Flash as Sensitometer

Discussion in 'Darkroom Equipment' started by Rafal Lukawiecki, Sep 17, 2012.

  1. Rafal Lukawiecki

    Rafal Lukawiecki Subscriber

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    On another thread I am discussing aspects of my film testing experience. As I am about to run the tests again, I would like to ask for any advice regarding the use a small electronic flash unit as a precise source of exposure, to contact print a 31-step Stouffer tablet onto the film I am testing. I would like to explore this option in more detail before I fall-back to using my enlarger, which I would prefer to avoid, due to its longish lamp warm up and start-up times, and the blue/green colour of light (Ilford 500H). At my disposal I have a few units that let me adjust their flash power. I am thinking of using a Nikon SB-800 as it has always performed well, and I have a reliable, and calibrated flash meter, Sekonic L-508, which I have used for the last 12 years. I suppose a studio flash head would be less precise, but I could use it, too.

    Please share your experiences of using a flash for this purpose, any gotchas, suggestions and so on. Ideally, I would like to find a fixed position for the flash unit, and a set of relatively fixed parameters, so I could use this set-up for ongoing film testing.

    I am also curious about the math of calculating the exposure for this application, though I realise I can arrive at that through additional testing.

    Many thanks.

    PS. It is hard to find second-hand EG&G sensitometers in Europe.
     
  2. ic-racer

    ic-racer Member

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    The most difficult thing is making some kind of box that will give even illumination. You can flash the film from across the room but that might not always be repeatable, but it certainly is a good way to get an even exposure across the stepwedge.

    These little "contact printers" usually go for little money on ebay. You could mount the flash inside one of those.
    http://www.ebay.com/itm/Vintage-Air...681?pt=LH_DefaultDomain_0&hash=item2ec1b84559
     
  3. Chan Tran

    Chan Tran Member

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    Yes I agree with flash it from across the room for evenness. I would use a flash meter and check to see if the illumination is repeatable. For calculation of the exposure I would also use the flash meter to measure lux-sec.
     
  4. Bill Burk

    Bill Burk Subscriber

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    Set flashmeter to ISO 100 and retract the dome or use flat disc.

    Time 1/30th (irrelevant but you have to set something).

    Try to get the flashmeter to read f/16.0... That will give you same as an EG&G.

    Now you want to reduce the intensity because that's too much for testing ASA 400 film.

    I have a 1.84 filter, and I also add .60 on top of that.

    So reduce the intensity of the flash or add filters to add up to density 2.44 (give or take 0.30).

    Is that about 8 stops? If so, you could try for f/1.0 at ISO 100

    If you have to use a filter, a physical filter is preferred over a neutral density filter. For example sheet metal with holes drilled or slots cut in it, placed over the front of the flash.
     
  5. ic-racer

    ic-racer Member

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    If you process some film like Ilford Delta 100 to ISO conditions then the exposure required to get a 0.1 density on the film is 8 millilux seconds (by definition). If you take the log of this (0.9) then you can simply add in the density of your step wedge at that point (like 2.8 log d or whatever that step is) and that gives you the output of your sensitometer light. In the example it would be 2.8 plus 0.9 = 3.7. If you take the antilog of that you are back to millilux seconds which would be 5011 millilux seconds.
     
  6. Rafal Lukawiecki

    Rafal Lukawiecki Subscriber

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    My plan, so far, was to have a fixed mount for the flash unit on the wall in darkroom, next to the enlarger, about 1-1.5 m above a simple glass/foam contact printing frame. I would hope that the illumination would be quite even this way, not needing a box at all.

    That little printer looks very cute, though.
     
  7. Rafal Lukawiecki

    Rafal Lukawiecki Subscriber

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    Thanks, Bill. What is the relationship between a reading on the flash meter of f/16 at ISO 100 and the flash output of X lux seconds, if I may ask? I am curious what I am aiming for at the exposure plane.

    As for reducing the intensity, couldn't I just dial the flash down until I read a suitably smaller f stop number, instead of using a filter?

    IC, thanks for explaining how to calibrate this set-up with a known film. Are ISO conditions for development simply what the manufacturer states or something more exotic?
     
