Average Gradiant to Contrast Index Conversion?

[Dan Rainer]

I'm currently experimenting a lot with sensitometry. I'm enjoying the process, but I'm finding the variation in the methodology to be really annoying. For example, I really like @Stephen Benskin's practical flare model in his paper "What is Normal?" and I also like the visualization of data with the BTZS Plotter software. However, the practical flare model uses Contrast Index and Plotter uses Average Gradient.

I'm assuming there's no simple conversion formula for the two variables?

 

[Bill Burk]

No way to convert them. Kodak picks two specific points for Contrast Index. Any other curve fitting won’t agree.

It’s easy to grasp that Kodak chose to measure the part of the negative that you are probably going to use.

 

[Stephen Benskin]

Bill is right, but in most cases the results will be similar. At least in more agreement than comparing either method with gamma. The main difference is how much of the curve is included in the evaluation. I like Contrast Index because the arc tends to include the part of the curve that will be used for a given Luminance range. It tends to represent reality better. In most cases the difference isn't enough to sweat over.

I'd be more concerned over BTZS. I never got into the weeds with BTZS. I don't like systems in general and I knew Phil Davis understood sensitometry; however, I've been reviewing the details (without owning the software) and have been concerned about what I've read so far. It could just be an initial reaction to the concepts and my dislike of the Davis' gimmicks and his predilection for making up acronyms. So far, not a fan of how he calculates flare or how it's presented, or the Fractional Density, or how his basic development models do not include flare. I'm just beginning to look at how Zone Spread is supposed to work.

 

[spookyphoto]

I can't believe I found posts on this subject. I feel like I am in a time warp I'm a retired photo-analyst who used to design film analysis systems and programs for the govt. and the micrographics industry some 40 years ago. Worked with $ 250K sensitometers and auto scanning densitometers. I wrote software for sensitometric analysis of every kind of film and paper on the planet. Ran many processing and developer studies. I wrote a subroutines to digitally calculate C.I., gamma, fractional gradient point, vertex ratio, Delta X, response at .1 above B+F etc. If I can be of any help let me know.

Below - $250K MTL Star sensitometer. When Kodak wanted to calibrate all their sensitometers they used to call us. Same with the Air Force and other agencies