Kodak Characteristic Curve - Log H Ref equals middle gray?

Discussion in 'Color: Film, Paper, and Chemistry' started by Oliver Roch, Aug 28, 2010.

  1. Oliver Roch

    Oliver Roch Member

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    Can anyone please explain me the value of Log H Ref given in most of Kodak Tech. Pub. Characteritic Curves? E.g. Ektar 100 E-4046 curve gives a Log H Ref of -0.84. Is it middle gray when shooting at box speed? Isn't middle gray normally placed at log 0?
    Compared to E-4040 Log H Ref -1.14 the Portra 160 VC seems to handle an enormous overexposure of nearly 7 stops.
     
  2. markbarendt

    markbarendt Subscriber

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    Old thread old unanswered thread, but I was searching for the same answer this morning.

    Found this: http://motion.kodak.com/motion/uplo...en_motion_education_sensitometry_workbook.pdf

    Page 6 seems to have the answer.

    Log H seems to be defined as "Exposure", I'm going to make the assumption that this essetially means "the camera setting". That seems to fit the curve, leaving room for about 3-4 stops down to the toe.

    Portra 160 http://www.kodak.com/global/en/professional/support/techPubs/e4051/e4051.pdf

    Portra 400 http://www.kodak.com/global/en/professional/support/techPubs/e4050/e4050.pdf

    I checked the TMax tech pub and it lacks the reference point.

    http://www.kodak.com/global/en/professional/support/techPubs/f4016/f4016.pdf

    So a couple questions crossed my mind.

    First question, asked three ways: How is zero defined? What does zero match in the real world? Is zero a reference for white in a print?

    Second: Why does the B&W curve lack the reference?

    Open to other thoughts here too.
     
  3. markbarendt

    markbarendt Subscriber

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    Oh and am I right in my definition of Log H?
     
  4. Bill Burk

    Bill Burk Subscriber

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    It's 10 to the power of 0 = 1. Given as lux-seconds.

    Log H is clearly explained on Page 6 of the workbook you linked... Yes it is "time-light" that you control when you change f/stop and shutter speed.
     
  5. Bill Burk

    Bill Burk Subscriber

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    I didn't answer the part about Log H Ref...
     
  6. markbarendt

    markbarendt Subscriber

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    So we have "exposure" understood but:

    "We" are still confused on what "0" represents.

    "We" are still confused about why "0" is at the thick (variable) end of the negative.
     
  7. Bill Burk

    Bill Burk Subscriber

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    Zero is 10 to the power of 0, which is 1, a whole lux second. So -1 and -2 are like tenths and hundredths of a second. More like the exposure that would be used on film...
     
  8. Photo Engineer

    Photo Engineer Subscriber

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    Hah, Bill, beat me to it while I was looking at the refs to refresh my memory. Good thing I refreshed the screen first.

    PE
     
  9. markbarendt

    markbarendt Subscriber

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    Cool, I get it.

    So, in layman's terms, zero essentially defines a standard amount of illumination. A whole lux second.

    That explains the longer curve left of zero for Portra 400, vs 160, and the difference in offset from 0 to Log H.

    Thanks
     
  10. Bill Burk

    Bill Burk Subscriber

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    Hey Mark,

    You're pretty much right...

    Just in case it hasn't sunk in yet. The scale of the x-axis is logarithmic. So it goes from left to right like 10 to the minus 3, 10 to the minus 2, 10 to the minus 1, 10 to the zero power, 10 to the first power. This works out as 0.001, 0.01, 0.1, 1, 10.
     
  11. markbarendt

    markbarendt Subscriber

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    Thanks Bill.

    My confusion wasn't actually with the scale. I simply didn't understand what the definition of the anchor point itself was; what it meant, why Kodak was starting the scale in the highlights.

    You said "pretty much".

    Is there something wrong with the definition of zero as 1 lux second of exposure?

    Is there something wrong with thinking of Log H as the intended camera setting, and by inference, the equivalent of an incident meter reading?
     
  12. ic-racer

    ic-racer Member

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    Anyone??

    My guess is that the reference exposure indicated at the top of each H&D curve is the exposure which will give the appropriate gray card density each film. Or the exposure that a "KODAK Gray Card (gray side) receiving same illumination as subject" produces on the film when the film is correctly exposed.

