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  1. #121

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    Sandy wrote, "BTW, I called Stouffer to discuss this situation and talked to one of their technicians. He told me that they neither test for nor attempt to control the UV absorption characteristics of their step wedges. He said that there was a UV densitometer on the premise and that they would pull it out and try figure out the reason for the discrepancy."

    Hey, that's great - thanks for taking the initiative and calling! I'm, of course, interested in hearing what they find too.

    When you get back to them, I hope this discussion has impressed a few thing on you that you will remember - please check with them and find out:
    1) What model of densitometer they used?
    2) How did they calibrate the densitometer?
    3) If they know - what are the peak wavelength and band-pass width of the filters?
    4) Did they have any control or check samples to compare readings with?

    Knowing these things will help us compare their results with ours.

    Kirk

  2. #122
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    For density measurements, I set the meter level to read 0 with nothing in the carrier, then set the scale pot to read the density of a 0.6 neutral density filter. There is nothing I can or should do about intermediate or higher densities, as the response of the meter is inherently logarithmic.

    For printing, I print the step wedge on the paper I intend to use. I judge which steps should be, say, zones 2 and 8 and set the meter to read those numbers in the corresponding steps of the wedge. This can be made to work even on stained negatives if a stained copy of the step wedge is used.
    Gadget Gainer

  3. #123

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    Quote Originally Posted by gainer
    For density measurements, I set the meter level to read 0 with nothing in the carrier, then set the scale pot to read the density of a 0.6 neutral density filter. There is nothing I can or should do about intermediate or higher densities, as the response of the meter is inherently logarithmic.

    For printing, I print the step wedge on the paper I intend to use. I judge which steps should be, say, zones 2 and 8 and set the meter to read those numbers in the corresponding steps of the wedge. This can be made to work even on stained negatives if a stained copy of the step wedge is used.
    How would this respond a step wedge that is developed in a proportional staining developer?

  4. #124

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    Patrick wrote, "...then set the scale pot to read the density of a 0.6 neutral density filter. There is nothing I can or should do about intermediate or higher densities, as the response of the meter is inherently logarithmic."

    Patrick - I would suggest using a high cal point that is a bit higher than 0.6. You're using an ND filter right? Glass or gel? The smaller Wratten filters are allowed to vary density within +/- 5% across the surface of the filter. Also, as you are calibrating at the bottom end of the practical range of densities you will encounter in film testing (0.6 is only two stops of density), you may have some larger errors for readings outside of your calibration range. Remember it only takes a little error in the setting of your high point to extrapolate to a larger error farther out - it tweaks the slope of the response line.

    It's best to have calibration points that "bracket" the range of values that you wish to measure. And since your diode has a pretty linear response, you can be reasonably certain that the values in between the cal points fall on your calibration curve. (But it is always best to use a check standard to verify!)

    If you like, I can send you a copy of my Stouffer Step tablet. I sure I've got some that will have densities above 2.0, probably more. Of course I can also "calibrate" it by reading the RGB-Vis densities with my densitometer. Let me know if you are interested.

    "For printing, I print the step wedge on the paper I intend to use. I judge which steps should be, say, zones 2 and 8 and set the meter to read those numbers in the corresponding steps of the wedge."

    So you make adjustments so the your meter reads in "steps" instead of actual optical density?

    "This can be made to work even on stained negatives if a stained copy of the step wedge is used."

    Well, that indirectly relates to this thread. And I'm sure Sandy would remind you of the complications involved in reading stained negs, especially when the spectrum of the measurments is not controlled.

    Sure, you could adjust it to read "effective stops" at two points, but I think the non-linear printing response of stained negs (at least on VC paper) will give you wacky results for the zones in between 2 and 8. Especially if you have no filtration on your densitometer - since it is going to be more red-sensitive than a densitometer than one that has a blue filter on it.

    To respond to Donald's question - Because of the lack of filtration and the spectral resonse of the photodiode, I think that any readings made with Patrick's desnitometer on a stained negative would essentially be missing the stain since the red and IR portion of the spectrum are "blinding" the densitometer, so to speak.

    Patrick - Thanks for the info.

