Calculating log relative density

Discussion in 'Darkroom Developing and Printing' started by BobW, Dec 23, 2003.

  1. BobW

    BobW Guest

    I've just been looking at the Ilford website at the datasheet for their
    Multigrade IV Paper.

    http://www.ilford.com/html/us_english/pdf/mgivrc.pdf

    It states that If you have access to a "photometer" it is possible to
    calculate the optimum contrast setting for a given negative by "measuring
    the effective density range of the image as projected on the baseboard".

    Now I tried this out yesterday, but am not sure if I got my calculations
    right.

    Using the enlarger "autoprobe" and with a fixed aperture (f2.8) I measured
    the exposure times required for the deepest shadows and brightest
    highlights.
    (I turned off the safelights while doing this - which seemed to make quite a
    difference to the highlight exposure time) . This was a fairly contrasty
    negative.

    The readings I got were;

    Shadows - 2.8 secs (bright areas of neg)
    Highlights - 40 secs (darkest areas of neg)

    So now I need to convert this into "log exposure units" so that I can use
    the table to derive the correct contrast setting.

    My method was to normalise the range i.e. 40/2.8 to get the number of
    "exposure units" (= 14.3), then take the log which gives 1.15. Looking up in
    the ISO table suggests a contrast setting of around 1.5.

    Was my method of calculation correct ?

    Thanks,

    Bob W
     
    BobW, Dec 23, 2003
    #1
    1. Advertisements

  2. Hi Patrick,

    Thanks for the reassurance. In practice this was all a bit academic, for
    example the negative in question also had large areas of midtone which
    needed teasing out, so I found that esthetically, the best contrast grade
    was 4 and I was happy to loose detail in the relatively smaller areas of
    highlight/shadow.

    Anyhow it's still another useful technique to have mastered and is another
    input to consider when printing. I suppose that this technique is most
    useful when printing negatives with large areas of highlight/shadow.

    Best regards and seasonal greetings,

    Bob W
     
    Robert Whitehouse, Dec 24, 2003
    #2
    1. Advertisements

  3. Hi Patrick,

    Thanks for the reassurance. In practice this was all a bit academic, for
    example the negative in question also had large areas of midtone which
    needed teasing out, so I found that esthetically, the best contrast grade
    was 4 and I was happy to loose detail in the relatively smaller areas of
    highlight/shadow.

    Anyhow it's still another useful technique to have mastered and is another
    input to consider when printing. I suppose that this technique is most
    useful when printing negatives with large areas of highlight/shadow.

    Best regards and seasonal greetings,

    Bob W
     
    Robert Whitehouse, Dec 24, 2003
    #3
  4. It still may come in handy. Now that you have seen that two density
    measurements do not always tell you how to choose a paper grade, you may
    refine your technique to give you a head start on how much and where to
    dodge and burn. You may also apply the same perceptions to exposure
    measurements for the camera exposure. On the other hand, you might get
    so bogged down in application of technique that you let photo ops go by.

    Either way, you have learned something valuable and that will lead you
    to learn more valuable facts and techniques. Have fun.
     
    Patrick Gainer, Dec 25, 2003
    #4
    1. Advertisements

Ask a Question

Want to reply to this thread or ask your own question?

You'll need to choose a username for the site, which only take a couple of moments (here). After that, you can post your question and our members will help you out.