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  1. #11

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    Sandy,

    Thank you for sharing that information. I noticed on your reply to my latest question in which I asked about the 1.4 density that Clay had posted on Ed Buffaloe's site that you mentioned that Clay's information was based on reading a negative through the "blue" channel on a densitometer. That you went on to indicate this reading translated to a 1.8 when read through the uv channel. That this reading was the cause for my excessive negative contrast when used for printing on Azo. Would the conversion of blue to uv that you addressed be an accurate linear conversion factor of "blue" to "uv" readings?

    Additionally, as I observe the Kodak spectral sensitivity charts for Azo the peak spectral response occurs in the range of 360-410 nm with drastic fall off at that point. It is my understanding that this is "near band uv". What is the factor that I am missing (considering the differing negative densities) in regard to the response of Pt-pd (which I have not used) and Azo. It appears that both process/materials respond to uv?

    Any information that you can share to clear up my murky view on the matter would be greatly appreciated.
    Art is a step from what is obvious and well-known toward what is arcane and concealed.

    Visit my website at http://www.donaldmillerphotography.com

  2. #12

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    Donald,

    Regarding your first question, it is not possible to give an exact linear conversion factor for the Blue to UV reading because it changes with increasing highlight density, and it also varies with films. However, in general it would be pretty safe to say that a blue channel reading of log 1.3 will read a bit more than log 1.7-1.8 in UV mode. However, a blue channel reading of 1.0 would read only about 1.25 in UV.

    About the issue of AZO sensitivity, it is true that this paper has its maximum sensitivity in the UVA range, and drops off rapidly in the near visible range. However, because of the speed of the paper most people do not use UV light to expose. The use of a UV lights of the type we use in alternative photograhy would result in impossibly short exposures. So in practice most people are exposing with lights that emit very little if any UV radiation and it is only the light in the blue range over about 420 nm that is effectively available to the process .

    That is at least what I believe to be the case based on my conversations with a number of AZO printers.

    Sandy

  3. #13

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    Sandy,

    Thanks for your timely response to my earlier question.

    I guess my questions still remain. If the spectral response of Azo is in the UVA or near band UV range, as Kodak indicates, then the reflector flood lamps that I and others use must be emitting some UVA energy (or so it would seem to me). Perhaps the percentage is small in relation to total light output and that would seem to bear out the problem with short exposure times using a purer uv source for Azo exposure.

    That still leaves me with the question of the differing density effects as you have described them insofar as Pyrocat developer is concerned. If, as information available would seem to indicate, then the relative UV effects on exposure by a stained negative would seem to apply to both Pt-Pd and to Azo. The relative "speed" of the two materials would seem to have no bearing on the fact that they both expose to UV.

    Bob Herbst in his tests (www.unblinkingeye.com) seems to show with step tablet exposure comparisons on Azo and Pt-Pd materials/processes that there is only 1/2 step greater density range recorded on Pt-Pd materials. That, if I am reading the results of his conclusions correctly, the silver chloride emulsion is capable of greater dmax then Pt-Pd and that Pt-Pd seems to show a greater ability to record and differentiate higher tonal values when compared to Azo.

    Would this ability to distinguish high negative densities by Pt-Pd be the sole reason for the greater negative contrast range used by Pt-Pd photographers? However, I still question why a seeming differing effect on the transmission of UV should not apply to Azo as you seem to indicate that it does.

    Thanks for any insight that you can share.
    Art is a step from what is obvious and well-known toward what is arcane and concealed.

    Visit my website at http://www.donaldmillerphotography.com

  4. #14

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    Donald,

    I thought my last response addressed directly your question. AZO has sensitivity well into the visible light range, and it is the radiation in the visible range that is providing exposure, not UV light. Your reflector flood lights probably emit little or no UV radiation, but the AZO material is exposed nevertheless because it also has considerable sensitivity to light in the visible range.

    What I am saying is this. You can expose AZO with a UV light source, which matches the greatest spectral sensitivity of the paper, but your exposures will be really short, too short for most contact printing conditions. Or you can also use the spectral sensitivity of AZO that is in the visible range and make the same exposure with lights of much greater wavelength, *but* your exposures will be much longer.


    Regarding Bob Herbst's tests, note simply that AZO is capable of a much greater reflective density range than platinum or palladium. The maximum reflective density possible with platinum or palladium is around log 1.5, whereas AZO is capable of readings approaching or in excess of log 2.0.

    Finally, I do not indicate that there is a differing effect on the transmission of UV light as it applies to AZO. If people were routinely using UV light sources to expose AZO, instead of light sources in the visible range, then we might assume that the density range requirements of negative for AZO and Palladium printing would be about the same. But the fact remains, folks are exposing AZO with light sources that produce little or no UV radiation.


    Sandy

  5. #15

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    Sandy,

    Thank you for your further elaboration on this matter. I did not wish to give the impression that I was being contentious of your statements. If I appeared to be, then I apologize.

    I did not understand what, to me, appeared to be conflicting statements. I was taking the Kodak released information to be the literal limits of the materials characteristics. I understand now that the Kodak information indicates the peak sensitivity and not the actual limits of the spectral response as I had previously assumed.

    Once again, thank you.
    Art is a step from what is obvious and well-known toward what is arcane and concealed.

    Visit my website at http://www.donaldmillerphotography.com

  6. #16

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    Donald,

    Bear in mind that I am just making observations and don't want to come off like some kind of expert. I don't know a lot about AZO but what I know for a fact is that most people are using light sources for exposure that put out very little or no UV radiation. And if you look at the spectral sensitity curve for AZO you will see that it has quite a bit of sensitity in the visible range at about 400 nm in the violet, and still quite a bit at 450nm in the blue.

    Yesterday someone contacted me in response to a message I left on the BTZS web site regarding BTZS testing of stained negatives. He had tried this with AZO using the blue channel of his densitometer and the resullts did not match the times he knew to be correct from practical testing for N development. I can see that this would be a problem wiht AZO since the exposing light used by most people does not match the maximum spectral sensitivity of the paper itself.

    Sandy

  7. #17

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    Sandy,
    Your information has informed me and filled some gaps that had existed in my understanding about exposure on Azo. Among those has been the matter of enlarging smaller format negatives on Azo. I know that Durst Pro USA has come out with a light source for that purpose. Michael Smith has indicated, furthermore, an individual is developing a light source for enlarging on Azo.

    My thoughts have been that if Azo responds only to UV (my misunderstanding) and considering the length of time that exposures take that it would take incredible wattages to accomplish this considering the restriction of the enlarging lens aperture and the diminishment of light intensity by the distance removed from the easel.

    Now I can see that considering the fact that Azo is very responsive to UVA (as indicated by your information of unmanageable short exposures in UV exposure units) that it would take a much smaller wattage device then what I had first thought. Once again, thank you for informing me.

    Best regards,
    Donald Miller
    Art is a step from what is obvious and well-known toward what is arcane and concealed.

    Visit my website at http://www.donaldmillerphotography.com

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