Originally Posted by smieglitz
I made an error of assumption. Whether or not it has any practical importance I don’t know at this time.
The issue is primarily this. Way back at the beginning of this thread I mentioned that I had not done any graphing by hand for years. I use the graphing plotter program for that. And the version that I use offers two methods for calculating slope, approximate CI and 0.1 over B+F. I assumed, having read Davis’s explanation of the two methods, that the 0.1 over B+F method was based on the Ilford B-Bar method. So very early on in this thread when you did a hand graph and found that “The calculations do appear to yield similar numbers for G-bar and CI,” I decided to test this myself using the plotting program with the data from a number of existing film tests to compare results with the approximate CI and 0.1 over B+F methods. What I found was what I reported, the calculations from plots using the two different methods gave equal values for DR and SBR, but the 0.1 over B+F method gave a slightly higher EFS.
What I wrote is not gibberish, as Stephen so impolitely put it (which rather surprised me since he made a point in his first message to tell us that he was polite), but solid data that resulted from two different methods of determining slope. Yes, I appear to have made a mistake in assuming that the 0.1 over B+F method was based on Ilford’s method for determining average gradient, but the data nevertheless showed a slightly higher EFS for this method, which shows me that different premises for determing average gradient can result in different values for EFS. And if anyone is interested I will be happy to show all the details of my film testing and the curves that resulted from the two methods of determining slope.
In looking at one of Stephen’s messages he appears to have criticized me both for finding some difference in the two methods, “suggesting a higher EFS approaches gibberish. These are methods to determine contrast – not speed, ” as well as for concluding that for all practical purposes there is no distinction between the two methods, “as for Sandy’s conclusion that there is no distinction between the two methods…well, that has just been shown to be wrong.”
And, just for the record, Davis’ plotter does provide a method for establishing a personal speed point which is for all practical purposes the same as EFS, and this is one of the data sets one gets from plotting the curves.
But more to the point. I am a practicing photographer much more interested in systems that allow me to expose and develop negatives with some degree of precision than in discussions of the theoretical aspects of sensitometry. And that is where the BTZS system comes in because frankly no one has provided a better system than Davis for exposing and developing with precision than BTZS. And it is also very clearly presented, a quality I admire greatly in writers.
Originally Posted by Jorge
You're confusing a meaurement unit with what is measured. CI is a unit of measure. It measures contrast.
A given negative may have no (measurable) contrast at all, but it may have been processed for a particular CI, and in that extended sense it 'has' a CI. If I process a roll of film for a given CI, but one of the negatives happens to be blank, it is no different from any other negative on that roll so far as degree of development is concerned, and CI is used as a way of measuring the degree of development. It's kind of like a reference point in geology. One refers to Pennsylvanian rocks that are not in Pennsylvania, because they were laid down at the same time.
In other words, CI is used, as a practical matter, as a kind of shorthand expression, to represent a degree of development.
No, you are the one confused and I do not plan to continue fieeding your idiotic trolls....
No respect, no respect at all....
Originally Posted by Jorge
I'm going to have to look into the intention of Davis' use of the term, but I am not optimistic. I think you might just be too deeply immersed into BTZS to be able to judge it objectively. Still, I'll be open minded, after all, we are talking about the difficult and confusing subject of photometry. BTW, the Focal Encyclopdia of Photography partially defines Luminance as "the property of a source or surface that most closely correlates with the subjective percept of brightness, which, due to adaptation and contrast effects of the visual system, cannot be used as a reliable measure."
Originally Posted by Jorge
The person who initiated the tread said, "Can someone point me to a resource that might help explain how the CI is determined or perhaps offer a little Densitometery for Dummies..." So far, I have offered a number of reading suggestions including the original paper on the subject written by the people who invented Contrast Index. I've explained how flare should factor in. I've given the poster an equation to determine CI for different LSLRs, and I showed him graphically how it is supposed to work in relation to SLSR, the negative, and the print. I've corrected a long misused method of G-bar (there is no arc). I've given information on the origins and intentions for the different method parameters. In addition, I had a graph comparing CI, G-bar, and the Zone System approach. No offense, but I think I've written more to respond to the original poster than anyone else so far. Jorge, I think you might have been skimming over my posts.
If I misrepresented what you said, I apologize. I got "slope so it can't be developed to a given CI" stuck in my mind and went with it. As for finding a higher EFS when using a method of contrast determination...well that's not possible and has no relevance to contrast determination (as long as there is enough exposure to measure). They are two different methods. One method determines contrast and another speed. Perhaps there was confusion in the way it was written and the way I read it. Terming what you said was gibberish wasn't polite, I just got pissed off by your dismissive attitude from an earlier post. Maybe we should both practice more patience and understanding?
As for the CI / blank film discussion, it's over as far as I'm concerned. What was meant to be a little fun thought got blown way out of control. If it's OK with you, let's make a fresh start of things - mutual respect, civility and all.
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Back to CI:
Originally Posted by Stephen Benskin
Kodak clearly uses the term to refer to degree of development:
It is much like saying that you develop your film to "N+1" or N-2", only with mathematical precision, as "N" can be quite different for different workers.
With CI, you're measuring the slope of the line connecting two defined points (strict definition) but it is also used to mean what affects that slope: the degree of development.
Kodak gives Contrast Index curves for various developers on its materials.
Maybe Kodak is 'idiotic' for doing so?
Thanks to all who have added comments..most have been very usefull.
If anyone thinks I have ignored there reply's please forgive me, it was not intentional. Since some posters are at a different stage in their understanding and have attained a level of enlightenment beyond what my interest are, decided to filter this thread and read what was relevant to my level of understanding - My Thanks to you.
There are those that can teach, and those with a great deal of knowledge, but have a problem presenting that knowledge to others. This thread is a good example of that IMO.
At this time I am asking Sean to lock the thread before this becomes another catalyst for a flame war.
BTW, the Focal Encyclopdia of Photography partially defines Luminance as "the property of a source or surface that most closely correlates with the subjective percept of brightness,
Exactly, luminance refers to the preceived brightness of the subject, Illuminance refers to the light falling on the object. This is clearly explained in Davis' book and the basis of what he calles subject brightnes range.
While you have given reading suggestions, they have been done in passing and were not specifically geared to explain in a simple way what is required for a beguinner. You have written all of this in an effort to defend an untenable position that blank film has a CI. All of this you have done it in such a complicated and convoluted manner that I would not be surprised anybody wanting to start learning sensitometry to acheive a better control would be put off and decided it was too much hassle. To what purpose does it serve to post graphics and curves to someone who is just strating and has no idea what they mean? Face it, you posted all this to show how much you knew, not in a sincere effort to help the original author of this thread.
Originally Posted by Stephen Benskin
OK, sounds like an excellent idea. The present path clearly has no productive end. However, there are a couple of issues I would like to pursue with you in private communications and if you are willing I would be appreciative if you would send your email address to me at email@example.com.
thread owner has requested this thread to be closed.