Under/over the lens MG filter testing

Discussion in 'Enlarging' started by tkamiya, Apr 28, 2011.

  1. tkamiya

    tkamiya Member

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    I had an unfortunate incident that damaged under-the-lens multi-grade filter. That is, I accidentally hit the top of film cleaner spray and spritzed some of the stuff on filter. That resulted in partial damage to the surface.

    Now I have a replacement.

    THAT allows me to do some testing on the effects of under-the-lens MG contrast filters!

    I always wondered how much, if any, degradation there is to have under-the-lens filters. I hear there is none. Then next question is, how much damage is too much. These filters are so easy to damage and hard to clean.... There has to be a limit where impact is negligible and tolerable.

    My idea is this.... Have some kind of reference negative and print set the enlarger head high to simulate high magnification printing. I'll use 5x7 paper to cut down on a cost. (will be printing just a part of the image)

    Then...

    1) print with no filter - reference #1
    2) print with over the film filter (undamaged) - reference #2
    3) print with under the lens filter (undamaged) - reference #3

    4) print with under the lens filter (already damaged by spray)
    5) print with under the lens filter (scratch some with sand paper)
    6) print with under the lens filter (severely scratch with sand paper)


    Would this test procedure sound reasonable? What kind of subject will make good test negative? I'm thinking something high contrast with sharp lines.
     
  2. michael_r

    michael_r Subscriber

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    But keep in mind when people say there is no image degradation by using a filter under the lens, there is an implicit assumption the filter is optically flat and free of blemishes, dirt, scratches etc. If not, all bets are off, even though the filter plane itself is out of focus.

    Regarding a test negative, I think an interesting test would also be a uniform textureless area. In addition to the obvious issues associated with scratches, damage etc, it would also tell you if things like smudges of finger oil (from careless handling) might effect how evenly light is transmitted, potentially showing up as uneven density.
     
  3. tkamiya

    tkamiya Member

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    well... I don't know about that. Even a brand new filters aren't blemishness. I see some light scratches and pitting new out of box. All of mine are clean but not perfect either. So my interest really is, how bad does it have to be before it shows up in prints. My hope is that it will have to be "pretty bad" before it has some impact.

    I'll probably do 8x10 prints as on smaller ones, anything will look good and prints are best evaluated as whole.

    Anyway, I'll do this shortly and post back the result.
     
  4. michael_r

    michael_r Subscriber

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    True, there are always small imperfections. I look forward to seeing the test results. These experiments are always interesting to me.

    Thanks
     
  5. Sim2

    Sim2 Subscriber

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    Nice opportunity for a useful test there. Interested in hearing your results.

    As an additional thought, what about if the filter is not exactly parallel to the neg, does that have a visible effect? Not expecting this to be tested but merely float the idea ion your direction :whistling:

    All the best for the testing.
     
  6. michael_r

    michael_r Subscriber

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    I've wondered about that as well, or if people use gels and there is some bowing or something.
     
  7. tkamiya

    tkamiya Member

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    I'll print one with clean filter tilted 45 degrees.
     
  8. MattKing

    MattKing Subscriber

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    Is there anything beside your enlarger that flare might reflect off of? If there is, this "45 degree" test might interact with that.
     
  9. tkamiya

    tkamiya Member

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    Good point. Yes. I will place a black board to prevent that.