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Thread: How to test for unknown film emulsion speed / ISO ?

  1. #1

    How to test for unknown film emulsion speed / ISO ?

    Hi all,

    I have a question that's been bugging me for some time and am hoping the answer is much more obvious than I realize. I recently learned how to make gelatin dry plate emulsions for large format glass negatives (bromo-iodide).. However as the speed for these dry plates can vary in terms of a few variables during their emulsion "cooking" time, namely time and temperature and fog density, there can be considerable differences in how I expose each batch.

    I'd like to determine the sensitivity of each emulsion batch as I finish preparing it so I can account for the exposure differences, but am foggy on how to go about it. In school I recall using the zone system to determine a personal ISO based on a given camera, film box speed, paper/chemistry combination. But where do I begin if I don't have a ballpark ISO to work from? Additionally, this film is primarily blue UV sensitive.

    My exposures with my recent emulsion batches have been about f/5.6 @ 3", if that's at all helpful. I'd appreciate any helpful approaches to how I might go about determining the speed of my emulsions. And please go easy on me if I'm being especially ignorant!!--it has been a while since I've been immersed in the zone system practice but I'm really eager to understand this and maintain some consistency in the exposures for my dry plates.

    Thank you in advance, LF photo community!

  2. #2

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    Re: How to test for unknown film emulsion speed / ISO ?

    Quote Originally Posted by catiecolvin View Post
    Hi all,

    I have a question that's been bugging me for some time and am hoping the answer is much more obvious than I realize. I recently learned how to make gelatin dry plate emulsions for large format glass negatives (bromo-iodide).. However as the speed for these dry plates can vary in terms of a few variables during their emulsion "cooking" time, namely time and temperature and fog density, there can be considerable differences in how I expose each batch.

    I'd like to determine the sensitivity of each emulsion batch as I finish preparing it so I can account for the exposure differences, but am foggy on how to go about it. In school I recall using the zone system to determine a personal ISO based on a given camera, film box speed, paper/chemistry combination. But where do I begin if I don't have a ballpark ISO to work from? Additionally, this film is primarily blue UV sensitive.

    My exposures with my recent emulsion batches have been about f/5.6 @ 3", if that's at all helpful. I'd appreciate any helpful approaches to how I might go about determining the speed of my emulsions. And please go easy on me if I'm being especially ignorant!!--it has been a while since I've been immersed in the zone system practice but I'm really eager to understand this and maintain some consistency in the exposures for my dry plates.

    Thank you in advance, LF photo community!

    https://en.wikipedia.org/wiki/Film_speed




    >> My first emulsion had ISO 0.6

    >> Having the emulsion also green sensitive is quite plain and cheap, throw some erythrosin B in the KBr solution. Also called E-127 food dye, some drops of 1:100 Ethanol dilution. You'll also gain speed.

    To determine film speed in the standard way is very well described in the Beyond the Zone System book, a must have, buy it, used it's near for free.

    Anyway, you need to try different developing times until your curve has the standard inclination (contrast), so at the end you plot a family of sensitometric curves to get the good one.




    To do things in the right way you need some instruments, this is my kit:



    The densitometer, $30, a RX medical surplus, of extreme quality, it includes a calibrated wedge. The luxometer, some $20, and the Stouffer wedge.


    So at the end you place the wedge on the plate, measure incident light with luxometer, and then you mesure obtained densities with the densitometer for each cell of the Stouffer wedge footprint.

    The wedge is graduated in half stops, so you will have an irradiation in lux*second for each cell, then you measure the obtained density for each cell. Then you plot the density for each ammount of received light. See graphs in page 8.

    You plot a family of curves with different development time and pick the standard one:





    Then see "Determining film speed" section here:

    https://en.wikipedia.org/wiki/Film_speed.


    Straight way:

    Expose your plate at ISO 0.3, shot a very contrasty escene with some 6 stops range. Spot meter each area and write down what are -3, to +3 areas. Correct your first ISO 0.3 by the right factor, for example if well exposed area was -2 then you have some ISO 1.2.

    You may have to correct development time to get desired contrast, with that development time you should repeat your test.

    When you have a calibrated emulsion you can compare it with another emulsion by making contact copies of the Stouffer wedge and see how many missmatched steps are there. Each cell is 1/2 step (usually) so for a mismatch of 4 cells also multiply or divide by 2x factor the ISO of the known emulsion. This will give you a first guess.

    Regards

    (Feel free to ask me what you need, if I can help)

  3. #3

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    Re: How to test for unknown film emulsion speed / ISO ?

    Hi Catie,

    For what it's worth, I use a sort of hack approach that has worked well enough for me. After developing a bunch of plates, I have a pretty solid idea of the development characteristics, and at this point my normal development is D76 1:1 for about 15 minutes. So I make a guess about the ISO of a new batch, meter an evenly lit, detailed scene, and vary the exposure around the metered suggestion for that ISO. That is, I expose a plate with the dark slide at roughly evenly spaced insertion points bracketing the time suggested by the meter. Once developed, I figure out which was closest and refine if necessary. Most time I get it with a plate, but it's easy enough to go up to 2-3 if necessary.

