


Originally Posted by sanking
How did you make the determination that the Kodak numbers are higher for both DR and SBR? When I compared the calculations with real plots for the two methods the DR and SBR were identical...Sandy King
Hi Sandy,
I based that statement on the information I referenced from the first edition of Davis' BTZS book. His explanation of the GBar there says to use an arc of 1.5 radius from the 0.10 above fbf density value on the characteristic curve. To approximate the Kodak CI, he further states to use a radius of 2.0 units from the same 0.10 above fbf. This yields higher DR and SBR values as illustrated in the last graph I posted and, in his book, the values do appear expanded when using the 2.0 Kodak value on the figures illustrating and comparing the methods. I realize this is just an approximation of the CI the way Kodak really determines it, but the numbers work out very similarly for the three methods. They do so at a difference in values for DR and SBR though.
Please take a look at the last graph I posted in this thread and let me know if I've made an error in the calculations. (The blue numbers and lines on that graph represent my interpretaton of Davis method for approximating Kodak's CI. The red lines and figures give Ilford's Gbar according to Davis' method.) Am I missing something in my interpretation of Davis' comparison?
This is one of those perplexing things about the literature to me. I fully understand how to determine CI using Kodak's method and I use that method when doing my version of the zone system (on the rare occasion when I choose to do ZS stuff).
However, when Davis or Schaeffer or James or Barnier or Crawford or Adams or whomever states "the negative density range for suchandsuch a process should be..." they all are using apparently different methods to determine/define the negative DR if you read and compare them closely, and this makes those statements irrelevant (and even detrimental) unless one adheres strictly to their method only and disregards anyone else's statements. There does not appear to be a consensus on how to determine DR  there does not appear to be a standard in that regard, at least in the popular literature I've read. Is there a formal standard definition of DR  say an ANSI or ISO standard perhaps?
OTOH, CI is straightforward as Kodak defines it. I know what it means, how it is derived, and, as a result, I know how to apply that number in my work.
Joe

Originally Posted by sanking
How did you make the determination that the Kodak numbers are higher for both DR and SBR? When I compared the calculations with real plots for the two methods the DR and SBR were identical. The only practical difference between the two is that the Ilford GBar methos suggests a slightly higher EFS than Kodak's CI. This might, depending on the paper curve of the process, result in some differnce in the printing characteristics of the negative. But for all practical purposes the difference between GBar and CI is irrevelant.
BTW, my reference is the 3rd edition of Beyond the Zone System and on page 29 Davis explains how Average Gradient, or GBar and CI (Contrast Index) are determined. It is same as one finds in Kodak and Ilford literature.
Sandy King
We are referring to the density points that are used to calculate the contrast. This has nothing to do with DR and SBR. They are the logE points used to determine the contrast.

Originally Posted by mikepry
That's it....kinda. With the BTZS you START with the paper. Don Miller has written about this in the past and like him, that is what attracted me to the whole process. All the other systems leave the paper more or less out of the whole process. With BTZS you make the exposure and then and only then you decide what printing process you will use. Once decided, you develop your neg for that exposure scale (of the paper) and voila. A negative tailored for your preffered paper for that particular image. I might suggest getting the BTZS Lite along with the BTZS book as it has a nice metering section in it that goes into the incident way of working. Good luck and if you have any questions there are a few of us here familiar with the system that will be more than happy to help out.
Thank You Mike, guess my statement was a bit oversimplified. You kind offer may be taken up...the main reason for the post was to pick the minds of those I thought could help...many of the good folks I expected to respond have and with good information, as always. Think I should at least pickup the book, already have the BTZS Lite, just not fallen into it at the level needed I think and will post additional questions as they come up. Have to admit, using the meters I have in incident mode in place of picking up a good spot meter is a pretty good motivation...would rather buy film/paper/metal than another meter...not that I have to, but have been using a 10 degree spot attachment which I think is probably OK, but not ideal.

I think this might fit under the densitometry for dummies category.
I use my densitometer very pragmatically  as a quick check on whether I am hitting my intended target. I record where I want a subject element to fall prior to exposure (zone 1 through 8) and then measure where it fell after development. This accomplishes several things  1. It constitutes a perpetial film test regimine under field conditions (I HATE film testing). 2. It tracks my skill as a photographic technician. 3. It gives me information on negative "a" that I can use to decide what, if any, changes to make in developing negative 'b'. 4. It increases my confidence in the technical side so I can concentrate on the artistic.
I don't know how to calulate CI nor the other items discussed above but I hit my intended densities with a high degree of consistency. That's all I am looking for.

