Durst AC 707 Autocolor

Discussion in 'Black and White' started by nestor_alvarez, May 11, 2018.

  1. Hello,
    About 3 years ago I changed my photography to 100% analog. I installed a darkroom in the garage and the results have been quite interesting and I can see my own improvement in developing / printing.
    One area which became totally frustrating is the control of contrast with the Durst AC 707. I use the table provided by Ilford papers in relation to the combination of magenta and yellow filters to reach a desired grade with VC papers. I used Ilford, Ultrafine Silver Eagle and Oriental Seagull. With all of them I get the same results: when I want to increase contrast (for example from grade 2 to grade 3) I get a print that is considerable with less contrast instead of more. I made a try from extremes (without filters, with filters at grade 1 and with filters at grade 5) and the results were amazing: the most contrasty print is the one without filters, then the one at grade 1 and the print at grade 5 is almost all white. I used the same exposure (21 sec at f11) for all of them as it is supposed that the combination of filters (between magenta and yellow) are predetermined to use the same exposure.
    Any help on this area is more than appreciatted!!!
    thanks, Nestor
     
  2. Try maximum magenta filtration on the enlarger, with no yellow at all, and certainly no Cyan.

    If that isn't contrasty enough, then there's something else amiss somewhere.

    An all-white print would indicate extreme under exposure.

    Remember that those filter guides are exactly that - a guide. You should make notes and adjustments for your own apparatus and way of working.
     
  3.  
  4. This is what I did yesterday at 0Y/170M and it came almost white. What is more troubling is that when I do a 75Y/10M (supposedly a grade 1) is more contrasty than a 34Y/45M (grade 3) up to the extreme that a grade 5 (0Y/170M) came almost white, loosing all details.
    All the rest is constant, same exposure at 21sec f11, same developer (BW-65 from Photographers Formulary), same time 90sec with same agitation at same temperature. All with Oriental Seagull VC paper
     
  5. Although I suppose it doesn't matter so much if you're not all "100%", it's called FILM if you want to be free of the taint of digital.:eek:
     
  6. "This is what I did yesterday at 0Y/170M and it came almost white."

    - Then you need to increase the exposure; regardless of what some obviously wrong filter guide says.

    You can't print well just by following an instruction book. It takes personal experience and adjustment according to your equipment.
     
    nestor_alvarez likes this.
  7. Thanks Joe. My next step will be to increase exposure when I increase contrast, even though the color enlargers with the three filters are supposedly made to keep the exposure constant at different contrast grades.
     
  8. Your results are surprising; what you describe should not be happening! Something is wrong! First, reviewing how variable contrast paper works (stylized view): Paper is coated with two emulsions: a low contrast emulsion sensitive mainly to green light, plus a high contrast emulsion sensitive mainly to blue light. Contrast is thus controlled by adjusting the ratio of the exposures applied to each.


    A yellow filter is a blue blocker. Setting the yellow knob higher results in higher yellow filtration, thus less blue light exposes the paper. This allows the green sensitive low contrast emulsion to carry the burden. The result is a lowered contrast print.


    Conversely, a magenta filter is a green blocker. Setting the magenta knob higher results in higher magenta filtration; thus less green light exposes the paper. This allows the yellow sensitive high contrast emulsion to carry the burden. The result is a higher contrast print.


    If no filter is deployed, the results will be about equal exposure action in both emulsions. The finished print replicates a “normal” fixed grade #2 paper.


    What could be wrong with your setup? Your notion that the filter settings prearrange the same exposure is false! All filters have what is called a “filter factor”. In other words, as you deploy more and more filtration, you also are blocking light energy. The more the filtration -- the less light the paper receives. Moving the filters induces a change in exposure. This change must be countered or under or over exposure results. We must counter filter changes just as we counter change induced by film density, enlarger height, and aperture setting. Exposure adjustments of the enlarger is an acquired skill.


    What I surmise: You are developing under safelight by inspection. When you observe the print has the desired density you then transfer it to the stop and fix. This is OK but ---- contrast is also adjusted by altering the developing time. More time in the developer adds contrast. Conversely, less time delivers a flatter print. Let’s for a few weeks change your developing method. For this training period, prints are plopped into the developer face down. Don’t peek! Time the developer, I think 90 seconds or perhaps 120 seconds will work for you. Pick a time and stick to it. At the end of the predetermined developing time, plop the print in the stop and fix. This method will likely solve your dilemma. Surely it will give you a better handle on what is happening. If you are to continue doing this stuff, buy an enlarging meter, it will save you time and money.
     
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  10. Thanks Alan! Very practical and detailed explanation. I am currently developing as you mentioned (about 90secs) and then place in stop and fix for 1min. Most of the time the paper is face down. I also make sure that the agitation and developer temperature is 20 deg C. What I am not doing is changing the exposure time at different filter grades. I did all (from grade 1 to 5) at 21sec f11. Do you agree to change exposure time, and in that case what percentage change from grade to grade do you recommend to start with?
    Thanks again for your explanation of how filters work!
     
  11. "Most of the time the paper is face down."

    - That's asking for scratch marks from the bottom of the developing tray. Always develop your prints face up, and don't use plastic-tipped print tongs.

    Besides, why would you want to miss that magical moment when the image first appears?

    Whenever I tried using print tongs I got scratches on the corner of the print; no matter how careful I was.
    So my technique was to have a disposable glove on one hand, and use that for developing. The bare hand was kept dry for placing paper under the enlarger, etc.

    90 seconds is the bare minimum development time for prints. Print developer is very sensitive to temperature, and if it's under 20 C it takes much longer than 90 seconds. Print development should be done pretty much 'to finality'. That is, the print shouldn't gain much more density, no matter how long the developing time. I used to keep my developer at about 75 F (24 C) to ensure this. A tray-heater is an essential piece of equipment if your darkroom is cold.

