Compensating for non-additive exposure (test strips / lamp ramp-on time)

Discussion in 'Enlarging' started by walter23, Dec 14, 2007.

  1. walter23

    walter23 Member

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    My testing has shown that there is a difference between multiple 1 second exposures and a single continuous exposure of the same total time (presuambly due to the incandescent lamp's response to switching on and off, slowly ramping up and fading off). The difference isn't huge but it's significant enough to matter to me in many cases.

    As a consequence, when I do test strips in multiple 1-second exposure overlays I tend to do my real exposure the same way (multiples of 1 second exposures) to compensate. With continuous exposures, as you'd expect, my times are somewhat shorter.

    Is there a good rule of thumb for reducing your exposure to compensate for this effect? I'd imagine it depends on your specific enlager or bulb, but maybe there's a correction that is good enough for most cases?

    What do you do about this?
     
  2. keithwms

    keithwms Member

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    Not quite sure I understand what you are saying, you are talking about exposing paper? If it's just a matter of ramping the lamp then you could shutter the lamp. My enlargers all have a red slide-in shutter that is paper safe. But that's too complicated. For test strips I just withdraw a piece of cardboard across the frame in increments of whatever timescale is relevant; I don't turn the lamp on and off at all. I don't understand why you'd want to do that.
     
  3. walter23

    walter23 Member

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    Keith, I guess withdrawing without turning the lamp off and on would work. I do usually mark the boundaries with a ball point pen though.
     
  4. keithwms

    keithwms Member

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    Well, you can premark with a pen and see those boundaries when the lamp is on, and then just withdraw the masking paper according to your boundaries.

    Not sure why you need sharp boundaries though; if the difference in exposure between one boundary and the next is wildly different then your overall exposure is probably too short. I usually have 5-10 sec differences between one test area and the next and just interpolate the best time from that. I don't have any fancy timers or whatever, I just count in my head. Works fine for low volume, slow, fun printing. If I had to make hundreds of prints then I would adopt more rigorous practices!
     
  5. Paul Howell

    Paul Howell Member

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    Although I do not do many test strips, when I use my cold light, which takes a second or 2 to warm up, I cover my entire test strip with cardboard, then with a metratone start my count, uncover the first exposure and move the cardboard for each exposure. Once I know my time, or times for burning and dodging, I once again cover the whole paper with my cardboard give the lamp a chance to warm up then remove the cardboard and start my count using the metratone rather than a timer. If I am lazy I just add a second to the time.
     
  6. Vaughn

    Vaughn Subscriber

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    "What do you do about this?"

    I don't worry about it. I try to keep my exposure times around 20 seconds to allow for relaxing dodging and burning. I do 3 to 5 second intervals, pick the best looking time and use that time as a continous exposure for my first work print. Then I look at the work print, decide on my burning and dodging scheme, decide it I want to alter the basic exposure and then go for my first attempt. Since it will be several more attempts and adjustments before I hit the final, it just does not make sense to me to worry about fractions of seconds or even a second or two so early in the process.

    Hitting the exposure button multiple times (especially at a second each!) would be a pain compared to one long exposure...especially if one had a bunch of dodging to do -- and it wears out the switch that much faster. But this is JMO and my technique...

    Vaughn
     
  7. walter23

    walter23 Member

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    I wouldn't worry about it either except that, at least with my enlarger and/or techniques, I see a noticable and undesirable difference. I guess with longer exposures and longer test exposure times (smaller f-stop) it would become less of an issue so that's a possible approach.

    I like the idea of letting the lamp ramp up and then uncovering test strip zones without switching the enlarger off. Will give it a try.
     
  8. Blighty

    Blighty Subscriber

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    I use an RH designs stopclock to make test strips. I select an area of the neg that has important highlights and/or shadows (I split-grade print) and make a sequence of increasing separate exposures of this one area. Obviously you'd have to mask off the exposed/unexposed bits as you went along, but the advantage of this system is that the correct exposure for test strip and print are identical.
     
