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  1. #11

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    Quote Originally Posted by dpurdy View Post
    Generally, you zero the film base on the film you are measuring. Though I am not sure that means its completely accurate at higher densities. The problem with a densitometer that I am facing is no one local will repair it and the cost of shipping my macbeth is more than I am willing to pay.
    If you zero on the film base, then you can't measure fb+f so I'd say you are wrong about that.

  2. #12
    dpurdy's Avatar
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    The idea is to remove the variable of film base fog and measure the exposure density. that is why you zero the film base. I am not wrong.
    D

  3. #13

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    so tell me how you measure fb+f then...

  4. #14
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    You don't really need to know what the base fog is beyond doing a visual and noting that it looks normal. Say if you are using some Delta 3200 that has been out of the fridge for a month you notice that the base fog is way heavier than when it was fresh. It is good to inspect the base to look for weird fogging. But when you are trying to figure density then you set the densitometer so that the base plus fog equals zero then measure the areas of density you are concerned with. The use of the phrase xdensity+base fog is merely being accurate for the sake of being accurate. Xdensity is your concern. If you want to measure a zone 7 test then you zero the base and measure the test and it should read 1.2. That could be described as 1.2+base fog. But the + base fog is unnecessary in practical terms. Every film you measure is likely to have a slightly or greatly different base fog from the other film. So every time you go to measure a density of concern you must first zero the film base to remove the variable of the base fog... which theoretically has no real affect other than overall even increase in density. What you are trying to determine is contrast above base fog.

    Dennis

  5. #15

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    well as it happens I do want to know what fb+f is becuase it useful for telling me if something has changed with film or developer. And to test it I have to zero my densitometer with nothing in it. Fortunately for me I have Heiland and its as simple as pushing a button when I am reading with or without film in it, so it only takes a second or two to zero. But if I had densitometer which which required adjustment via screws or some other system then I might think that I would zero it with nothing in it because its pretty simple to subtract fb+f from any reading I make of actual zone negs.

  6. #16
    dpurdy's Avatar
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    You are making it too hard. I think once you start doing it you will see what I am trying to say.
    Have fun...DP

  7. #17
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    A modern digital transmission densitometer that uses a microprocessor and A/D converter - anything made after 1977 or so - doesn't/shouldn't need calibration. A transmission densitometer measures how well something transmits light with respect to nothing - and nothing is, well, nothing - the absence of any calibration standard.

    A transmission densitometer should self calibrate by taking two measurements from the photodiode with nothing in the densitometer's light path: one with it's light on (the 'blank' reading) and one with it's light off (the 'dark' reading).

    Density is then:

    OD = - log10 ((sample - dark) / (blank - dark))

    Where 'sample' is the signal from the photodiode when it is measuring whatever it is measuring.

    The story is different for reflection measurements - there calibration is needed as a reflection measurement is the ratio of the sample signal to something that is 100% reflective (or through mathematical juggling the ratio of the sample reading to a reading of something that is of a known reflective density).

    If you don't need to share data with other densitometers it doesn't really matter what you calibrate the meter too as long as you always use the same thing. Using glossy photo paper if you calibrate to bright base white at 0.15 and blackest black at 2.3 you will be close enough that it really doesn't matter. 1.57 OD will be _your_ 1.57 OD: call it 1.57 MD (My Density). That 1.57 MD equals someone else's 1.68 OD doesn't matter a hoot because you are always comparing two things: how does your print compare to your black/white calibration photograph. If you use the manufacturer's calibration tablet you are just comparing your print to the manufacturer's tablet.

    To extend on the self calibration theme - you can make a very usable Zone print step tablet by

    o Using #2 1/2 glossy paper find the exposure for the first pure white and
    the first black - the place where it doesn't get any whiter and the place
    where it doesn't get any blacker.

    o Using the settings from your f-stop timer (or a time stops chart) divide
    the exposure range found above into 10 equal parts. If pure white was 3.3
    stops on the timer/chart and pure black was 7.5 stops then each zone
    step from white to black is .47 stops.

    o Made a set of prints at zone step intervals. For the above example
    make a set of prints at 3.3, 3.8, 4.2, 4.7, 5.2, 5.6, 6.1, 6.6, 7.1 and 7.5
    stops of exposure.

    o You now have a zone step tablet. Keep it on file and use it for comparing
    it's tones to your prints' tones.

    Adams defines zones as the tones resulting from breaking the HD curve into ten equal expsoures. So you actually have a _true_ zone system tablet.

    The same thing can be done for negatives to make a transmission zone step tablet, but you have to decide what you want black (the black part of the negative) to be - it is a good idea to pick a black that makes a pure white with #2 1/2 paper when a just-barely there density in the negative produces a blackest black on the print.

    Older analog densitometers usually have lots of little fiddly adjustments because analog isn't very good at math and analog logarithmic circuits are very sensitive to temperature fluctuations (and any other event that may come along). The same is true of quasi-digital densitometers made before 1973 that have analog circuits but a digital display instead of the older meter/needle movement . '73-77 vintage digitals can go either way - uProcessor or analog signal processing.
    Last edited by Nicholas Lindan; 12-21-2007 at 06:19 PM. Click to view previous post history.
    DARKROOM AUTOMATION
    f-Stop Timers - Enlarging Meters
    http://www.darkroomautomation.com/da-main.htm

  8. #18

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    Quote Originally Posted by dpurdy View Post
    You are making it too hard. I think once you start doing it you will see what I am trying to say.
    Have fun...DP
    But then, when the fog level goes wonkers for some reason, and you don't have much experience with a densitometer OR visually inspecting negatives, then you get an undeclared variable that wrecks havoc on calculations.

    Ongoing B+F measurement is a good indicator of processing drift and chemical exhaustion.

  9. #19
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    I have trouble with the phrase "zeroing the film base"; zero the densitometer, calibrate, then read the film base.

    My x-rite 301 is a transmission densitometer and it requires nulling, or zeroing with no film, then it requires calibrating by measuring a known denisty to see if the measurement calibrates, i.e., to read +/- .02 of the known calibration reference. It it does not read to within that tolerance, them I can adust it to do so, then repeat the calibration to see how it reads. When it is calibrated, simply read the fb+f of the film and there you have it. IMO, I think you should buy an already "calibrated" step tablet and use that as the known reference value; seems to me that if you get an uncalibrated tablet, then you won't be sure that your densitometer is actually reading correctly because the tablet itself is not calibrated. It may provide you with a density but how do you know it is correct unless the density you read is within a known reference value according to its tolerance?

    This is just my own thinking on it---I'm not an electronics expert. I have since tested some film with it and things are doing fine.

    Chuck

  10. #20

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    Tobias TB+ Autozero

    I've a Tobias TB+. It will autozero on the first reading
    taken after turn on. So fb+f is measured by first zeroing
    on nothing. To rezero for image density readings,
    read fb+f then press the 'store' switch. Only
    densities above fb+f will display. Dan

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