    F/11 is also quite a small stop to be printing at, and shouldn't be necessary if you have a decent 6 element enlarging lens. Although extending the exposure time gives more opportunity for dodging and burning, a shorter exposure can be used for straight prints.
     
    Last edited: May 12, 2018
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  12. P. S.
    21 seconds is a remarkably precise time. How did you arrive at that?

    You might want to look up 'test strips' and how to produce one to get a sensible exposure time.

    The difference between 21 and 20 seconds is absolutely invisible in a print.
     
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  13. Thanks Joe regarding the recommendation to leave the print in the development tray more time instead of less. I do not have a problem with cold temperature as I live in Miami and usually what is difficult for me is getting the developer at 20C as it will normally go to 24 or 25 C alone. In order to keep it at 20 C I need to place a bag with ice into the tray and measure the temperature until it reaches 20C. I am printing basically 6x6 so f11 is two stops before maximum aperture.
    I arrived at 21 sec after the second test strip were I considered the 20sec strip a little underexposed and the 22sec kind of over, so I decided 21sec.
     
  14. Best if you change aperture leaving the time constant. This is because paper displays more reciprocity failure that film. Thus a change in exposure time requires a slightly greater time change than you would think. However time change is easy - I have computed a table of increases for you. This table is based on no filters deployed and a 20 second exposure with no filters deployed.


    White - 20 seconds
    1 - 25 seconds
    1-1/2 - 31 seconds
    2 - 32 seconds
    2-1/2 - 34 seconds
    3 - 40seconds
    3-1/2 - 50 seconds
    4 - 100 "seconds
     
  15. "This is because paper displays more reciprocity failure that film. Thus a change in exposure time requires a slightly greater time change than you would think."

    - Hmmm. In 40 years of B&W and colour printing, I never noticed such an effect. Nor did any electronic metering aids, density wedges, etc. that I used take account of reciprocity failure. Yet they still managed to predict the correct exposure needed.

    Surely, if reciprocity effects were so severe with printing paper, then varying the contrast grade would just be matter of changing aperture to lengthen or shorten the overall exposure? And then who would need VC paper?
     
  16. @ rodeo_joe ---

    It’s a fact that photo papers display more reciprocity failure than film. Prints are viewed by light playing on the paper from an adjacent lamp. This light transits the emulsion, its intensity is reduced, then the light hits the undercoat (baryta), now it's reflected, so that it again transits the emulsion, to arrive at our eyes. These two transits of light inside the emulsion result in the paper reacting 2X to exposure changes. Whereby a doubling of the exposure playing on film results in a 1 f-stop change (0.30 density), the same change to paper results in a 2 f-stop change = 0.60 density delta. Pictorial papers react twice as forceful as film. Predictable however this adds complications to the equation especially for color papers with multiple emulsion layers, each responding differently. When we write software to counter, it is titled “slope control”.


    The reciprocity curve (plot) for film requires that we add time when the exposure is lengthened and add time when the exposure is shortened. Center of the reciprocity curve for film is about 1/50 of a second. Paper reciprocity curve centers on about 10 seconds. High speed photofinishing printers operate at about 1/25 second. For this application the curve is lopsided. For dense negative we elongate the time, reciprocity then forces us to add even more time. Conversely, thin negative require shortened time. Because the reciprocity curve is centered at 10 seconds for papers, we are now forced to shorten (not lengthen) the exposure due to reciprocity (odd but true).


    Generally contrast adjustments are not based on exposure time changes. Some exception, variable contrast roll paper used in high speed photofinishing of black & white printing, color integrative film, these uses time to optimize contrast. 50+ years’ photofinishing and manufacturing enlargers, printing light meter analyzers, test films and papers, high speed photofinishing printers etc.
     
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  17. Joe, what a lesson. Thanks!. All the information provided is very valuable to polish my art. I have experience with reciprocity in films but not in paper. Your information will help me out in this area on papers.
    Yesterday I tried to calibrate my Durst probe / meter to a negative that I printed with the right exposure / contrast according to my view. But I did not succeed. If I cannot do it after another tries I will buy an enlarger meter.
     
  18. Yeah, I have to ask if you are making good test strips to determine exposure and development time.
     
  19. Yes, I do not have a problem with that, my problem is that when I change the filters (Y and M) to increase contrast, according to the Durst instruction manual it is not needed to change exposure as all the possible combinations render an equal exposure. But I get a less contrasty instead of more.
     
  20. Forget trying to keep the exposure constant for the time being.

    Try this:
    Place a 1" strip of paper under the enlarger, covered with a piece of opaque card. With no negative in the enlarger and the lens at f/8 wind up the yellow filtration to 50. Turn on the enlarger lamp and create a test strip with about 10 steps at 2 seconds each.

    Develop by inspection until you can see all or most of the steps. Note the development time where the strip no longer gains density quickly. If the strip is too dark or (unlikely) no steps show, then you need to repeat the process and close the lens a stop, or increase the step time.

    Now repeat the process with 0 yellow and 100 magenta. Same steps and same development time/temperature.

    From these strips you should be able to judge three things:
    1) What development time to use.
    2) Approximately what exposure time and f-stop you need.
    3) Whether the contrast is actually changing, and how.

    The yellow-filtered low contrast strip should show more steps between pure white and solid black. Say, 8 steps.

    The magenta-filtered high contrast strip should show fewer steps between white and black. Maybe only 5 steps.

    If that's not the case, then something is wrong with the filters or the paper or something else. The something else might be a darkroom that leaks light, or an unsafe safelight.

    I take it you can see the colour of the enlarger light changing when you turn the filter wheels?
     
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