  9. Dan Henderson

    Dan Henderson Member

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    like Blighty, I use the RH timer and split grade print. I made a test strip mask with a 1" wide strip in the center, and marked my easel blades in 1" increments with luminescent tape so that I know how far to move the mask each time. I position the strip at one end of the area I want to test, and expose for 2 seconds, then move over and expose for 2.82 seconds, then another for 4 seconds, 5.66 seconds, 8, 11.3, 16, 22.6, and 32 seconds. Those are 1/2 stop increments that are automatically generated by the RH timer. It takes longer to make the full test strip print this way than successively uncovering strips, but each strip accounts for lamp warm up, and whichever strip I judge as right on the test strip print will get exactly the same exposure on the full print, and ends up saving me time and materials in the long run.

    Dan
     
  10. Nicholas Lindan

    Nicholas Lindan Advertiser Advertiser

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    There is an application note on this subject on the Darkroom Automation web site:

    http://www.darkroomautomation.com/support/AppNotePH212LampDelay.pdf

    The effect is minor with an enlarger using a standard PH212 bulb and amounts to a 50 millisecond error in exposure time, equal to a 1 second error in 20 addiditive exposures.

    If you are seeing significant differences with a standard enlarging bulb the problem is with the timer. Some digital timers are quite innacurate and have their own turn-on delay: a 1 second time may actually be 0.8 seconds and a 10 second time may be 9.8 seconds. Obviously with a clock-work timer such as a GraLab the accuracy in setting the timer is +/- 1/2 second and making precision additive test strips can be a problem.

    Cold light heads have a significant turn-on delay that does need compensation. However, the compensation time required changes with the lamp temperature and the time between exposures.

    You might try a new timer ...
     
  11. vet173

    vet173 Member

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    With my cold light I give a 10 second warm up then the exposure time. It has helped with consistency. I think if I had to print for a living, I would add a shutter to the enlarger and leave it on. I haven't had any such problems with condenser with a 212. Now I use a color head on my V-xl.
     
  12. BobNewYork

    BobNewYork Member

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    I recall reading somewhere that the problem is known as "the Intermittancy Effect" i.e. ten one second exposures will not give you the same density as one ten-second exposure. When I do test strips I set the timer for the longest exposure I think I'll need for the whole strip, fire up and progressively cover the test strip at three second intervals. That way each different portion receives its exposure in one fell swoop rather than in incremental chunks. It's always worked for me - maybe worth a try.
     
  13. Joe VanCleave

    Joe VanCleave Member

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    I've found that you should be able to get accurate test strips by using a sheet of opaque card with a slot cut into it; then set the enlarger's timer for each time duration required for each section of the test strip, and expose through the open slot. This has the effect of taking into account all the variations in your timer and lamp (i.e. lamp warmup time, timer on/off variations, etc.)

    ~Joe
     
  14. Thomas Bertilsson

    Thomas Bertilsson Subscriber

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    Alternative to alleviate headache

    Your alternative is: Don't do test strips!. Think about alternatives. Instead proceed by doing one print that's a little too dark, one slightly too bright and then interpolate a good base exposure.
    I consistently get to a very good work print with three sheets. Once there I figure out adjustments in contrast grade and dodging/burning. That gets me close in four sheets and usually completely there in five.

    Believe it or not, this approach saves me paper, because when I do test strips I can't see the whole print at a certain exposure time, so I might miss high/low values because of that, and that always causes a lot of swearing later on.
    Just a completely different alternative for you.

    - Thomas
     
  15. Akki14

    Akki14 Member

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    But I do test strips so I know where to go for a print that is too bright or too dark. Not everyone knows or remembers this vague information for every kind of negative/print they have so test strips are valuable even just as a ballpark figure. I do test strips and still need to do a few prints but that 1 test strip isn't the same as 1-2 sheets of paper. I try to use quite large test strips, though, 1/2 - 1/3 of a paper and place it where it can get highlights and shadows in shot.