    It's not as accurate as Pere's approach, but it's worked well enough for me. At some point I'll do a more formal development test, but the latitude of this stuff is such that I haven't needed to yet.

    My original two batches were in the ISO 0.4 range. My current one (slower introduction of the silver nitrate, longer ripening) is more like ISO 1, which is a big help. Looking forward to pushing it a bit farther.

    Robert

    p.s. Picked up some 11x14 plate holders not so long ago at a photo show; have a couple of process lenses that should cover and am trying to jury-rig a box camera to try them out on. Should be fun...

  4. #4

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    Re: How to test for unknown film emulsion speed / ISO ?

    I would make a sensitometer from a step wedge (Stouffer sensitivity guide) and a simple light source that you can control, like an enlarger with stabilized power supply and timer... Instead of trying to determine the meter candle seconds by metering, I would try to estimate the meter candle seconds by experimenting with some fresh manufactured film. Then I would use that setup for the unknown emulsions, and compare the sensitivity of known to unknown.

    If I was experimenting with home-made emulsions... I would modify all (or at least some) of my plate-holders to cover a small strip on an edge (reserve part of an edge of the plate to expose the Stouffer sensitivity guide)... And before developing the plate, I would expose that sensitivity guide into that small reserved area.

    In other words, I would put a Stouffer scale on every plate (or at least put the Stouffer scale on many plates).

  5. #5

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    Re: How to test for unknown film emulsion speed / ISO ?

    Quote Originally Posted by Robert Brazile View Post
    That is, I expose a plate with the dark slide at roughly evenly spaced insertion points bracketing the time suggested by the meter.
    Your option is a sound way to test it, I like it, I'll use it...

    Just I'd propose a refinement, this is after making that "bracketed" exposure of the scene with the dry plate, it could be good to rightly expose a Kodak/Ilford sheet of known ISO, and then picking the dry plate strip that best matches the commercial sheet, so we'll have a good correction factor as we compare with a calibrated material.


    I guess that it would be suitable an scene with a remarkable dynamic range difference from top to bottom,is we move the dark slide from left to right, as in this case each strip will contain a wide dynamic range to better compare the strip to the sheet in all latitude of the sheet and of the dry plate, so a bright sky with some clouds and ground with different shades would help.

    Perhaps this comparison should be made with the sheet exposed with a blue or cyan filter (depending on if we compare with color blind or erythrosin orthochromatic), and later also considering the filter factor for the effective speed.

    The other option is to use a gray scene without much color saturation.

    ... or even better, using an ortho sheet of known ISO to compare with an ortho emulsion.

    In DIY emulsion moving from color blind to ortho is an straight move... E-127 !!!

  6. #6

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    Re: How to test for unknown film emulsion speed / ISO ?

    Quote Originally Posted by Bill Burk View Post
    I would make a sensitometer from a step wedge (Stouffer sensitivity guide) and a simple light source that you can control, like an enlarger with stabilized power supply and timer... Instead of trying to determine the meter candle seconds by metering, I would try to estimate the meter candle seconds by experimenting with some fresh manufactured film. Then I would use that setup for the unknown emulsions, and compare the sensitivity of known to unknown.

    If I was experimenting with home-made emulsions... I would modify all (or at least some) of my plate-holders to cover a small strip on an edge (reserve part of an edge of the plate to expose the Stouffer sensitivity guide)... And before developing the plate, I would expose that sensitivity guide into that small reserved area.

    In other words, I would put a Stouffer scale on every plate (or at least put the Stouffer scale on many plates).
    Hello Bill,

    This is also a very good way, perhaps better that using an scene, but still it requires a densitometer, or scanning the sheet and the plate at the same time to compare levels in the pixels.

    Metering the lux of the light source is also very straight. A 0.01-300000 range Luxometer today is $25, this is a very useful and affordable instrument (better buy one with 0.01 lux capability). Anyway IMHO your method is the best if one do not want to perfrom the ISO plots and calculations.


    One thing I see is that probably the plate and the sheet exposures have to be made with different light intensities, because we need a some 1s exposure in both cases. More than 1s provocates reciprocity failure in the darker cells of the stouffer wedge (failure is not because exposure time, but because low flow of photons). And Exposures lower than 1s have accuracy problems with enlarger timers, and if using a filament bulb there are uncontrolled effects because warming times.