  8. laser

    laser Advertiser Advertiser

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    It would be helpful if you could state your purpose including your film application's exposure time, spectral characteristics, illuminate levels etc.

    Even without this information I would suggest following approach KISS: keep it simple.

    The most accurate and reliable mechanisms can be very simple: ½ to 1/100th of a second use guillotine drop shutters. Gravity is quite constant in one location. For 5 seconds and greater use synchronous motors in timers i.e enlarger timers. They can be very repeatable. Accuracy is not as important as repeatibility if you are only using one time duration.

    It seems to me that a common electronic flash is far too variable. EG&Gs are good but they are specifically designed for the purpose. Typical flash units are designed to perform to very different criteria.

    Your Ilford light source has its own problems.

    A simple solution assuming you are using materials that do not have RLF in the 5 second range: use a low wattage light bulb controlled by a electro-mechanical timer that is repeatable in the 5 second range. Many enlarging timers can satisfy this need. Put the lamp in a housing like a safelight. Add neutral "filter material" if the light is too bright. Bond paper provides about a stop of density per sheet or increase the distance (start at 10x the diagonal of your exposure plane) between the light and film.

    To check out the performance expose ten pieces of film and process them identically. Processing at the same time would be best.



    Bob
    see my website: www.makingKodakFilm.com
     
  9. Rafal Lukawiecki

    Rafal Lukawiecki Subscriber

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    Unfortunately, my flash meter, L-508, only shows f stop value, there is no readout in lux-sec.
     
  10. Rafal Lukawiecki

    Rafal Lukawiecki Subscriber

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    Thanks for the guillotine suggestions, Bob. I think it would be a bit out of my mechanical skills comfort zone at the moment. It might be easier to try and locate a used EG&G in the future, but I'd love to have a solution sooner.

    The purpose of the device is for re-testing 320TXP to establish my N, N-1, N+1 development times, at this stage, and perhaps to evaluate speed too. I describe my issues, including my flawed attempt at deriving CIs, on this thread. Longer-term, I would like to use this set-up to continue testing other materials, but all would be fairly standard B&W film.
     
  11. Bill Burk

    Bill Burk Subscriber

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    You can turn down the flash if you like.

    I've tried to find out the meter-candle-seconds of my EG&G but here on APUG, we haven't concluded exactly what I've got. So I don't know the math. In my case I believe that the No. 96 ND filter is a little "yellow" so "sensitometrically" I am getting more density than I calculated. Especially in blue light which film is most sensitive.

    This is why I recommend a physical neutral filter. IC-Racer can show you pictures of what I mean, he has cut some out of acrylic.

    But you don't need the accuracy of illumination to get results that are good enough for process control testing.

    The goal is easy: You want to expose enough but not too much. You want to reduce the light output to the point that the graph drops down near zero (above B+F) in the toe. In case you cannot reduce the light that much, it's OK if your lowest toe reading falls around 0.10 (above B+F).

    To repeat my earlier post, if the flashmeter indicates f/1.0 at ISO 100 - you are pretty close to what you need.
     
  12. albada

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    The "ready" light comes on in electronic flashes when the capacitor is about 1/2 or 2/3'rds charged. Furthermore, the maximum voltage applied to the capacitor depends on how charged your batteries are; i.e., the voltage of the supply. So for consistency, I suggest (1) waiting at least one minute after the "ready" light lights up, and (2) powering the unit from AC instead of batteries.

    Mark Overton
     
  13. Bill Burk

    Bill Burk Subscriber

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    Good advice. With the variable power manual flash setting, if it's well-designed, you probably won't use all the charge per flash.

    For process control purposes... I'd keep the flashmeter handy and test the flash output several times just before you get the film out to test. Do this a few times until you are confident in the flash output stability. Write down what you got (or the average).

    Then do the flash for film test... Later, if something doesn't come out right, you'll have a record of how much flash output you were getting.

    If your light consistency falters, even by a stop, you can still get an idea what your contrast index is for any specific development run.