    Kirk

  5. #125

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    Quote Originally Posted by Kirk Keyes

    This could be demonstrated by combining a Wratten 18A filter with your tungsten light. The 18A passes UV from about 320 to 400 nm, centered around 360 nm and filters out visible wavelengths up to about 700 nm where it starts to pass IR. This alone would be ideal for testing your Pt/Pd paper as it will not respond to the IR. You could try it with some panchromatic film, as long as it was not sensitve past 650-680 nm. I have no doubt that you will loose a lot of film speed when compared to the tungsten light alone, daylight corrected or not. Please let us know what you find.


    Kirk
    Yes, I had already figured out that the transmission characteristics of the 18A filter would be about perfect for this type of testing.

    But are UV filters only made in glass? The prices I have seen for18A filters are fairly intimidating.


    Sandy

  6. #126

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    Glass - The 1980 Kodak book says, yes - only glass. I missed that. Well that takes the fun out of doing a fun little test...

    And as to our previous discussion, it is probably not float or crown glass, but something a bit more fancy.

    Did you see this web page - http://msp.rmit.edu.au/Article_01/06.html
    They say that Tiffen makes a similar filter and B&W as well - B&W 403. They show a Schott UG1 that looks perfect for this test as well.

    I haven't done it, but check around www.edmundoptics.com web site too.

    Kirk

  7. #127
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    [QUOTE=Kirk Keyes]
    Patrick - I would suggest using a high cal point that is a bit higher than 0.6. You're using an ND filter right? Glass or gel? The smaller Wratten filters are allowed to vary density within +/- 5% across the surface of the filter. Also, as you are calibrating at the bottom end of the practical range of densities you will encounter in film testing (0.6 is only two stops of density), you may have some larger errors for readings outside of your calibration range. Remember it only takes a little error in the setting of your high point to extrapolate to a larger error farther out - it tweaks the slope of the response line. "
    I can always cut the 0.6 and double it or triple it. That is probably the best way to test linearity of the log function.

    "It's best to have calibration points that "bracket" the range of values that you wish to measure. And since your diode has a pretty linear response, you can be reasonably certain that the values in between the cal points fall on your calibration curve. (But it is always best to use a check standard to verify!)"

    I hate to say this, but you are wasting your time telling me what I have known for years, and have learned not to worry too much about. In order to use these precise measurements, you must know more about your materials than you can know. You and I are never certain that we are measuring the effect of a developer or the effect of the variations of the film.

    "If you like, I can send you a copy of my Stouffer Step tablet. I sure I've got some that will have densities above 2.0, probably more. Of course I can also "calibrate" it by reading the RGB-Vis densities with my densitometer. Let me know if you are interested."

    I have and have had a number of Stouffer tablets, calibrated and uncalibrated.


    "So you make adjustments so the your meter reads in "steps" instead of actual optical density?"

    No, the meter reads in zones according to custom, but because of the non linearity of paper, it cannot read all zones exactly right. It can be set, however, to read any 2 zones of a negative and assure me that those zones will print as such on my paper.


    "Well, that indirectly relates to this thread. And I'm sure Sandy would remind you of the complications involved in reading stained negs, especially when the spectrum of the measurments is not controlled. "

    The spectrum of the measurements is not what controls the response. It is the part of the spectrum transmitted by the negative that the paper is able to "see" and has no direct bearing on what we measure.

    "Sure, you could adjust it to read "effective stops" at two points, but I think the non-linear printing response of stained negs (at least on VC paper) will give you wacky results for the zones in between 2 and 8. Especially if you have no filtration on your densitometer - since it is going to be more red-sensitive than a densitometer than one that has a blue filter on it. "

    You are telling me I might be able to do things I have already done. Are you also telling me that knowing the exact densities of a negative in any and all parts of the spectrum will somehow cause the paper to respond obediently? The best you or anyone can do on a straight print is to set two printed densities where you want them, unless both the paper and the film are linear, or just happen to complement each other's non-linerities. Measurements of anything do not mean a whole lot unless they are usable. We can use them to test theories or to make prints. Prints are seldom used to test theories, and theories do not always make good prints. Now I'm telling you something you already know.
    Gadget Gainer

  8. #128

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    Also check out the Hoya U-360, U-340, or U-320 filters.

  9. #129

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    Sandy - Here you go:

    http://www.bhphotovideo.com/bnh/cont...ku=8207&is=REG

    That's cheap enough I might get one to play with!

    Kirk

  10. #130

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    Look for a Tiffen "Hot Mirror" filter to go with it for blocking the IR. Under $50 for sizes up to 55 mm.



 

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