    As we need 1 second exposures for a 50 ISO sheet and for a 0.5 ISO plate then we need to vary light power, by probably stopping diafragm, or moving enlarger head up/down. At that point the $25 luxometer can be useful to test what real lux ratio we have, but not strictly necessary. An Andriod/iPhone luxometer App can also be used. Smartphones have a photocell in the face for screen auto-brightness. That smartphone luxometer works different, as it's very directional (no white dome), but it will work with limitations, as perhaps it won't have accuracy for the low lux required to expose the sheet.


    I thing your method is the best, anyway Robert's one is also good to see what happens in practice with real subjects, because the different latitudes that may happen will have a pictorial effect that's not as easy to evaluate from Stouffer's contact prints.

    Regards.

  7. #7

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    Re: How to test for unknown film emulsion speed / ISO ?

    I would be remiss if I didn't say that great variations in speed from batch to batch are totally avoidable. Consistent time and temperature will eliminate all but a fraction of the variability. The variability you see is probably, mostly, from variations in the UV intensity at any given exposure situation. A basic dry plate emulsion should be between ISO 3 and 12, in full, midday light -- the lower number in the winter, the higher in the summer, with a range in between. The old timers used the charts that came with each brand plate, and/or developed their own. Here's one from Hammer plates: http://thelightfarm.com/cgi-bin/hamm...gesize=Smaller
    The trick of testing speed with the dark slide (just as you would test paper under an enlarger in the darkroom) is all you really need, and you shouldn't have to do it more than a couple of times in order to calibrate your recipe. Because the plates can be developed by inspection, you've got all the bases covered. When in doubt, bracket. It's always good insurance with handmade materials. 2 cents, d
    Denise Ross
    www.thelightfarm.com
    Dedicated to the Craft of Handmade Silver Gelatin Paper, Dry Plates, and Film

  8. #8

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    Re: How to test for unknown film emulsion speed / ISO ?

    Quote Originally Posted by dwross View Post
    I would be remiss if I didn't say that great variations in speed from batch to batch are totally avoidable. Consistent time and temperature will eliminate all but a fraction of the variability. The variability you see is probably, mostly, from variations in the UV intensity at any given exposure situation. A basic dry plate emulsion should be between ISO 3 and 12, in full, midday light -- the lower number in the winter, the higher in the summer, with a range in between. The old timers used the charts that came with each brand plate, and/or developed their own. Here's one from Hammer plates: http://thelightfarm.com/cgi-bin/hamm...gesize=Smaller
    The trick of testing speed with the dark slide (just as you would test paper under an enlarger in the darkroom) is all you really need, and you shouldn't have to do it more than a couple of times in order to calibrate your recipe. Because the plates can be developed by inspection, you've got all the bases covered. When in doubt, bracket. It's always good insurance with handmade materials. 2 cents, d

    Hello Denise,


    I'd like to ask you some questions,


    > What ISO has the "TLF #2 Full-Ammonia Bromide Emulsion, 2nd ed.", ? (it will be my next try)


    > What speed increase is due Steigmann's ?





    Just one comment... beyond the dark slide way to bracket the test exposure, I found useful to retrieve little samples during digestion/ripening and save it in order to evaluate the fog/speed performance form different digestion times, making contact of the Stouffer's...

    Would be this useful for adjusting TLF #2 ? or the recipe is optimized enough so no need ?


    One thing else, do you have any guess why Acros (soon a defunt for LF...) could reach that LIRF performace ? May it be because dye sensitization ? using dye sensitizers from color films ? Dopping agents ?


    Thanks in advance,

    Best regards

  9. #9

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    Re: How to test for unknown film emulsion speed / ISO ?

    Hi Pere,

    I developed TLF#2 with Steigmann's in mind (first, I had to resurrect Steigmann's from the old lit!), so I don't know what the speed would be without it. Based on other emulsions, I could guess anywhere from 10 - 25% of the speed is due to the gold sensitization.

    Additional testing is never a wrong thing to do. It basically comes down to what you enjoy and the trade-offs of your own time management and priorities. I've gone the route of pouring two plates every ten minutes of ripening, clear into over-ripened times. Interesting once or twice, but it doesn't really generate enough material to do rigorous testing. It's nice to make a full batch of identical plates at a time and then shoot them in as many different conditions as you can find. TLF#2 went through two full years of testing, saving plates from dozens of batches to use and compare at different times of year, altitudes, and light conditions. Also, different developers. Developer choice is as important a variable as anything else. It's all great fun for those of us with rat lab tendencies, but honestly, just as much can be learned by simply making lots and lots of plates and going out as often as you can and photo'ing up a happy storm.

    Sorry, I don't know anything about Acros.
    Denise Ross
    www.thelightfarm.com
    Dedicated to the Craft of Handmade Silver Gelatin Paper, Dry Plates, and Film

  10. #10

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    Re: How to test for unknown film emulsion speed / ISO ?

    Quote Originally Posted by dwross View Post
    Hi Pere,

    I developed TLF#2 with Steigmann's in mind
    Thanks so much !

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