    Once you setup the "system," it should be so easy to use that you can run a test shot in with every few batches of film you develop -- just to keep tabs on your contrast index (verify that you get what you aimed for).
     
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  15. polyglot

    polyglot Member

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    Meh. If I measure the output of any of my flashes (Minolta 5600 and Bowens 1000DX), they are consistent shot-to-shot within the 1/10 stop resolution of my flash meter, which is IMHO quite good enough when you're testing against a wedge with 1/2 stop resolution. I'd expect any high-quality thyristor flash to be as good as mine.
     
  16. RalphLambrecht

    RalphLambrecht Subscriber

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    that is what i do. because according to my lightmeter my strobes are within 1/10stopfrom flash to flash. i spotmeter for the center bars of the stouffer 45and ,and take it from therethe stouffer is taped to a thick milky piece of neutral plastic, and this approach has worked for years, just need to seal against the flash to avoid flare. go for it.
     
  17. Rafal Lukawiecki

    Rafal Lukawiecki Subscriber

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    Ralph, are you contacting the Stouffer to the film? That would be my intention, following the issues I encountered doing the WBM tests, described on another thread. You mentioned that you seal against the flash to avoid flare—are you using a camera or a duplicator? I am not sure why significant flare would occur when contacting. Many thanks.
     
    Last edited by a moderator: Sep 18, 2012
  18. RalphLambrecht

    RalphLambrecht Subscriber

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    i'm using a self-made slide duplicator, flash from one side and photograph from the other.then, i cover the duplicator with a black cloth to make sue i have no light leaks when photographing into the flash so to speak. i have a picture of the set up somewhere.
     
  19. Chan Tran

    Chan Tran Member

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    Make a reading at your box with the flat diffuser. Count the number of stop from f/1.0 for example if it displays 5.6 3 then 5.6 is 5 stops from f/1.0 plus .3 then it's 5.3
    The Lux.second is (2^5.3)*2.5=98 lux.sec
     
  20. Rafal Lukawiecki

    Rafal Lukawiecki Subscriber

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    Thank you! I was looking for the formula, I'm glad it is so simple.

    On another note, I have been testing the repeatability of my flash with a meter, which shows down to 0.1 of an f/stop. It seems to depend on the selected power output. At lower outputs, it is less precise, hitting up to +/- 0.2 f/stop third of the time. At a higher output it is much more accurate, hitting the same level 5 out of 6 times, and deviating -0.1 once in six.

    I wonder if a tungsten bulb controlled by a spare enlarger timer would be still a better choice in the long term, but perhaps I have what I need to complete the re-test now.
     
  21. polyglot

    polyglot Member

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    The exposure time is generally either too long or too difficult to control exactly. So you either run into reciprocity failure of the film (exposures >1s) or you have inaccuracies due to warmup/cooldown (different intensity and redder spectrum, both of which will have film-dependent effects on sensitivity) of the bulb consuming a significant fraction of the exposure duration (< 0.5s). A very bright tungsten bulb (bright enough that a 0.25s exposure is enough to do your sensitometry) with a mechanical shutter should be a very usable combination though.

    Tungsten bulbs also have a redder spectrum than daylight, to which film is less sensitive (in a very film- and source-dependent manner) so then you have to correct for that somehow. Strobes are a much neater and more-controllable solution.
     
  22. ic-racer

    ic-racer Member

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    Even a $2000 sensitometer from ebay won't be adequate for ISO film speed testing. For example the ISO 9001 certified Agfa Avisense cost $24,000 US dollars. (http://www.agfa.com/docs/sp/aerial/avisense2000_2004-06-15_en.pdf)
    Likely you just need to do relative film speed tests and make control strips to manage development times. In that case, a $20 "contact printer" hooked to an enlarger timer should work fine with a little modification. These show up on ebay all the time. I'd run it at one second. That is the exposure time the Wejex sensitometer uses.
    mark.php?c=OPS&p=pce119844.jpg
     
    Last edited by a moderator: Sep 18, 2012
  23. Bill Burk

    Bill Burk Subscriber

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    There's always tradeoffs...

