A quick question on exposure and contrast

Discussion in 'Enlarging' started by Krzys, Jul 16, 2010.

  1. Krzys

    Krzys Member

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    If I find the exposure time for a print at grade 2 and then want to increase the contrast grade will I then have to increase the exposure time as well?
     
  2. Kevin Caulfield

    Kevin Caulfield Subscriber

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    It depends on what sort of filtration you are using. Sometimes you need to increase the exposure time for grades 4 and 5.
     
  3. Krzys

    Krzys Member

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    The color filter dials on my LPL color condenser enlarger. I was just wondering if there was a need for increased exposure then is there some kind of rule to work by.
     
  4. Ian Grant

    Ian Grant Subscriber

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    Colour filter dials on a condenser enlarger ??? Never heard of that before. Colour heads are all diffuser.

    Depends which system of filtration your using, single filter - then yes exposure definitely needs increasing, dual filtration you may need a slight shift in exposure as you ioncresae the contrast.

    Ian
     
  5. Krzys

    Krzys Member

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    You are right about the condenser...I got confused by the C7700 model name
     
  6. Mike Wilde

    Mike Wilde Member

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    There are ways to calibrate a dichroic head for a constant expossure for a tone, when shifting grades. It relies on using a step wedge, and printing about 16 patches. I usually print them 2x2" so they can easily be pasted into a record book.

    If you have one a step wedge at hand, shoot me a PM and I will dig up and post the proceedure to you.

    Basically the end result is you dial in extra Y&M (kind like ND in colour with all three filters using CYM) to decrease the amount of light getting through the filters when you are printing though thin flilter settings.
     
  7. Ian Grant

    Ian Grant Subscriber

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    Ilford have already done that and the result is their Dual filtration recommendations :D

    Ian
     
  8. pentaxuser

    pentaxuser Subscriber

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    So the answer, krzys, is to look up the dual fitration on the manufacturer's tables that come with the paper. Most makes have such filtration tables but I suspect some may have been more meticulous than others in arriving at them. However I have found that while within a short range of grades say 2-3.5 the same exposure produces the same or very close to the same exposure results, the variation gets larger for the more extreme grades.

    Mike's idea of using a step wedge to caibrate your own enlarger and paper makes sense to me, although I have never done it. If I have understood Ralph Lambrecht's tables on his site Darkroom Magic he is saying that not only is the Ilford and Agfa dual filtration different from what he believes to be correct but more crucially that dual filtration, no matter how well you have calibrated for it does not ensure same exposure. He helpfully gives another table which states what the increase and decrease in exposure should be in I think 1/12ths of a stop for changes in grade.

    I am hoping all will be more fully revealed when I get his book which I have ordered.

    So dual filtration values and same exposure may be good enough for the middle range of grades. Alternatively even with a colour head you can dial out all the filters and fit an Ilford under the lens filter system which is 12 filters for grades 00 to 5. I think that Ilford claim that each filter is so designed to give the same exposure up to grade 3.5 and then you double exposure for grades 4,4.5 and 5.


    It is as you will see quite a complicated subject - well it is for me :D:

    pentaxuser
     
  9. Mainecoonmaniac

    Mainecoonmaniac Subscriber

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    It depends....

    It depends. If you printing in MG paper, probably. But it always better to do a test print. When you change contrast, try to print for your highlights and see how the shadows look. If you want minimal hassle on test printing, I used to use Ilford paper and filters. If I remember correctly, you don't have to change printing times within grades 0-3, but if you go from those grades to grade 4 and above, you double your print time. If you go the other way, you half your print time. I think those filters from Ilford have neutral density filter to compensate for the different grades. I use a color head to print BW and I always make another test print to check on range of tones.
     
  10. MattKing

    MattKing Subscriber

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    If you are using the dual filter approach, as at least one other poster has said, it depends.

    The dual filter tables are all referenced to a particular tone of grey. So they are accurate for that tone, but that may not be the tone that you care about the most.

    I would still use them as a good starting point though.
     
  11. Nicholas Lindan

    Nicholas Lindan Advertiser Advertiser

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    If you are using Ilford paper and Ilford filters then you can change contrast with the same time. Obviously, only one shade of grey will remain constant in tone.

    MGIV FB WT #2 -> #3 1/2 will keep a 0.9 OD grey (a bit lighter than ZIV)
    MGIV FB #00 -> #3 1/2 will keep an 0.45 OD grey (ZVI, skin)
    MGIV RC #00 -> # 31/2 will keep an 0.55 OD grey (ZV - ZVI, 18% - skin)

    For other tones you can find the correction from the paper speed charts published by Darkroom Automation http://www.darkroomautomation.com/support/index.htm

    The charts give the paper speed - that is the amount of exposure in stops - for each grade of paper and each zone-system tone.

    For example, with MGIV RC you want to change from #2 to #3 contrast, keeping the shadows (ZIII) the same:

    • For #2 the speed for ZIII is 8.36
    • For #3 the speed for ZIII is 7.89
    • The difference is -.47, or -1/2 a stop
    • If you reduce the exposure 1/2 a stop (divide by 1.4) then ZIII will stay constant as you go from grade #2 to #3.

    Second example, with MGIV RC you want to change from #2 to #1 contrast, keeping the light highlights (ZVIII) the same:
    • For #2 the speed for ZVIII is 5.38
    • For #1 the speed for ZVIII is 5.24
    • The difference is -.14, or -1/8 a stop
    • If you reduce the exposure 1/8 a stop (divide by 1.1) then ZVIII will stay constant as you go from grade #2 to #1.

    The stops table http://www.darkroomautomation.com/support/stopstable.pdf will help with adjustments to time if you aren't using an f-stop timer.

    However, if you are using a color head your filtration and time settings may not match Ilford's filters. A good place to start is with Ilford's suggestions for filtration for 'constant time' printing. If you want to calibrate your system a good approach, if you don't want to use a step tablet, is to make a zone grey scale for each of the contrast grades you use most often http://www.darkroomautomation.com/support/zonestrp.htm
     
  12. Greg Davis

    Greg Davis Member

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    I don't remember where I read it, but when using a color head with B&W paper, increasing or decreasing the yellow has no change on the time, but the magenta requires a 0.1 second increase for every point added. Try it and see if your results are in line with this.
     
  13. Ian Grant

    Ian Grant Subscriber

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    It's not as simple as that :D

    But close to the mark.

    I use a single filter setting, and I fly by instinct, I dial in a bit of extra Magenta, or go Yellow -quite rarely when needed :D and can guestimate the exposure difference quite accurately.

    But that's years of printing experience, reading other peoples poorly exposed negatives :D

    Ian
     
  14. RalphLambrecht

    RalphLambrecht Subscriber

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  15. Greg Davis

    Greg Davis Member

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    As I said, I just read it somewhere, but it does give you somewhere to start.