    Example: Ekatar 100, an exposure to the film of Log H -0.84 should produce a density of 0.77 to 0.87 (correct gray card density)
    Example: Elite 200, an exposure to the film of Log H -1.14 should produce a density of 0.8 to 1.0 (correct gray card density)
     
  13. markbarendt

    markbarendt Subscriber

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    Kodak defines Log H as "exposure", not as a reference point.

    Kodak's grey card instructions include using one of several offsets to find the actual "exposure" so I'd say the grey card value itself isn't the target.
     
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  15. Photo Engineer

    Photo Engineer Subscriber

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    The target is a step wedge. The exposure is a set value that will be equivalent to what results in the lux values shown on the X- Axis. If exposure time varied, then you would have reciprocity enter into the equation.

    PE
     
  16. Bill Burk

    Bill Burk Subscriber

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    That part is exactly right. I just wanted to emphasize that the horizontal units aren't linear.
     
  17. markbarendt

    markbarendt Subscriber

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    As a practical matter, in normal daylight conditions, will an incident meter used normally give us Log H/exposure?
     
    Last edited by a moderator: Jul 9, 2012
  18. Photo Engineer

    Photo Engineer Subscriber

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    Mark;

    IDK really. I know that the light used in the exposure of the examples made by EK is adjusted for color temperature for either daylight or tungsten. Meters generally are calibrated for either illuminant (AFAIK) but it depends on the scales on the meter as to what units you can derive from them. It will take some degree of calculation to get Log H / exposure from the meter. Going from a curve back to the meter reading might be easier.

    I've usually found that the conversion between the EK curves and reality are useless. The curves can be internally compared and the "real curves" can be compared. In this instance, "real curves" = pictures or a step wedge exposed in-camera. i have used this method in film work as well as the "backward calculation" that I mentioned in the last paragraph.

    PE
     
  19. markbarendt

    markbarendt Subscriber

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    Thanks PE.
     
  20. ic-racer

    ic-racer Member

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    Kodak does not define "Log H Ref"

    I'm pretty sure "Log H Ref" to be an exposure reference point. Specifically, the exposure produced by a gray card when the film is exposed per manufacturers instructions. EDIT: I thought it might be speed point but see below for ISO 5800.
     
    Last edited by a moderator: Jul 9, 2012
  21. markbarendt

    markbarendt Subscriber

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    http://motion.kodak.com/motion/uplo...en_motion_education_sensitometry_workbook.pdf

    See page 6

    Also see PE's answer above about it being tied to step wedge and specific time.
     
  22. Photo Engineer

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    Basically B&W = Color Neg = Color Reversal as far as densitometry and speed measurement are concerned. A 400 speed film must be 400 speed and etc. Neutral = Equal sensitivity to all regions of the spectrum under a given light with either Daylight or Tungsten balance (temperature in degrees K).

    So, given a method, it applies everywhere. The curves look different but mean the same in "engineering" terms.

    PE
     
  23. ic-racer

    ic-racer Member

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  24. Photo Engineer

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    And basically the same method is used for reversal films except that you work from Dmax and not Dmin.

    We had several members of that committee working in our division as you might guess. And, our "workshops" were given by these people. We called the classes "Ding Dong School" and we had to take them to be "certified".

    PE
     
  25. Bill Burk

    Bill Burk Subscriber

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    I've been avoiding the word "Ref".

    From the label on the Characteristic Curves...

    Portra 160
    Log H Ref: -1.051

    Portra 400
    Log H Ref: -1.44

    I will guess the meaning of Log H Ref:

    I agree with ic-racer, it appears to be a middle-gray value. It is about 1.26 to the right of the black and white speed point (In ZS terms, figure the speed point is Zone I, this puts the "Log H Ref:" about 4 stops higher, somewhere in the vicinity of Zone V). I am looking at a graph and I can't tell with sufficient precision whether that is the incident meter aim, or the 18% gray card.

    I am thrown off by the older Portra 160 Log H Ref: -1.14 because everything else is pretty close to 1.26 from the speed point. I believe it might be an old typo - (maybe the old chart had a copy/paste error from a 200 speed film). If it's not a typo, then the old Portra 160 just barely missed being 200.
     
  26. markbarendt

    markbarendt Subscriber

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    I don't disagree with the thought that the "Ref" falls in the mid-tones.

    I found this, this morning. http://wiki.magwerks.com/wiki/images/b/bb/NIST_SP250-37.pdf

    It does appear to be a measure of exposure time, in relation to a standard illuminant, and in the Kodak cases the scene is a step wedge.

    What I can't find is a way to convert it to an "EV" or lux value.