into the lamp mount. Any heat from the lamp has to get through the filters, assuming you have a dicroic model, and the mixing box before it gets to the negative anyway.
    It's most likely the focus mechanism slipping.
     
  5. patrickjames

    patrickjames Member

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    As said above, use a glass carrier and focus with the lens set at the aperture with which you are going to print.
     
  6. ic-racer

    ic-racer Member

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    I have had cases where 11x14 and 16x20 RC paper settles a little after closing the easel. Looks nearly identical to negative popping. Easily 'cured' by a short pause before hitting the timer. I'll admit it took me a while to figure that one out. Watching the reflection of the safelight on the paper right after I closed the easel lead to the diagnosis.
     
  7. RH Designs

    RH Designs Advertiser Advertiser

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    I had a similar problem with an LPL 7700 recently, caused by the lens board slowly sliding downwards by gravity alone. Tightening the adjustment screw that controls the tension on the focus knob cured it. I don't know if the 4500 uses a similar design, but it would be worth checking.
     
  8. David A. Goldfarb

    David A. Goldfarb Moderator Staff Member Moderator

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    Also, focus with a scrap sheet of the paper you're printing on under the grain focuser, if you're not already doing this. Some people believe this is unnecessary, but it seems an easy enough risk to avoid, and most people wouldn't want the groundglass on their camera to be off by the thickness of a sheet of paper.
     
  9. MPandolfo

    MPandolfo Member

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    Right David, but it's more like the subject "to be off by the thickness of a sheet of paper" , pretty negligible.