Originally Posted by smieglitz
Hi Sandy,
Please take a look at the last graph I posted in this thread and let me know if I've made an error in the calculations. (The blue numbers and lines on that graph represent my interpretation of Davis method for approximating Kodak's CI. The red lines and figures give Ilford's Gbar according to Davis' method.) Am I missing something in my interpretation of Davis' comparison?
This is one of those perplexing things about the literature to me. I fully understand how to determine CI using Kodak's method and I use that method when doing my version of the zone system (on the rare occasion when I choose to do ZS stuff).
However, when Davis or Schaeffer or James or Barnier or Crawford or Adams or whomever states "the negative density range for suchandsuch a process should be..." they all are using apparently different methods to determine/define the negative DR if you read and compare them closely, and this makes those statements irrelevant (and even detrimental) unless one adheres strictly to their method only and disregards anyone else's statements. There does not appear to be a consensus on how to determine DR  there does not appear to be a standard in that regard, at least in the popular literature I've read. Is there a formal standard definition of DR  say an ANSI or ISO standard perhaps?
OTOH, CI is straightforward as Kodak defines it. I know what it means, how it is derived, and, as a result, I know how to apply that number in my work.
Joe
Joe,
It has been so long since I actually graphed a curve by hand I don't even remember how to do it. I just drop the numbers in my plotting program and get the data spoon fed.
However, what I have found is that you if you follow the method described by both Kodak and Ilford for plotting contrast, CI or GBar, you will get the same DR but the suggested SBR for the ES of your process will be slightly different.
There is of course no single DR value that corresponds to the ES of all processes so it could be defined and determined differently than the way it is derived from the CI or GBar plot. In fact, for anything other than silver gelatin printing it probably should be determined differently. For another process you would just need to determine how much compaction of the shadows and compression of the highlights is appropriate for the type of work you do and the process. Obviously the DR value that you come up with will be a lot different for a long toe and long shoulder palladium print that it would be for regular silver gelatin process, and different again for a process like carbon that has a perfectly straight curve. So in that sense there is no standard DR as the range of toe and shoulder possibilities with alternative processes is very wide.
Sandy

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Originally Posted by sanking
...However, what I have found is that you if you follow the method described by both Kodak and Ilford for plotting contrast, CI or GBar, you will get the same DR but the suggested SBR for the ES of your process will be slightly different...
Sandy
Makes sense. Since CI (or Gbar) as used by Davis equals the quotient of DR and SBR, if DR is held constant you would need to have different SBRs for variable CI. Two of the three would have to vary if the other was constant.
Thanks also for the comment about standard DRs and various processes.
Joe

Originally Posted by smieglitz
Since CI (or Gbar)
What of Gamma. I've a Rodinal data sheet which expresses
contrast by the term gamma. Many years ago that term and
contrast were familiar to me; CI, only a few years ago and
now Ilford's Gbar. Dan

Originally Posted by dancqu
What of Gamma. I've a Rodinal data sheet which expresses
contrast by the term gamma. Many years ago that term and
contrast were familiar to me; CI, only a few years ago and
now Ilford's Gbar. Dan
Gamma is the term that was widely used until in the past to describe the slope of a characteristic curve. However, it is calculated by directly dividing the DR of a negative by the SBR, which only works if you have a straight line. This is rarely, if ever the case, with modern films, so CI and GBar, which includes some of the toe and shoulder in the calculation, have almost universally replaced Gamma. The difference between Gamma and CI/GBar depends a lot on the curve itself but can be quite great in practice.
Sandy King

I've never used gamma as a measure but as I understand it, the problem with that measurement (and why it has largely been abandoned) has to do with where you measure the slope of the characteristic curve. As Sandy has indicated, gamma works fine where the slope is a straight line, but if you take the typical film curve, the slope is constantly changing. IIRC gamma is equivalent to the slope representing the tangent of the curve at a given point. Since the tangent changes depending on where on the curve you measure, you can get different values for gamma from the same curve.
Gbar and CI OTOH nail down the contrast measurement in the shadow region and restrict it to a certain range. With Ilford's Gbar, the reference in the shadows is the point on the curve 0.10 above fbf and for the highlights, an intersection point on the curve at a radius of 1.5 density units from that shadow point. The Gbar or average gradient then becomes the slope of the line between these two specific reference points. Kodak's CI measurement actually takes the fbf value as a floating anchor point which helps restrict the measurement to two specific points at radii of 0.20 and 2.20 from the fbf anchor on the characteristic curve.
Joe

Originally Posted by photomc
As the new owner of a densitometer, attempting to get a better understanding of how it is used to determine the contrast index (CI) of a negative. Can someone point me to a resource that might help explain how the CI is determined or perhaps offer a little Densitometery for Dummies...
Thanks,
Let me try to untangle this for you.
CI or Gbar or gamma are all measures of negative contrast using certain reference points of exposure, not density. You measure the density at (exposure) point A and at (exposure) point B (the distance betweent the points differentiates the various systems) and derive a ratio. This can be done only with a calibrated test strip. Once you have measured the developed strip, you can then determine the CI or Gbar or gamma. All film of the same type developed in the same way as the test strip will have the same CI or Gbar or gamma. A blank film or a fogged film, it does not matter, because the CI or Gbar or gamma expresses a degree of development and nothing else. If you develop the TMY test strip for, say, 8 minutes in DK50 1:1, and you measure the CI as 0.56, then all TMY developed for 8 minutes in DK50 1:1 will be developed for a CI of 0.56 (aside from emulsion changes).
What you need to do first is to determine what CI is recommended for your process, and then get some test strips to process. You will need the densitometer to measure these. Aside from this, there is no use for the densitometer.

