Measuring negative densities with spot meter

Discussion in 'B&W: Film, Paper, Chemistry' started by Dan Henderson, Aug 6, 2005.

  1. Dan Henderson

    Dan Henderson Member

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    I am attempting to fine tune my negative exposure and development, using a spot meter instead of a densitometer. I am following the "Tone to Zone" article (a link to which I think I found at this site). The process makes sense except for one thing:

    The article states that 1/3 stop difference on the light meter equals .10 density, with .30 = 1 stop. The article goes on to say that the Zone VIII negative used to refine development time should be 1.20 density units above the Zone I negative, or 4 stops less on the light meter since 1.20 density units equal 4 stops.

    But if the Zone I and Zone VIII negatives were exposed properly, there should be 7, not 4 stops difference between the two.

    Anyone who can enlighten me will be immediately inserted into my will. Not that my estate will be worth much, but its the thought that counts, right?

    Thanks in advance.
     
  2. photobackpacker

    photobackpacker Advertiser Advertiser

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    Hi Dan:

    The zones are based upon a customary curve. It may or may not match your materials. Here is a listing of the densities by Zone of a characteristic curve. These are representative and are a good guide for a diffusion light source.

    Do you have a good step tablet? If not, I recommend you get one. I have had rank beginners compare the step tablet to a negative on a light table and call out with very adequate accuracy what the densities are. This worked better for me than the spotmeter process. You can get a 21 step tablet that is calibrated for a reasonable price from this company. http://www.stouffer.net/Stoufferhome1.htm

    Good Luck.
     
  3. Jorge

    Jorge Inactive

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    The average photographic paper can only acomodate 4 stops, so you have to "shrink" the 7 stops difference into the 4 stops the paper can handle. So the best thing to do is aim for a target density for highlights with detail of about 1.2

    In theory if we had a paper that had an exposure scale of 2.1 then you would be correct and we would be looking for a target density range of 2.1. But few processes have this straight one to one relation ship. For example pt/pd which has a very long scale usually works best with a density range of 1.45, pure palladium works best with 1.8.

    I think printing out paper has a very long range close to the 2.1, but I am not sure.

    Anyhow this is why the 1.2 density range is recommended. IMO You would even be better served shooting for 1.1 above zone I.

    Make sure I get the spot meter in the will... :smile:
     
  4. photobackpacker

    photobackpacker Advertiser Advertiser

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    Oops - Forgot the densities.

    All of theses are net densities (minus base + fog)
    I .10
    II .18
    III .31
    IV .48
    V .70
    VI .94
    VII 1.14
    VIII 1.30
    IX 1.40
     
  5. dancqu

    dancqu Member

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    You may wish to use an EM-10 enlarger spot meter; about
    $30 new. I've calibrated the EM-10 against the 21 step
    step wedge.

    With your negative in the same position as when
    calibrating the wedge, read the number on the EM-10
    dial and compare with your calibrations.

    To all, where are all the EM-10 densitometerists? Dan
     
  6. noseoil

    noseoil Member

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    Dan, the heart of BTZS type "SBR" numbers is what you are seeing now. Paper can only see a narrow range of values, film can see much more, our eyes see more than the film. How can we cram all of our ability to see light onto a narrow range of values which is the paper? Many films are capable of "seeing" a long range of values, but what good is this if it doesn't match the paper's scale?

    You might try a step wedge printed on your favorite paper. How many log numbers does the paper actually see? This is the first question to be answered by your meter. Once you have this number, how much exposure & development is required to make the film work with this paper? tim
     
  7. Lee L

    Lee L Member

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    Dan,

    I don't see any answers yet in the most basic of terms, so I'll try that to see if it helps with the concept.

