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  1. #1

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

    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?
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  2. #2
    keithwms's Avatar
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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.
    "Only dead fish follow the stream"

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  3. #3

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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.
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    keithwms's Avatar
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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!
    "Only dead fish follow the stream"

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  5. #5

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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. #6
    Vaughn's Avatar
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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. #7

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    Quote Originally Posted by Vaughn View Post
    "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
    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.
    The universe is a haunted house. -Coil
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  8. #8
    Blighty's Avatar
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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.
    Norman is an island.Time and tide wait for Norman.

  9. #9
    Dan Henderson's Avatar
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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


    web site: Dan Henderson, Photographer.com

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    I am not anti-digital. I am pro-film.

  10. #10
    Nicholas Lindan's Avatar
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    Quote Originally Posted by walter23 View Post
    My testing has shown that there is a difference between multiple 1 second exposures and a single continuous exposure of the same total time ... Is there a good rule of thumb for reducing your exposure to compensate for this effect?
    There is an application note on this subject on the Darkroom Automation web site:

    http://www.darkroomautomation.com/su...2LampDelay.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.

    Quote Originally Posted by walter23 View Post
    What do you do about this?
    You might try a new timer ...
    DARKROOM AUTOMATION
    f-Stop Timers - Enlarging Meters
    http://www.darkroomautomation.com/da-main.htm

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