sunlight vs.electronic flash for BW film speed tests

Discussion in 'B&W: Film, Paper, Chemistry' started by wiedzmin, Mar 8, 2013.

  1. wiedzmin

    wiedzmin Subscriber

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    Will electronic flash with correct filtration affect B/W film speed testing vs. using only natural sun light?

    I found following (average values):

    daylight around noon with clear sky and electronic flash have around 5,500 K

    morning / afternoon sunlight ~ 4300K-4500K - flash with #3409 Roscosun 1/4 CTO (converts 5500K to 4500K) should be within that range

    It looks like color temperatures should match pretty close. Are there other factors except just light color temperature which will make a difference?
     
  2. AgX

    AgX Member

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    Yes, the reciprocity of the emulsion is of influence if the exposure with flashlight would be significantly shorter than with sunlight.
     
  3. wiedzmin

    wiedzmin Subscriber

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    Do you mean exposure as a flash duration ? If yes it looks like Nikon SB-28 (which I can use) has ~1/1000 s at 1/2 power. Will that be OK?
     
  4. Gerald C Koch

    Gerald C Koch Member

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    The color temperature of daylight can vary considerably depending on the position of the sun and atmospheric conditions. It can go from below 5000K to over 12000K. For this reason artists favored studios with northern facing windows. In addition it varies with altitude with the temperature incrreasing for each 1000 ft. It is not a very reliable reference source.
     
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  5. wiedzmin

    wiedzmin Subscriber

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    yes you are right, that's why I was thinking that maybe using electronic flash with correct filtration close to morning or afternoon daylight temperature can be used for film speed / development test. I used reference to the sunlight bc I'm going to use film under sunlight, only test would be performed using flash.

    I'm trying to do the film test as described in Way Beyond Monochrome book using stouffer step wedge. I was trying to use sunlight but always light changes (or I'm too slow) all the time at my location and I was not able to get the consistent results. That's why I was thinking to use flash with gel on it to be close to the sunlight.
     
  6. wiedzmin

    wiedzmin Subscriber

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    I checked technical information for a few films and it looks like below 1/1000s should not be a problem. Just to be sure I can use flash at full power what should lower SB-28 flash duration, based on tests I found, to ~1/300s. Please see attached (I did not know that for TRI-X Kodak recommends +10% development at 1/1000s).

    If I will take care about high intensity reciprocity failure and gel flash to lets say 4500K (bc I rarely take photos at noon). Will that be OK to test film speed / development time in that environment if I'm going to take "real photos" outside in the sunlight using results of the test performed inside with flash like described above?





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  7. Ken Nadvornick

    Ken Nadvornick Member

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    Interesting idea. Are you reasonably certain that the flash unit you intend to use has an acceptably repeatable output duration?

    Ken
     
  8. wiedzmin

    wiedzmin Subscriber

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    Good point, I tested it using flash meter. Flash was on the tripod facing wall, I measured with meter against the wall facing flash. I fired 30 times at 1/2 power - same readout on flash meter. I will test using full power what will allow to decrease flash duration below 1/1000s.
     
  9. ic-racer

    ic-racer Member

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    The typical 'in camera' exposure index test checks that the system of lens, aperture, film, shutter and meter will produce an adequate exposure of the film. Using a flash for the test negates some of the benefit of the test.
     
  10. kevs

    kevs Member

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    Why would you bother 'testing' films? That's the manufacturer's job. Read their data sheets—why would they lie about this stuff? Test your light meters and cameras' shutter speeds. Don't waste you film with urban legend nonsense, go out and make photographs—that's what film is for!
     
  11. wiedzmin

    wiedzmin Subscriber

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    at Ken Nadvornick - flash at full power test - 95% the same, 5% 1/10th stop lower - ok, thank you for suggestion

    at ic-racer - I'm using the same camera, film etc , only difference is that I would like to use flash with gel to match color of the sunlight in afternoon ~4500K, could you explain more what using flash will negate?
     
  12. wiedzmin

    wiedzmin Subscriber

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    what I'm trying to do is to perform the test to customize film speed and development

    there are many methods, I decided to try the method described in the book "Way Beyond Monochrome" chapter Creating a Standard (page 211) + excel spreadsheet provided by the book author at http://www.darkroomagic.com/DarkroomMagic/Darkroom.html. There are 3 methods suggested there. I have a medium format calibrated Stouffer 31- step tablet, therefore I decided to try the method called Elaborate and Precise. I also have older densitometer which fortunately agrees with calibrated stouffer for visual, and blue channels within 0.02.

    I primarily take photos in sunlight that's why I decided to use sunlight to do the test but...
    Problem I faced was that I was not able to run the full test (5 rolls of film) because light at my location (due to trees, windows position etc) was changing, even during the same roll of film.

