CI of Blank Film

[Claire Senft]

Useful?

has this been a useful discussion?

 

[Kirk Keyes]

has this been a useful discussion?

Did you learn something? If so, then yes.

 

[smieglitz]

has this been a useful discussion?

Well, I've certainly learned alot, but not about photography.

I sense you may be wondering whether the concept of CI is useful in a practical sense. The answer to that question is yes. While this "debate" has been largely about irrelevant theoretical aspects, the concept of CI has practical application.

As others have mentioned, CI and commercial control strips are used to monitor and calibrate processing in large lab situations and with automated processing.

I find the concept useful in home darkroom use also since it lets me determine the "degree of development" I've given test rolls (my own personal control strips as Jorge has mentioned). Once I have three tests (two of which are extreme over- and undrevelopments from the third "normal" or recommended) measured for CI, I am able to constuct a time vs. temperature development curve in order to gauge the proper development condition for a given desired result. (Actually, I vary developer dilution vs time in my personal work, but most others change the time so I use that example.) Once the curve is worked out, the time for any CI will be apparent from the curve. If I can match the CI to a film/paper combination and desired contrast range through experience or other tests, I can predict which time will give me, for example, N+2 for that film. Essentially, I can work out a personal time (or dilution) vs contrast chart for my own method and don't have to rely on the manufacturer's suggestions (although they may be useful starting points for me).

Kodak and Ilford publish these Time vs CI (or G-Bar) curves for various developers and their film line. As a result, users can chose certain developers to produce a given development in a certain time period or can deduce developer activities from the CI vs Time charts.

Nowhere in that practical use of CI information does the consideration as to whether a blank film has a certain CI enter into the equation.

Joe

 

[Ornello]

Make sure to measure the developer formula, dilution and temperature before you do.

You already have the agitation technique down.

Yup. Added the boric acid too, to keep the time long enough....

 

[Ornello]

Ornello tiwce you have posted a copy of a Kodak document. I thank you for you help. My monitor squeezes the document down somewhat and it becomes somewhat difficult to read. The document is about characteristic curves. It makes several mentions of gamma. No where in this document can I find CI mentioned.

Questions:
Is CI mentioned in the document as posted?

If not, how does it help the discussion?

CI and gamma serve exactly the same purpose. CI is a modification of gamma. Gamma used only the straight-line portion of the curve (not the toe or shoulder), and this resulted in somewhat skewed data, as good negatives do use some of the toe. Because of the variations in toe length among various films, films developed to the same gamma did not always print equally well on the same grade of paper. Since CI measures part of the toe, films developed to the same CI print equally well on the same grade of paper. In all other respects, CI and gamma are the same sort of thing: a measure of contrast. What Kodak says about gamma 'It is a measure of the degree of development" is therefore true of CI.

 

[Donald Miller]

. In all other respects, CI and gamma are the same sort of thing: a measure of contrast. What Kodak says about gamma 'It is a measure of the degree of development" is therefore true of CI.

This sounds like a subjective interpertation. Where in Kodak literature does it specifically link the two?

 

[Ornello]

This sounds like a subjective interpertation. Where in Kodak literature does it specifically link the two?

Link what? CI and gamma? Kodak discusses the limitations of gamma in their discussion of CI back in the 60's or 70's, when they introduced the new measure. The function of the two is identical. CI is a modification of gamma. The modification is to the region measured, that's all. In all other respects, they are the same. The values for CI tend to be a little lower because the toe tends to have a lower slope.

 

[dancqu]

The film doesn't "have" a CI, it's been developed to a CI.

I don't think that last can be proved. Dan

 

[Ornello]

I don't think that last can be proved. Dan

Sure it can. Just develop a process control strip together with it.

 

[NER]

What nonsense. As others have correctly stated, a negative of uniform value, exposed or not, has no contrast by itself, and hence no CI. It has only a particular measurable density. CI, on the other hand is "the slope of a straight line joining TWO points on the characteristic curve that represent the approximate minimum and maximum densities used in practice." This unequivocal definition along with very clear and simple instructions for determining CI are given by C.I. Jacobson, and R.E. Jacobson on page 40 of their book titled "Developing" (18th revised edition, Focal Press, March 1972). CI does, of course, have practical use in relating the contrast of a negative to the scale of the paper upon which it may be printed, and this is perhaps most readily apparent from a study quadrant diagrams. I apologize to those who already understand these fundamental concepts, are not confused by the differences between "density" and "contrast," and who are as annoyed as I am by the enormous amount of energy wasted on this topic in the preceding 24 hours, to say nothing of the impolite tone of some of the responses we have been subjected to.

 

[Claire Senft]

Actually I do understand.

I do understand and value the concept of contrast index. I thank you for your interest. In actual practice I find average gradient easy to measure while with contrast index, at best, I can only closely approximate it.

