Film speed test using a light meter

Discussion in 'B&W: Film, Paper, Chemistry' started by Doug Bennett, Jan 21, 2004.

  1. Doug Bennett

    Doug Bennett Member

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    I've arrived at the point where I want to establish personal film speeds for my 2 or 3 most used films. I don't have a densitometer, and after some asking around town, haven't been able to locate one to use.

    So, I had this thought. If the goal is to establish a Zone 1 density, could one use a light meter to do this? Would this work:

    1. Meter a gray card in even, diffused light. This should be Zone V;
    2. Make a range of exposures from Zone I to Zone IV;
    3. Make an exposure or two with the lens cap on. This should establish base + fog. Process the film.
    4. Meter through the base + fog frame, then through the Zone I frame. If Zone I meters one stop different from the base + fog frame, is this now a true Zone I?
    5. If the difference is more than 1 stop, it would seem that the film was rated too slow. If less than 1 stop, than too fast.

    What's the downside of this? What am I missing?
     
  2. L Gebhardt

    L Gebhardt Subscriber

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    The Beyond the Zone System book has a section on using your spot meter as a densitometer. It should be fairly easy to setup. If you have a step wedge you can check your accuracy.

    I use my Epson 2450 scanner for this purpose. I calibrated the scanner settings with a step wedge. I read the density with Picture Window Pro. It works great.
     
  3. mikepry

    mikepry Subscriber

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    Doug,
    What I have done in the past is to get a .010 density filter from kodak(the gel one that is 3" square) and take that on top of a film that was not exposed but developed with your other test sheets and use your spot meter on a tripod and read through the .010 filter and blank neg at a diffused light. I use the globe under our kitchen ceiling fan and it has worked every time. Take that reading and write it down. Then take readings of all your other negs and the one that is closest to the original reading will be your .010 above film base plus fog. It will also be your new personal film speed. I have done this several times and it is slick. An old timer showed me it and he called it a poor mans densitometer. Hope I haven't confused the hell out of you!

    Best,
    Mike
     
  4. Leon

    Leon Member

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  5. dr bob

    dr bob Member

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    I put in an article here at APUG a while back concerning this topic, but I cannot find it anymore. I'd be glad to discuss this with the originator via email or pm (or chat - if I can figure out how to use that).
     
  6. Doug Bennett

    Doug Bennett Member

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    Thanks for the link, Leon. It was interesting, as is the whole web site. Maybe I'll sort of do both methods at the same time, and see if they lead to the same conclusion.
     
  7. Paddy

    Paddy Member

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    Here's a simple and effective method to determine Zone I/ film threshold - density.

    1. Shoot 5-6 blank/unexposed frames.

    2. Shoot a series of exposures around Zone I (= -4 stops below grey card base meter reading). Shoot from 1 stop below Zone I, to 2 stops above Zone I, using either 1/2 or 1/3 stop increments. Shoot the rest of the roll at Zone V, as this helps to simulate a normally exposed roll of film.

    3. Develop at products' recommended "standard" time.

    4. use the unexposed/clear strip of film to make a test contact exposure strip, to find out the minimum amount of time required to get a maximum paper black.

    5. once you have established the minimum contact time required for #4, then contact print your Zone I exposure test (#2)

    6. If your minimum contact time for #5 is correct, then the frame from your Zone I exposure/threshold test, which prints as the first perceptible grey above paper black, will indicate both adequate Zone I threshold exposure, and also your corrected E.I. film speed. For example: You shot APX 400, but the frame that printed as the first perceptible grey above paper black was exposed as Zone I + 1 stop. This means that you needed to double the exposure to acheive a correct Zone I density, 4 stops below Zone V. therefore your corrected film speed is E.I. 200.

    Of course, that's only the first half of the testing process, as determining a correct development time is the other half of this matter.

    Good Luck!
     
  8. john_s

    john_s Subscriber

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    Doug,
    Referring to your first post, I think you're on the right track. However, I think that the density you want over FB+F for Zone I exposure is not one stop as measured by the light meter, but a third of a stop (=.1 density units).

    This is if you accept that .1 over FB+F is the right Zone I negative density, which it commonly is held to be.

    One problem of doing this is that the gradient of the characteristic curve is pretty low at this part of the curve. So it's very hard to discriminate between adjacent settings if you have used, say, 1/3 stop increments in exposure. Or in other words, the speed you'll choose is very dependent on the shape of the foot of the curve.

    I have seen it suggested that it is better to look for Zone III being 0.4 density over FB+F, on the grounds of this part of the curve being much more linear. Zone III will theoretically be more influenced by development, but it was suggested that it was not by much.
     
  9. john_s

    john_s Subscriber

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    Dr Bob,
    That article of yours was very good. I have a copy of it, and I've just re-read it.

    I had trouble finding it on the APUG site too. Are they somewhere else now?
     
  10. dr bob

    dr bob Member

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    I'm Not sure. I found it once backing through the discussion forum on "articles". I guess they havn't been transfered to the new format yet. Let's ask Sean?