Originally Posted bysmieglitz

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.

Sandy