    Flash is closer to daylight in terms of spectral distribution, though it is "discontinuous" meaning it has spikes of certain wavelengths instead of a smooth curve. That brochure from IC-Racer shows the zig-zag curve and describes how they dealt with that issue.

    At the low settings, your flash duration might be very short, less than 1/1000th second - so your results from too short a flash might be affected by Reciprocity Failure. (Recommend brighter flash and find a way to block some light).

    Tungsten doesn't have as much blue light as flash - an 80B blue filter can compensate.

    For tungsten, I think you can easily rig a reliable shutter for repeatable times (The best time is between 1 second and 1/1000 second to avoid Reciprocity Failure). Think about jerry-rigging a camera body to the enlarger lens mount.

    ---

    As IC-Racer says - all you really need speed testing for is to see how one film compares to another. Consistent exposure will give you a relative speed test - where the results give you an EI to believe. When you get a value that is within 2/3 of a stop of what you expected... That is good enough. We can write pages of threads trying to find that last 2/3 stop - You would have to follow every ISO/ASA specified procedure to get absolutely certain film speed results (including taping film to a plate of glass inside a Thermos bottle).

    My favorite short-cut is to develop to the "ASA Triangle" and then mark that down as if it corresponds to box speed. My shortcut is within 2/3 stop of reality.
     
  24. Rafal Lukawiecki

    Rafal Lukawiecki Subscriber

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    I'm trying to calculate the needed exposure to place step 28 of my 31-step Souffer, 2.6D, just above fb+f. Could someone confirm if I got my numbers right? I gather that:

    Opacity of a density of 2.6 is 10-2.6 = 0.002511
    Exposure necessary at film plane is 0.8/ISO so 0.8/320 = 0.0025 lux s
    Exposure metered above just above the Stouffer needs to be 0.0025/0.002511 = 1.13 lux s

    Looking at the earlier given formula to convert a light meter f/stop reading to lux s:

    Number-of-stops-to-right-of-f/1.0 = log2 (exp/2.5)
    so, log2 1.13/2.5 = -1.15

    which would suggest my meter ought to indicate an f/stop of 0.7, a surprisingly low amount of flash exposure, which it may not be able to read, I would think. I suppose I could just keep reducing down to f/1.0 and then fiddle with a filter of some sort to reduce the flash output even further and perform a test with film.

    I plan to have a go at it today, so please shout if you feel I got it wrong. Thank you.
     
  25. Bill Burk

    Bill Burk Subscriber

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    Without doing calculations, since it's Saturday morning and I have to head out...

    I added the 2 stop ND filter (it's the filter that gets my exposures to be 0.1 near the last few steps) over my flashmeter and metered the EG&G with meter set at ISO 400.

    It recommended f/2.0 at ISO 400, you might have set the meter at ISO 100 so that you can get EV readings. If so... That accounts for the two stops difference in the plan.

    So this is the light "without" the Stouffer scale... Just at the glass of your contact frame...

    If you choose to aim for f/0.7 instead of f/1.0 that isn't significant... It validates your calculations.

    Once you get a strip developed, you can always "count steps" to adjust exposure up or down. On your scale 0.1 density is 1/3 stop, so for each step you count from the step you wanted to be on... is just 1/3 stop more or less depending whether you want to go up or down the scale.
     
    Last edited by a moderator: Sep 22, 2012
  26. Rafal Lukawiecki

    Rafal Lukawiecki Subscriber

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    Thanks, Bill. Here is my Irish "sensitometer", foot pedal operated. Balancing the higher output of the flash, for repeatability, with an ND cut from Lee lighting ND gels, I am sure to get some interesting spectral distribution, but it still beats my enlarger head (500H) for this purpose.

    I measured some 400 pops over the film area, placing the meter in different positions, and it is repeatable within 0.1 f/stop at the surface. The central area might be 0.05f/stop higher than the outskirts. Ideally, the light source would have been further away, to eliminate this fall-off, but this will have to do for this test. I'll do some pre-tests, to check the exposure, and then I'll expose the test sheets. I'll report when I have them analyzed, and, hopefully, plotted out.

    image.jpg