A though for Ornello.

Since gamma and contrast index are the sme sort of thing, would you give me a Deardorff brochure in my quest to learn more about Leica?

 

[Ornello]

I do understand and value the concept of contrast index. I thank you for your interest. In actual practice I find average gradient easy to measure while with contrast index, at best, I can only closely approximate it.

A though for Ornello.

Since gamma and contrast index are the sme sort of thing, would you give me a Deardorff brochure in my quest to learn more about Leica?

I don't follow you. CI is a refinement of gamma, using slightly different measurement points. In all other respects, the concepts are equivalent.

It is also used as shorthand to represent 'degree of development' (in so many words from Kodak) simply because there is no shorthand term for 'degree of development' that is mathematical in nature.

If I am using DK-50 to process my film, and I develop it for 8 minutes, I can look on the CI charts that Kodak supplies to see what what CI that corresponds to (suppose that it's 0.56). I can then go to another developer and look for the same CI value. I can do all of this without even exposing a roll of film. That's the beauty of a CI (or gamma or G-bar) measurement. I can get very similar contrast for my film with the different developer.

See:
CI curves

Now note, just because I'm developing for a given time in a given developer does not mean that I am getting exactly the CI that Kodak shows on the curves. The point is, that any individual variation in my technique (agitation pattern, etc.) will carry over to the other developer as well.

 

[Claire Senft]

thank you Ornello

I already inderstand the CI concepts. It is meant to address the shortcomings and to be an improvement over gamma as a measurement of contrast. It is very difficult for the layman to measure because it requires a special protractor. If one has, as do I, a transmission densitometer, one can measure average gradient. If one increases the base fron 1.5 units to 2.0 units, that was used in the average gradient measurement, one gets a close approximation of contrast index.

I appreciate the spirit in which you posted the document about gamma but I found it curious that it was being offered and never even mentioned contrast index.

So how did your film turn out?

 

[Ornello]

I already inderstand the CI concepts. It is meant to address the shortcomings and to be an improvement over gamma as a measurement of contrast. It is very difficult for the layman to measure because it requires a special protractor. If one has, as do I, a transmission densitometer, one can measure average gradient. If one increases the base fron 1.5 units to 2.0 units, that was used in the average gradient measurement, one gets a close approximation of contrast index.

I appreciate the spirit in which you posted the document about gamma but I found it curious that it was being offered and never even mentioned contrast index.

So how did your film turn out?

The document pre-dates the change from gamma to CI, which occurred in the late 1960's, I believe.

Kodak still uses the term 'gamma' in certain applications:

http://www.kodak.com/global/en/professional/support/techPubs/p255/p255.jhtml

My film turned out exactly the way I wanted it to.

 

[gainer]

I already inderstand the CI concepts. It is meant to address the shortcomings and to be an improvement over gamma as a measurement of contrast. It is very difficult for the layman to measure because it requires a special protractor. If one has, as do I, a transmission densitometer, one can measure average gradient. If one increases the base fron 1.5 units to 2.0 units, that was used in the average gradient measurement, one gets a close approximation of contrast index.

As you have the tools to get the H&D curve, you do not need a special protractor. You need a straight edge with 3 marks on it. Call one 0, the next is at 0.2 log exposure units, and the third is at 2.0 log exposure units. You move the straight edge along the graph, keeping the 0 mark on the B+F level until you find the place where the other two marks fall on the curve. Draw the line and calculate its slope from rise over run. That is the contrast index.

You can fold a piece of the same log-log graph paper you use for the plot and simply make the marks on the folded edge.

If anyone wants semi-log or full log graph paper, send me a blank CD and I will send you the setup files for a program I wrote for Windows that you can use to print your own.

Snail mail to:
Patrick A. Gainer
HC 77 Box 86
Glenville WV 26351.

 

[Ornello]

As you have the tools to get the H&D curve, you do not need a special protractor. You need a straight edge with 3 marks on it. Call one 0, the next is at 0.2 log exposure units, and the third is at 2.0 log exposure units. You move the straight edge along the graph, keeping the 0 mark on the B+F level until you find the place where the other two marks fall on the curve. Draw the line and calculate its slope from rise over run. That is the contrast index.

You can fold a piece of the same log-log graph paper you use for the plot and simply make the marks on the folded edge.

If anyone wants semi-log or full log graph paper, send me a blank CD and I will send you the setup files for a program I wrote for Windows that you can use to print your own.

Snail mail to:
Patrick A. Gainer
HC 77 Box 86
Glenville WV 26351.

Yes, but...how does one expose the film so precisely? Using a camera won't work, because of flare, etc. You have to use process control strips....

 

[Jorge]

Yes, but...how does one expose the film so precisely? Using a camera won't work, because of flare, etc. You have to use process control strips....