    So I came up with the idea to use flash with gel (to match 4500K) instead of sunlight. However in WBM page 218 we can read that "...Film has a different sensitivity to different wavelengths of
    light. Therefore, select a light source with a color temperature representative of your typical subject matter and setup. In other words, use daylight or daylight bulbs if you are a landscape photographer, and use photofloods or flashlight if you mainly work in the studio".

    That made me think if simple gelling flash to light color temperature of daylight will do the trick and it wont negatively affect the tests.

    That's why I decided to ask experts for help.
     
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  13. markbarendt

    markbarendt Subscriber

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    Your idea should work fine.

    The only caveat I see is that aperture becomes your sole exposure control. Flash is fast, a slow flash is maybe 1/1,000th of a second so typically the shutter speed has no effect on the flash exposure, just the ambient exposure.
     
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  15. wiedzmin

    wiedzmin Subscriber

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    thank you, I'm going to try it then.
    yes I'm aware about that shutter speed wont affect flash exposure. Is your concern that without shutter affecting exposure I may not have enough aperture steps to perform the test? Valid point, good thing about the test method I'm trying to use is that you photograph stouffer step wedge using the same aperture/shutter. 31-steps of the step tablet will represent 31 exposures within one frame.
    Details are here (more is in the book) http://www.darkroomagic.com/DarkroomMagic/Darkroom_files/FilmTestEvaluation.pdf
     
  16. Bill Burk

    Bill Burk Subscriber

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    wiedzmin,

    Ralph Lambrecht is well respected here, and you may get a chance to correspond with him here or at DPUG.

    If your flash is 1/1000th second and assuming it is valid to compensate for reciprocity failure by adding 10% to development time, guess how that will affect your test results? Your test results will tell you to develop 10% longer than otherwise. I don't consider 10% that significant, because I could just as well be off 15% for other reasons. But reciprocity is a point to consider. Maybe you can find a studio flash with a powerful, 1/100th second flash.

    As far as light source issues, everything is a compromise. Electronic flash compared to Daylight is considered a compromise because its spectrum is "discontinuous". But some lab-grade sensitometers ARE made with electronic flash because as a compromise, it is not that bad. Or maybe you can use Tungsten as a compromise, with 80B daylight filtration.

    I find Film Speed tests more difficult to nail down. I "develop to the ASA triangle" and then take the point where that curve crosses 0.1 density, as the rated speed of fresh film in standard developer. Once I have that benchmark, I can try a different developer or use expired film stored in unknown conditions, and know by comparison what speed I am getting. This to me, is a great value of film speed testing.

    The other great value of film testing is knowing when you are starting to get "out of control".
     
  17. wiedzmin

    wiedzmin Subscriber

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    Bill,

    thank you for your input - very helpful.

    I will try to reach out to Ralph but you answered all questions already.

    Flash - 1/1000s is at 1/2 power. I will most likely use it at full power and flash duration should go down to ~1/300s. I agree, I would like to avoid any adjustments caused by reciprocity failure. I do not want to add another variable :smile:

    light source - thank you for the info. This is what bothered me. I knew that it could not be that simple that just match the color temperature and all will be set. I hope that if some sensitometers use flash than it may not be that far off. Using tungsten - what would you use flash or tungsten ? Small problem is I do not have any tungsten lights (except standard household bulbs), Regarding filtration for tungsten I assume that would be 80B filter on the lens?

    Film testing in general - when you say "know by comparison what speed I am getting" - do you mean you measure densities of that other film/dev combination and compare it with your "standard"? That expired film - that can be any film or is the same kind of film (only expired) you established your "standard" for? Sorry for probably naïve questions. I was following your threads like Hitting ASA Triangle Does Not Mean You Got Full Film Speed, unfortunately I'm not there yet to understand/ appreciate all nuances. A lot of home work to do...

    "The other great value of film testing is knowing when you are starting to get "out of control". " - this is what I would like to accomplish as well

    thank you
    Tomasz
     
  18. Ken Nadvornick

    Ken Nadvornick Member

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    Don't want to steal ic-racer's thunder, but I think what he's referring to is that in a film speed (EI) test you normally want to include all of the (major) real world variables that will have a significant impact on your real world exposures. Soup-to-nuts, as it were. And two of the biggest variables with potentially large error spreads are your shutter and your meter. (Especially your shutter.)

    By formulating the test using electronic flash, you will effectively remove both of these potential error-inducing variables from your results. Exposures will be determined by aperture alone without regard to the shutter (camera body or lens) you will normally be using in the field. Likewise, your regular continuous-light meter (in camera or handheld) will not be used during testing either since the light source is electronic flash.

    For example, I've done EI tests also. One of the things I did in preparation was to use a Calumet shutter tester (no longer available new) to pin down the actual speed of the shutter I intended to use. That became my standard against which other shutters were later calibrated. At least to the extent necessary to give reasonable, usable results. No need to go overboard.