    You're exposing the film to light that has a range you can't always control. That's very often near the 7 stop, or 2.1 (7 stops * 0.3) density range you mention. However, papers can't handle this range, so you develop your films so that they have less than a 1:1 correspondence of density to the light they receive. The gamma, or average slope of the line in a graph of the film's density vs. the amount of light striking the film, is a number that reflects that correspondence of film density to exposure. The slope of this line is most often in the range of 0.55 to 0.65, so an increase in exposure of 1 stop doesn't yield a 0.30 increase in negative density. Instead it yields an increase of about 0.55 to 0.65 times 0.3 density units per stop. So for each increasing stop of light striking the film, you develop to reach just under 2/3 stops of negative density in order to fit the image within the range of standard printing papers.

    Try this with your example: fitting 7 stops of exposure light into the 4 stop range that paper can handle requires that you get density on the film that is 4/7ths of a stop (a factor of 0.57) of negative density for each stop of light striking the film. So each 0.3 (one stop) increase in the subject should only provide an increase of 0.57*0.3, or 0.17, density units on the film so that the image can "fit" on your paper.

    So what you're trying to do in all this is to develop the film so that the range of stops striking the film (SBR) is multiplied by some fraction (the gamma of the developed film) so that it fits within the range in which the paper can show detail.

    Hope this helps.

    Lee
     
  8. Galaxens President

    Galaxens President Member

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    The approximate 4 stops is the input range (paper exposure range) that gives the output range (print reflektive density range) of about 7 stops.

    Let's say the normal scene has a range of 7 stops (film input range). To give the proper paper input range (paper exposure range) when printing, the film output range (negative transmission density range) for that input has to match the paper input range (paper exposure range) of 4 stops to make deep blacks with shadow detail and bright whites with highlight detail in the print and show the 7 stops range (print reflektive density range) of the original scene .
     
  9. Jorge

    Jorge Inactive

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    Well this is where it gets a little complicated. The reflective ability depends very much on the paper and process used. The problem is that many interchange density range with paper scale. For example, pt/pd has a very low reflective scale on average but it requires a negative with a long density range, or better said at the printing stage the tones get compressed. OTOH we have silver fiber bases papers that can have a Dmax (maximum reflective density) of up to 2.4 when toned. Now, theoretically if a paper has a Dmax 2.4 we can print 8 different steps of gray one stop apart. The problem is that this response is not linear, so we have to look at the "curve" of the paper. IOW how well can the paper separate gray, black and white tones.

    I hope you will agree that a paper that has a Dmax of 2.4 but is unable to separate tones and everything looks black from 2 to 2.4 is of not much use above a density of 2. This is an extreme example but it is the easiest way to explain it. In reality most people who know about the reproduction icicle, use as "maximum black" the point where the paper reflects 90% of the possible maximum density. Why this number? because it is the point at the shoulder of the curve where blacks can separate detail, it allows you to have "accent" blacks with no detail and gives you what is called a convincing black.

    As you can see when we start to choose the portions of the curve that are straight to reproduce then we start to leave out values that have no use for us when printing. So even if a paper has a scale from 0 to 2.4 we might just be using anything from 0.3 to 2.1, which is about 6 stops.

    I hope this helped.
     
  10. Donald Miller

    Donald Miller Member

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    I don't know how accurate a spot meter would be to determine reflective densities. On paper, the reflective densities can be separated by less then 1/3 stop differentiations. This is especially true of regions in the toe and shoulder of the paper.

    In other words, using a step tablet exposed on paper, the step tablet would be in 1/3 (.10 log) stop increments but the corresponding reflection density on a print, in the toe and shoulder regions of the paper, may show this as .04 differentiations (for example).

    Neither the paper or film are linear throughout their range. The most effective way of approaching this is to determine the characteristics of a paper and then determine the characteristics of a film so that the characteristics of the film can be brought into conformance with the paper characteristics.
     
  11. dr bob

    dr bob Member

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    I am the author of the article here on the use of a spot meter to measure negative (transmission) densities. The use of the spot meter under the conditions specified gives repeatable though not necessarily accurate data. I have NEVER been able to get repeatable data from reflective (print) readings. I attribute this to my inability to furnish a reliable and repeatable light source. There are just too many uncontrollable variables for this type of use IMO anyway. I’m sure with the application of funding and considerable thought, it can be done, but by then wouldn’t it be just as economical and much easier to purchase a densitometer?