True or False... (kind of a trick question, just for fun)

Discussion in 'B&W: Film, Paper, Chemistry' started by ic-racer, Aug 19, 2008.

  1. ic-racer

    ic-racer Member

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    Two prints with identical total base exposure time (same enlarger, negative, paper, lens etc.). The print with an extensive 'burn-in' around the edges can have a LIGHTER center.
     
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    True or false? Yes.

    If you did the base exposures and followed that with the burning in, then the center of the burned-in print would be identical to the non-burned in, but appear lighter because of the darker edges. So the answer would be false.

    If the edges of the burned-in print were burned in while making the base exposure on that print then the center would be lighter because you'd have been dodging it while burning in the edges on the base exposure, so the answer would be true.
     
  3. ic-racer

    ic-racer Member

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    Total exposure time to the center of each print is identical.
     
  4. Steve Smith

    Steve Smith Member

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    It's definitely true or false.
     
  5. Robert Kerwin

    Robert Kerwin Member

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    Lighter than what? The center of the other print? Take a knife and cut out the two centers, then compare them side by side on a sheet of white mat board. Better yet, have someone else do it for you and not tell you which patch came from which print.
     
  6. eric

    eric Member

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    E, Both A and C are correct.
    or
    F. Neither B or D is correct.
    G. All the above is correct.
    ...this is how I got through college and look at me now! :smile:
     
  7. scootermm

    scootermm Subscriber

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    how about...
    H. The question is dumb.

    :smile:
     
  8. gainer

    gainer Subscriber

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    All questions are dumb. The people who ask them are at least smart enough to ask. Sometimes it boils down to asking the right people to answer dumb questions.

    You could take two pieces of white drawing paper that look to be the same brightness and paint a black border on one of them. See how the width of the border changes the apparent brightness of the central portion. A change in the brightness of any part of the field of view can cause a change in the adapation level of the eye. This adaptation has two mechanisms: the obvious one is the iris, which has a rapid response but a limited range. The other is chemical, more like printing out paper. You see that if there is a small light leak in your darkroom. At first you can see nothing when the light goes out. After a while, you begin to wonder if you should have loaded that developing tank without a changing bag.

    The eye has a logarithmic response to change of illumination, so that the just noticeable change is nearly constant over a wide range. For that reason, brightness comparisons should be done side by side, not one after the other.

    It's a complex subject over which I spent many hours during my years of study of human factors at NACA-NASA.
     
  9. 2F/2F

    2F/2F Member

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    With identical processing, and exactly the same exposure, False. However, both of these things are difficult to get exact. Are you actually comparing the center of one to the center of the other? Or does it just look brighter on the burned one because you are comparing it to the edges?
     
  10. srs5694

    srs5694 Member

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    They could be different because of reciprocity effects, if the lens aperture or ND filtration lengthened the exposure time significantly. There could also be effects based on filtration for contrast on VC paper, but IMHO that would be violating the spirit of the question.
     
  11. ic-racer

    ic-racer Member

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    Last edited by a moderator: Aug 19, 2008
  12. ic-racer

    ic-racer Member

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    This is what I meant by "kind of a trick.." The 'Exposure Time' is the same on the center of both but not necessarily the "Exposure."
     
  13. 2F/2F

    2F/2F Member

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    Obviously, same exposure to the middle of the print. It's not kind of a trick. It is very straightforward.
     
  14. ic-racer

    ic-racer Member

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    Ok, another hint. Think of the order of things. ie Burn-first vs Base exposure-first.
     
  15. Steve Smith

    Steve Smith Member

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    If you burn in first then do the base exposure, the light has already been on for a while so the base exposure may be brighter (is that how they work?). In that case the centre of the print would be darker, not brighter though.


    EDIT: But now I have seen your other post with the graph, I see light intensity drops with time so if you burn first then do the base exposure, you will indeed get a lighter un-burned in centre area.

    Steve.
     
  16. Stephen Benskin

    Stephen Benskin Member

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    A dark surround tone will make the middle appear lighter. A light surround will make the middle appear darker. It's called simultaneous contrast. Perception is a very important aspect of photography, so not a bad question.
     
  17. Frank Szabo

    Frank Szabo Member

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    False - it's called optical illusions, rather similar to some of the tricks illustrated with lines we see sometimes.
     
  18. ic-racer

    ic-racer Member

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    EXACTLY!

    I have now posted a pair of example prints that show it on the other thread. http://www.apug.org/forums/forum43/53354-coldlight-performance-time-intensity-temperature.html
     
  19. 2F/2F

    2F/2F Member

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    "Burn-first vs Base exposure-first."

    No. Why would I do that? That's not the same exposure for the middle section?
     
  20. ic-racer

    ic-racer Member

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    The burn is on the edges only. Center exposure time is the same on each.
     
  21. 2F/2F

    2F/2F Member

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    I understand. What I am saying is that I would not do the burn first, in order to make the center more consistent print to print. I was asking why someone would do the burn first.

    As for the lamp heat affecting exposure, if I remember your IR measurements graph, it only takes noticeable effect on density in exposures of some time...far longer than any exposures I have made except with lith printing or very large prints from fairly dense negs.

    Thank you for your testing, BTW. It is quite interesting info. What did you use to determine the reduction in lamp brightness at a certain exposure time?
     
  22. ic-racer

    ic-racer Member

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    Yes, I usually don't either, but (I didn't go in to detail in the example photograph) but it was a split grade exposure, so I had an extensive burn before the second main exposure. (I did not do the burn on the test print, thus the change in lamp output from test-print to final print).

    Exactly, I agree, and I think a compensating timer or special control unit is not a nesscessity.

    Simply put a light meter on the baseboard aimed up at the enlarger lens. Safelights off, of course.

    In 30 years I had never encountered anything like this, so I wanted to share the experience. I have never seen a graph like I posted in any discussion of coldlights, but it IS a characteristic response of these lamps.