Is there an on-line explanation of how CI, SBR and DR relate?

[John D Gerndt]

I am having a hard time laying hands on a book recommended to me to explain the above terms. Is there an on-line article that explains these terms?

Many thanks,

[Bill_1856]

I've been doing serious darkroom work for over 50 years, and I haven't the faintest idea of what CI, SBR, or DR mean.

[Francis Abad]

The book to purchase is Beyond the Zone System by Phil Davis (4th ed.). It will go through the whole process of the meaning and uses of CI (contrast index), SBR (subject brightness range) and DR (density range).

SBR is the range of the low and high exposure values measured in a scene plus 5.

DR relates to the density range of the paper, expressed in log units (how black does it get to and how white does it become, something like that in layman's terms).

CI relates to negative contrast as matched to certain types of printing process/paper. For example, to get a proper negative to sufficiently make use of the tonal scale of AZO contact printing paper one needs to develop the negative to achieve a CI of 0.71 or thereabouts.

These are all just terms to identify the characteristics of the materials we use in order that we can forget about fumbling in the darkroom - i.e. the ability to predictably succeed in making negatives that are easy to print in the paper or process we decide to use.

[Jay DeFehr]

Am I correct in understanding that all of the terms describe the range in brightness or density? SBR for the subject, CI for negatives, and DR for paper?

[sanking]

Not exactly.

SBR (subject brighness range) is for the subject, CI (Contrast Index = modified form of gamma) applies to the negative, as does DR (density range). The corresponding value for papers is ES (exposure scale). As Francis mentions, a good source for reading about this is Davis' Beyond the Zone System.

Sandy King

[D. Poinsett]

Here’s the condensed version.

Measure SBR with a spotmeter or deduce from readings. Fit the SBR to the negative DR via CI. Control CI by development time. The negative DR should match the desired paper exposure scale (ES) in order to represent the SBR in the full tonal range of the paper. The ES will depend primarily on paper grade.

Here’s the less condensed version.

The subject brightness range (SBR) is the difference between the lightest and darkest area of the subject. Choosing this range depends on aesthetic choice but basically boils down to choosing which parts of the subject you want white in the print and which parts you want black. SBR is usually expressed in stops and measured with a spot meter. (It can be done with an incident meter too but that’s another conversation. Also sometimes you cannot measure SBR directly, ie, snow, black cat, etc. That too is another conversation.) Subjects in bright sunlight have a high SBR, 10 stops or more sometimes. Subjects in light from an overcast sky or low diffuse light have a low SBR, 4 stops or less sometimes. An SBR of 7 stops is typical.

The contrast index (CI) of the negative relates film exposure to negative density. There are other measures of negative contrast too (G, and avg G for example). High CI makes means small change in film exposure makes a big change in negative density. Low CI means large change in film exposure makes a small change in negative density. CI is usually controlled with development time.

The difference in density between the areas on the negative that represent the darkest and lightest values in the SBR is called the negative density range. For any given film, the density range is a function of contrast index and SBR. Negative density can be measured with a transmission densitometer (light is transmitted through the negative).

Printing paper also has a density range. It is the difference between white and 90% of max black. Paper contrast is commonly indicated by Grade and/or ISO numbers and relates the print exposure needed to produce the density range of the paper. High grade numbers have high contrast: small change in print exposure makes a big change in print density. Low grade numbers have low contrast: big change in print exposure makes a small change in print density. Print density is generally measured with a reflection densitometer. The print exposure scale can be measured with various light meters but is most easily checked with a transmission step wedge (a negative with controlled density steps).

When the density range of the negative matches the exposure scale of the paper, the SBR of the original subject will represented by the full tonal range of the paper in the print. If the negative density range and paper exposure scale are not matched, the SBR of the original subject will either exceed the tonal range of the paper or fall short. This usually results in prints with one or more of the following: blown highlights, lack of shadow detail, or a lack of tonal separation (muddy). While these results can be chosen for aesthetic effect, they are usually the result of unintended mismatch of negative DR and paper ES.

Make the match by working backward from the print and forward from the subject. Your favorite paper has some grade that requires a exposure range to achieve the full tonal (density) range. This determines the print exposure scale and negative density range you need. Measure the SBR, expose the film, and adjust development time to achieve a CI that will produce the desired negative DR. Voila.

I did not address overall film but it obviously plays a roll in the process. Just make sure there is enough exposure to record the desired detail in the shadows. A little over-exposure is not a problem. Under-exposure is.

Note that burning and dodging a print locally alters the effective density range of the negative. Heroic dodging and burning can save nearly unsalvageable negatives but it is not much fun. Variable contrast paper can also restore the density range and exposure scale match if it’s not too far off.

As others have mentioned, see Davis’s book for more comprehensive treatment.

[John D Gerndt]

I offer many thanks to all who gave freely of their information/knowledge. The Phil Davis book is the book I am having a hard time laying hands on. There do seem to be two copies in Michigan libraries but I cannot get to them. I am not in a position to buy from the amazing “A” source.

It seems too that I would need a densitometer to calculate CI and DR and that the condensed version of an ES is the single number known as a paper grade.

I do have a spot meter left over from my student days (along with those unpaid student loan balances) so I can establish a SBR and I have a home made (from Kodak ND filters) step wedge.

So I can eyeball the DR resulting from a given development, maybe even confirm my guess with the spot meter and I can try to match the CI (of unknown value) with the ES (unknown) of some grade of paper. The bugger is to know what time and temperature development (DR being the only measurement I can get a SWAG for) goes with what SBR? IS this whole endeavor hopeless without a densitometer?

I have multiple requests in for the Phil Davis book and you can be sure I will read and study it, but even $200 is hard to come by in my world, a calibrated densitometer seems out of the question. I spend all my disposable income (and some that is not quite disposable) on film, paper and chemistry. Am I spinning my wheels?

My prints are mostly good but I want to get better. This forum is my best resource. I throw myself on its mercy.

Humbly,

[Paul Giblin]

Go to your local library and ask them to borrow the book from another library via interlibrary loan. They will be happy to do it.

-Paul

[Michael Mutmansky]

John,

A decent densitometer doesn't need to be too expensive.

Send me a private email, and I may be able to help you out in this respect.

---Michael

[Francis Abad]

John, if you need starting times for SBRs 6 to 13, with Efke PL100 and Classic 200, developed in Pyrocat HD, using BTZS type tubes, and destined for AZO contact printing, I would be more than happy to list them for you via email. You can play with these times and see what suits you using trial and error. Pyrocat is very forgiving and in no time you would have your own development times matched to your own particular variables.