Some Data Questions

[yeknom02]

I had two questions arise when looking for some new film to try...

The first seems pretty simple. I was looking at the data sheet for Fomapan 100 and I saw the curves for Fomadon Excel/Kodak Xtol—see p. 3. (This is great since Xtol is my developer of choice for most situations.) The curve has Dmin, S and G as a function of minutes of development time. Dmin seems pretty simple, minimum density (though I'm not entirely sure how useful a statistic it is.) I'm wondering what S and G are—it seems that S might be film speed? Anyway, I was hoping if anyone had some insight as to how I could use this data, and whether there are similar curves out there for other films.

The second question is shorter, but probably much more difficult to explain to me: What is the contrast index? The only thing I've been able to find is that you want a certain value for a condenser enlarger, and a certain other value for a diffusion enlarger. What I'm interested in is whether it's actually a measure of image contrast and whether it's controllable with choice of developer and things like agitation strategy, temperature, and time.

(For example, I like the look of HP5+ pushed to 1600 because it gives me more contrast. I'd also be interested if I can replicate that level of contrast with a different film and/or developer, maybe to get a finer grain. So maybe looking at the CI is a way to do it?)

 

[Stephen Benskin]

Attached is THE paper on Contrast Index.

 

[Mike Wilde]

'Way Beyond Monochome' has a description similar to the 1966 paper, with templates to build your own measuring devices.

I think there is an excel spreadsheet on Ralph Lambrecht's web site as well that aids in understanding gamma and CI.

 

[yeknom02]

Mike,
I have WBM II, though I just moved so it's still in one of my book boxes. I have the spreadsheet that I think you're referring to. No densitometer, though. I didn't know CI was related to the film curve.

I'll have to pull it out of storage to look over it, as well as read that article from Photo Sci Eng. I hope that CI isn't something I'm going to have to test for weeks to find out.

 

[dpgoldenberg]

Here is a short answer to the original question:

The contrast index (CI) is, as the name suggests, as measure of negative contrast. It is the average slope of the characteristic curve measured over two specifically defined points, one in the toe region of the curve and the other in the (more or less) linear region. The technical details have to do with the way in which the two points are defined, and they are explained in the paper Stephen posted, along with the rationale for defining the contrast index in this particular way.

The usefulness of this (or other measures of contrast) is that if you know the CI for one film-developer combination that you like, and then can generate the same CI for another film-developer combination, you will probably like that, too.

Kodak, who invented and promoted the CI, provide values for their products. If you do your own film test, you can use the characteristic curve to determine the CI, using methods others have referred to. Another good reference is the Kodak Basic Sensitometry Workbook:
http://motion.kodak.com/motion/uplo...en_motion_education_sensitometry_workbook.pdf

I hope this helps.

David

 

[Mike Wilde]

While not as precise, the human eye is very good at matching similar shades of grey. Not so great at dense blacks though.

no denistometer work around: Spot metering with your camera focused on a white sheet of paper on the baseboard of tha projected neg, set to read a spot of interest. Then slipping a step wedge in the neg carrier at same carrier height, aperture etc. and slid the step wedge unti the meter reding of the area of interst in the neg is matched. You may need to mask off the open are of the neg carrier if you are using and undersized step wedge.

I started out this way with a 1/2"x5" step wedge before an old denistometer was gifted to me.
I guess the same technique could even be extened to scanning the neg, and scan the step wedge with the exposure locked to the first scan value.