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  1. #31
    Bill Burk's Avatar
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    Quote Originally Posted by Mr Bill View Post
    I presumed that the silver-colored interior and the sample-holding chamber were somehow designed to make the light distribution more even. (And possibly the striped attenuator, laying in the bottom, was a factor; all of my testing used one of these.)

    When first working with a new EG&G sensitometer (in my job, around 1980?), I was skeptical about eveness of lighting, due to the flat sample holder.
    Hi Mr Bill,

    Sure the chamber is silvered and it helps but when you look down at the unit going off... you can clearly see the bulb spiral. Now the bulb is bigger than a point. I could have taken its shape into account on my sketch and that would knock the error down a couple percent.

    To find the real deviation, all it would take is to remove the step wedge and fire off a shot through the glass. Then measure the resulting density for evenness. The differences could be factored into the densitometer reading for each corresponding step.

    Again, this is amusing to me - not a serious consideration. I believe the EG&G is suitable for the job.

    Michael,

    You might consider taking a "body cap" and jerry-rigging a camera mount to get accurate half- or quarter-second xposure time.

    Unless you are working with an unknown film speed, like expired Tri-X (to mention a real case - I get EI 64 on Tri-X expired 1987)... Your chief benefit from the home-made sensitometer is going to be the curve shape and gradient (however you choose to measure it). The shape and gradient doesn't change with over or under-exposure, it just shifts left and right on the graph.

    You can figure out the illumination by working backwards from results. Develop long enough to get 0.62 gradient with a standard developer. No matter where the curve falls left or right, the Log-E value directly under the crossing point of the curve and 0.1 over B+F ... for practical purposes that satisfy me ... is the illumination that gives the rated speed of a fresh film.

  2. #32
    Stephen Benskin's Avatar
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    Quote Originally Posted by Bill Burk View Post
    You can figure out the illumination by working backwards from results. Develop long enough to get 0.62 gradient with a standard developer.
    Bill, I think it's important to make a clear distinction here. The ISO parameters produce a gradient of 0.615 within the parmeters of log-H range 1.30 and density range 0.80. Depending on the toe shape, the average gradient of the film can be very different.

  3. #33
    Stephen Benskin's Avatar
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    Quote Originally Posted by Leigh B View Post
    OK. Sounds like a solution looking for a problem.

    Is this a BTZS thing?

    - Leigh
    Maybe you should explain how you test. Do you contact the step tablet within the camera? Is your question one of accuracy of the exposure vs accuracy of the characteristic curve?

  4. #34

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    Quote Originally Posted by Leigh B View Post
    OK. Sounds like a solution looking for a problem.

    Is this a BTZS thing?

    - Leigh
    God no! No BTZS for me. Never liked that system at all.

    Really what I'm getting at is some kind of no-flare baseline would be useful for me - because as much as I understand the effects of flare, I still don't understand how to know if a given situation has flare, how much etc. I was re-reading Hutchings's book several weeks ago and something he says in there bothered me regarding testing - he says don't meter a grey card surrounded by either white or black and expect to get a good reading. Now, I'm not saying he's right or wrong. But it just raised these old flare questions in my mind.

    In the past, when I tested film/development (35mm is easiest), I did it the old fashioned way I thought best: in-camera, by photographing a brightly lit white card and changing shutter speeds/apertures to get all the different densities, measure the developed frames with a densitometer and done.

    I'm rethinking this now that I'm going back to sheet film since in-camera testing is somewhat more difficult with a view camera.

    I never liked using wedges for film testing though. For one thing, you don't get a long enough range. I test from threshold density to about 15 stops higher. The very high densities, and the shape of the curve up there, are very important to me given the conditions I'm usually photographing under.

    But I figured maybe it would be a good learning experience (if nothing else) to first try contacting a wedge the way the science guys do.

    Stephen and others are of the opinion, for example, that since flare is difficult to measure, you should eliminate it from film tests. I'm still not sure about that. I understand that might the best way to get a film's H&D curve from a legal perspective , but contacting is about as far as you can get from actual shooting conditions, particularly with a view camera. This was to be the topic of another thread though - what is the best way to test your film/development.

  5. #35
    Stephen Benskin's Avatar
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    Quote Originally Posted by Bill Burk View Post
    Todd-Zakia sketch an intermittent illuminance in their diagram of the monochromatic sensitometer. A sector wheel "constructed so as to produce a stepped series of illuminances on the sample when the wheel is rotated sufficiently rapidly to avoid the intermittency effect."

    I think they are ruling out the spinning sector wheel as far as speed testing in the ISO standard.
    That type is only intermittent if it doesn't make the exposure in a single pass. An additional shutter is added and synchromised to the revoving disk like with the Kodak Type IIb. The situation whether to use a intermittent or non-intermittent sensitometer has to do with recipocity.

    I stand corrected in my belief that the light source was what made a sensitometer intermittent or non-intermittent. It appears that non-intermittent sensitometers have an advantage when the light source isn't very stable as was the case in the early days.

    Of course it was Jones that classified the types. From Theory of the Photographic Process:

    Type I. Intensity-Scale Sensitometers
    Intersity variable, Time constant

    A. Exposure intermittent
    1. Wedged Exposure
    2. Stepped Exposure

    B. Exposure non-intermittent
    1. Wedged Exposure
    2. Stepped Exposure

    Type II Time Scale Sensitometers
    Intensity constant, Time variable

    A. Exposure intermittent
    1. Wedged Exposure
    2. Stepped Exposure

    B. Exposure non-intermittent
    1. Wedged Exposure
    2. Stepped Exposure

  6. #36
    Stephen Benskin's Avatar
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    Quote Originally Posted by Michael R 1974 View Post
    God no! No BTZS for me. Never liked that system at all.
    Calling sensitometry "BTZS" is like calling evolution "Darwinism."

  7. #37
    ParkerSmithPhoto's Avatar
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    Quote Originally Posted by Stephen Benskin View Post
    Is there a way to convert the meter's readings into lxs?
    You'd have to ask Nicholas on that one. It reads in stops, which are easily converted to optical density. It's helped me get my negatives much more accurately developed.
    Parker Smith Photography, Inc.
    Atlanta, GA

    Commercial & Fine Art Photography
    Portrait Photography

  8. #38
    Nicholas Lindan's Avatar
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    Quote Originally Posted by Photo Engineer View Post
    Ilford makes a very nice on-easel meter as well.
    It's not really a meter - it's a comparator that tells you when two light levels are the same.

    It is possible to calibrate the dial on it - but first you would need an accurate meter.

    The DA meter doesn't read in lux. Lux measure light as an 'average' human eye sees it. Approximated lux are useful for checking illumination for OSHA compliance, but not very useful in a darkroom.

    The DA meters are calibrated to an arbitrary standard. In general, a reading of 10 stops with an incandescent source will produce max black on Ilford MGIV RC in one second, a reading of 9 stops of light will take 2 seconds, 8 stops of light takes 4 seconds.
    DARKROOM AUTOMATION
    f-Stop Timers - Enlarging Meters
    http://www.darkroomautomation.com/da-main.htm

  9. #39
    Photo Engineer's Avatar
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    I know it is a comparator, but it allows you to compare exposures from yesterday with those of today so to speak. It works.

    PE

  10. #40
    Stephen Benskin's Avatar
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    This is obviously a case of someone missing interpreting what I was saying about precision in testing. The meter sounds like a nice tool for repeatability in the darkroom. I was talking about accurately determing exposure values.

    Lux is pretty useful in sensitometry though.

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