    I followed a similar procedure for the meter I used. But meters can be a little more difficult. Most are non-linear across their range. And given the differences in sensor types, age, calibration drift over time, etc., most are not similarly non-linear. One of the ways to factor this particular potential source of error into testing is to make your meter reading using light levels similar to those you will be using in the field. Meaning, film speed tests (the so-called Zone I test) would be metered against a black surface, while the development time tests (Zone VIII) would be metered against a white surface. By not including your normally used meter in the testing chain, this error might later—and frustratingly—creep into your real-world exposures.

    None of this will necessarily invalidate your tests. But it's always good practice before running an experiment to try to anticipate in advance the possible sources and sizes of error which may creep into your system. That way when things don't turn out perfect you might be better positioned to interpret your data and still glean useable information from it.

    After the inital "Oh my god, that can't be right" it's always nice to be able to look back at your notes and discover "Ahh. Never mind. I can explain that. Factor it out and this test is still valid. I don't really need to redo everything."

    Ken
     
  19. wiedzmin

    wiedzmin Subscriber

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    Ken,

    you are right test which not simulates real life scenarios or does it in very limited way wont be very accurate or it wont make any sense at all. I'm thinking to use flash because in test described in WBM book, how I understood it, author of the book tries to eliminate aperture/shutter speed influence by using stouffer step wedge. So I was thinking if I use flash it wont make a difference for that particular way of testing. Do I assume correctly? What of course is not eliminating problem of exposure variations in taking photos in real life with shutter speeds which are not exactly what we think.
    Shutter speed test is a good idea.

    How did you compensate for discrepancies in shutter speeds (if you found any) - you adjusted ISO or aperture? Calumet shutter tester - how does it work? Does it display values on lcd or you have to connect it to the computer microphone input and use audio software to read it?

    Tomasz
     
  20. Bill Burk

    Bill Burk Subscriber

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    Hey Tomasz,

    I'll try to explain this bit. My usual film is 400 TMAX, so I have done lots of tests of that film and the toe of the curve crosses 0.1 density at a fairly consistent place on the x-axis.

    So I just assume that is ASA 400.

    In the "Hitting ASA Triangle" thread, I found my usual benchmark had 2/3 stop uncertainty. I developed Panatomic-X and compared to the benchmark it indicated greater than 32 ASA speed. Now that is highly unlikely. More likely I never really get ASA 400 from 400 TMAX (Other issues: Neutral Density Filters are non-neutral. Different spectral responses of 400 TMAX and Panatomic-X. Dirty test wedge that I'm not willing to spend $100 to replace right now).

    So let's simplify for a moment and ignore the 2/3 stop problem - I can always go back later and fix that.

    I have enough 400 TMAX film curves that I can take one that meets the ASA triangle. If I draw a vertical line where that curve hits 0.1 density, all I have to do is declare "That is 400" - everything else compares to that.

    Now develop any film and draw its curve. If the toe of the film curve crosses the 0.1 density 0.3 density units to the right, is 1 stop slower than 400.

    So you can pick up a brick of unknown Tri-X, develop it "as much as possible" to fit the ASA triangle. Now in relative terms, where the curve meets 0.1 density, you can tell how much slower that film is due to fog or age.

    The sensitometer I use has a flash that is 1/100th second. It's like a manual flash, always puts out the same amount of light. So the graphs should compare.
     
  21. wiedzmin

    wiedzmin Subscriber

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    Bill,

    thank you for explaining it. I think I'm getting it.

    Do you mean several tests or you just know it and can do it in one attempt?
     
  22. Bill Burk

    Bill Burk Subscriber

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    First time I always do a test of 5 developing times.

    Test exposures on different sheets or few inches apart on roll film and cut them apart for different development times.

    Later, as I go along, I just include a test exposure in with regular runs and "double check" what I was aiming for versus what I got.
     
  23. wiedzmin

    wiedzmin Subscriber

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    got it thank you
     
  24. wiedzmin

    wiedzmin Subscriber

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    I'm going to do tests using flash at full power to lower flash duration + gel to bring color temperature to 4500K. Film I'm going to test will be Fuji Neopan 100 Acros (120). I have 3 developers available xtol, hc-110, id-11. I 'm not sure which one I will use yet. Any suggestions are very welcome.

    I will post results here early next week.

    thank you very much for all input and help !
     
  25. wiedzmin

    wiedzmin Subscriber

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    This is result of my test. I finished last night.

    film used: Fujifilm Neopan Acros 120
    developer: HC-110 1+63
    temperature 21 C
    light source electronic flash Yongnuo YN-560 II with 1/4 CTO to convert to 4500K (attached pdf with flash power vs. flash duration test) - I used 1/125s shutter speed to photograph step tablet.

    I have no previous experience with film testing - any advice, suggestions would be most welcome.

    If test data looks all right I will do the last test to determine normal EI.

    thank you
    Tomasz
     

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  26. PeterB

    PeterB Subscriber

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    That data appears fine Tomasz. You have a good portion of the toe for each curve which is something I failed to achieve during my film testing (I had to extrapolate the toe area on my 5 curves and enter 'dummy' data).

    Peter