No you dont, you can contact print a step wedge to a known light source. As long as this is consistent it is all that matters. How do you think Kodak does it?

 

[Ornello]

No you dont, you can contact print a step wedge to a known light source. As long as this is consistent it is all that matters. How do you think Kodak does it?

With highly accurate, calibrated machines of some kind...

 

[Claire Senft]

Thank you Patrick

The only reason I believed that a special protracter was necessary was due to Phil Davis's statements in the fourth edition of the BTZS book. It really doesn't matter to me since I find average gradient completely suitable for my needs but much appreciated was the helpful advice that you gave me and the friendly spirit in which it was given.

How goes the oboe playing?

 

[dancqu]

The document pre-dates the change from gamma to CI,
which occurred in the late 1960's, I believe. Kodak still
uses the term 'gamma' in certain applications:

And Agfa uses the term 'gamma' for all applications? I've
Rodinal, purchased four years ago, and the data sheet
uses that term.

BTW, I gave a quick look at two Phototechnique articles;
one each, Phil Davis and a Howard Bond. Both use
Average Gradient. Bond goes into detail. Dan

 

[Kirk Keyes]

Yes, but...how does one expose the film so precisely? Using a camera won't work, because of flare, etc. You have to use process control strips....

Or you can make your own - use a step wedge that has been calibrated and you enlarger. Correct the enlarger to daylight, and then you can measure the exposure with your incident light meter with a flat diffuser (spot could be used, but incident is more direct). Use the look-up table that came with your incident meter to convert incident readings to lux, multiply by exposure time (shoot for 0.4 to 1.0 seconds) and then figure out the exposure of each step on the test neg from the known densities of the step wedge. This will give exposure values in lux-seconds, which I see Kodak uses in their charts.

Davis covers much of this in the BTZS book (although slightly differently).

Or, pick up an EG&G sensitometer. I have a friend with one. They show up on eBay occasionally.

Kirk - www.keyesphoto.com

 

[gainer]

The only reason I believed that a special protracter was necessary was due to Phil Davis's statements in the fourth edition of the BTZS book. It really doesn't matter to me since I find average gradient completely suitable for my needs but much appreciated was the helpful advice that you gave me and the friendly spirit in which it was given.

How goes the oboe playing?

My first born son is getting a degree in music from Virginia Wesleyan in May and I am going to join him in his senior piano recital. We will do a movement of the Poulenc oboe sonata. I am furiously making reeds that will allow me to recapture what I once had. It's like trying to get another picture like the one you thought was your masterpiece. Magic reeds don't do it any better than magic paper or film.

I must have stayed up too late last night. You don't use log-log paper to plot the H&D curve, as you are plotting density against log exposure when you use either the step number or the actual density of a step density wedge.

As to flare, there are various opinions. You can get a flare-free curve and make some kind of a theoretical correction when you go to use the data in practice, or you can photograph a reflection wedge with the camera you plan to use and find out how flare affects your CI. A lot depends on whether you are doing it for your own use or are planning to pass the data on to others.

 

[Claire Senft]

Good For You Patrck

The session with you son sounds as if it will be a very nice event for both of you. Knock their socks off Pat! Geez from reed making to rocket science, you have had some experiences.

I have the graph paper that I need. I am more than satified in using average gradient. I was interested in finding out if those that were so certain that blank film had a CI thought that a developed film with a single density on it was any use from a practical standpoint. If you read thru the thread from the beginning, as you may alredy have done, you will see that it was not a pretty conversation with a hostility being involved.

 

[sanking]

As to flare, there are various opinions. You can get a flare-free curve and make some kind of a theoretical correction when you go to use the data in practice, or you can photograph a reflection wedge with the camera you plan to use and find out how flare affects your CI. A lot depends on whether you are doing it for your own use or are planning to pass the data on to others.

Davis in Beyond the Zone System describes a method for testing for flare and applying the data in field use in diverse SBR scenes. And in WinPlotter there is a data entry that allows one to compensate the curve for the value of flare indicated.

Assuming that you use a very efficient lens hood, a compendium bellows for example, the amount of flare is fairly low with modern lenses in low and medium contrast scenes, say SBR 5, 6 and 7, but can be quite high with SBRs of 9 and higher.

Sandy

 

[Kirk Keyes]

You guys probably aren't interested in hearing this, but Stephen Benskin has done some programming that does some really nice tone reproduction graphs - here's a sample of a 2 quadrant graph that he posted on photo.net last year.

http://www.photo.net/bboard/big-image?bboard_upload_id=17658784

He can plug in differing levels of flare, as well as different H&D curves for the film. When a 4 quatrand curve is made, you can see how the exposure at the lens is modified as it is recorded on the film, is then transferred to the paper, and then that is compared to the original scene. I find it really interesting.