Questions about negative contrast

Discussion in 'Darkroom Developing and Printing' started by Peter Irwin, Dec 19, 2003.

  1. Peter Irwin

    Peter Irwin Guest

    Questions about film contrast.

    The recent discussions have left me somewhat in need of clarification.

    I put down points which I think I know, but some of which
    will need correction.

    1) Gamma is the slope of the H&D curve, a single figure quoted for
    gamma is for the straight-line portion of the curve. This seems to
    be still used as the main figure for cine film, but for still film
    Contrast Index and G-bar are now regarded as more useful.

    2) G-Bar, as used be Ilford, is the average gradient of the H&D
    curve from the DIN speed point (0.1 above base fog) to the point
    representing an exposure of 1.5 ( five stops) above this.

    3) Contrast Index, used by Kodak is similar except that it uses
    the old ASA data point and an exposure 2.0 above this
    The two are similar in practice.

    4) On a film with an average sort of toe in the H&D curves, the
    figure for G-Bar or CI will be a little over 8/10ths the gamma
    of the straight line portion.

    5) If you increase exposure to get out of the toe region, the
    effective contrast of the negative is going to be somewhere
    between the CI figure and the old fashioned gamma.

    6) The obvious homebrew way of measuring this is to take
    a series of pictures at different exposures of an evenly
    illuminated uniform surface. My results will be influenced
    by the accuracy of my shutter and diaphagm (probably not
    too bad but not perfect) and possibly by things like lens flare.
    This would seem to be good enough to tell me something, even
    if it isn't perfect.

    Something as simple as a handheld light-meter and a light table
    ought to tell me something about negative density. There must
    be some pitfalls in this or people wouldn't need to make fancy
    densitometers.

    A bunch more questions will occur to me soon, but I'd be
    glad of any comments.

    Peter.
     
    Peter Irwin, Dec 19, 2003
    #1
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  2. Peter Irwin

    Paul W. Ross Guest

    Go out, shoot some pictures and print the negatives. If you can print
    with a 2 1/2 VC filter and everything from highlights to shadows are
    to your satisfaction, you are on target.

    Best move is to pick a film and developer combination and STICK TO IT
    until you are comfortable with the results.

    Me -- TMax 100 or Agfa filem, Rodinol 1:25 and published time and
    temperatures values. KEEP THINGS SIMPLE. It is like golf -- ONLY
    change one thing at a time, as the various effects are not
    orthogonal...

    Bracket exposures -- film is cheap.

    Sorry... I couldn't resist....
     
    Paul W. Ross, Dec 21, 2003
    #2
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