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  1. #1
    JeffD's Avatar
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    Light color and black/white film testing

    I would like to conduct some indoor film tests in which a grey card is illuminated by a lamp, and film exposed and densities checked.

    I understand that modern black and white films are somewhat pan chromatic, or, supposedly the lamp light "color" used to illuminate the grety card shouldn't matter much, or does it?

    How much variation might I have from typical tungsten light, compared to outdoor light in regards to densities registered on film from the same brightness levels?

    If there is a significant variation, is there an inexpensive, readily available lamp bulb I could purchase that might make any differences negligable from a practical photography perspective?

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    127
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    I just ran a quick test using my filtersimulation app (www.dctsystems.co.uk).

    Using my hp5 spectral data (which is pretty rough - anyone got better?), then compared to a D6500 light source, then none of other lighting standards (D5500, D7500, a, b or c, all normalised at 550nm) produced a response significantly different. That is more than 0.15 stops (most were much less).

    On the other hand using an efke25 othochromatic model, Illuminant A is half a stop down on D6500.

    My film data sucks, as haven't found a good source of spectral responses - the graphs provided by manufacturers are of poor quality at best. At worst it's not even clear what they're plotting (are ilfords plots log or linear? are they responses or sensitivities? I've no idea). So I could be way out, but at least its a starting point.

    I'd suspect an equal or bigger problem is what do you mean by "the same brightness levels"? As measured by your eyes? Your meter? your film? Your meter has a spectral response too...

    While my results sugguest it should be OK, you could get a dayliight bulb (any hobby place sells them), You could of course use flash?

    Ian

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    JeffD's Avatar
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    Quote Originally Posted by 127
    I'd suspect an equal or bigger problem is what do you mean by "the same brightness levels"? As measured by your eyes? Your meter? your film? Your meter has a spectral response too...

    Ian
    By brightness level, say exposing a grey card at zone 1, or 4 stops underneath my pentax spotmeter reading.

    Basically, I am wanting to know if my film speed and development tests will carry over outside, if the tests are conducted under tungsten lighting, indoors. I am not really anal on this- I know there will be some difference. I would be really happy if it was under a third of a stop....

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    What I have always read is to use the light that you are going to be photographing in: daylight for outside work, tungsten for continuous indoor lighting, flash for studio etc. I do wonder though, especially with outdoor photography...

    Outdoor light temperature varies enormously during the day as the sun rises and sets and as clouds cover the sky etc. We normally pick film speed based on shadow detail, but again, light in the shadows is generally much bluer than direct or diffused sunlight as it is mainly lit from the open sky, not the sun. Plus, that shadow is often under vegetation, imparting a strong green component to the light - and other times it is not, so no green tint... Which combination of these lighting conditions are you supposed to use to determine film speed?

    Then, as Ian points out, any light meter has its own, unknown, response. Add in all the human errors of placing your shadow reading on the wrong value, variations in emulsion, variations in processing (time, temperature, agitation, freshness of chemicals etc)... It's a constant amazement to me that anything useable comes out of the developing tank at all...

    On the other hand, normal tungsten lighting IS very red and the only time you get that outside is when the sun is low on the horizon so it's probably the worst-case scenario as far as setting film speed is concerned - except that this is also one of the best times to make an image outdoors: it does not get any simpler the more you think about it....

    One major problem is that I have no idea to what extent all these variables, pulling in their various directions, will effect the outcome.

    When we next get some sun, I think I'll use up a roll of 120 FP4+ and see what differences I get in speed between an 18% grey card under a bush and an 18% grey card in my kitchen lit by a 100w lamp...

    Cheers, Bob.

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    Quote Originally Posted by JeffD
    By brightness level, say exposing a grey card at zone 1, or 4 stops underneath my pentax spotmeter reading.

    Basically, I am wanting to know if my film speed and development tests will carry over outside, if the tests are conducted under tungsten lighting, indoors. I am not really anal on this- I know there will be some difference. I would be really happy if it was under a third of a stop....

    Jeff,

    First of all I wrote an article appearing in the current issue of PHOTO Techniques Magazine about how Zone I being four stops under the meter reading point is incorrect and causes a 2/3 stop overexposure.

    Second, the idea of testing under tungsten light has more to do with the spectral sensitivity of the meter's photo cell. Before 1960, meters were calibrated at a rather low color temperature. This lead to two film speeds for each B&W film: one for interior and one for exterior. Remember that at different times, different photo cells were widely used. During each period the ANSI Meter Calibration standard tended to reflect the most popular meter's spectral bias. Testing under tungsten light will probably reduce the accuracy of the exterior film speed, but if you are doing the Zone System approach, you are off anyway so the slight difference should bother you.

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    JeffD's Avatar
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    I guess, since the variables are so overwhelming, it is not too important if I film test with indoor lighting. It would be nice if there were a type of bulb that gave an aproximate intermediate color between "average daylight" and "average tungsten", although I don't know if something fits the bill. One of my reasons for testing indoors is to have a fairly consistant type of light from test to test. I want to get a good zone 1 test, to get my film speed, then carry out 4 or 5 different development time tests to get a good feel for how I can manipulate contrast...

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    Stephen Benskin's Avatar
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    Jeff,

    As long as you are aware of the variables, you are in good shape. Most industrial lighting shops can help you with what ever you want. Aim for a light source around 4700 K. This is good enough. The standard indicates a 2855.6 K source with specific filtation to bring it up to 4700 K, or as they say, "selectively absorbing liquid filter."

    Good luck.

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    I believe most B&W film is balanced for tungsten.
    Don't forget your yellow filter. Dan

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    Quote Originally Posted by dancqu
    I believe most B&W film is balanced for tungsten.
    I'm sure Stephen can tell us what the ISO standard uses for expsoure - I'm guessing it is daylight balanced at 5500K or 6500K.

    Instead of shooting frames at differing settings in a camera, I use a step wedge under the enlarger and contact the wedge onto the film. One expsoure is all that's needed. I make the exposure under the enlarger to which I've added an 80A (blue color balancing) to the enlarger. I measured the color temp of the enlarger - it's now 5750K with the filter. I use my light meter to measure the exposure at the baseboard and then calculate film speed from the resulting film density plot. Pretty easy to do.

    Kirk - www.keyesphoto.com

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    Quote Originally Posted by Kirk Keyes
    I'm sure Stephen can tell us what the ISO
    standard uses for expsoure - I'm guessing it is daylight
    balanced at 5500K or 6500K.
    That ISO is for speed. For color fidelity in a gray scale
    rendition, tungsten is used. Ilford, IIRC, uses 2850K.

    That temperature is used because of the limitations set by
    the green and red sensitisers. In other words a panchromatic
    film is no faster than its green and red sensitivities.

    So, for outdoor shooting don't forget your yellow filter. Dan

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