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# Thread: Densitometery and Contrast Index (CI)

1. Originally Posted by photomc
As the new owner of a densitometer, attempting to get a better understanding of how it is used to determine the contrast index (CI) of a negative. Can someone point me to a resource that might help explain how the CI is determined or perhaps offer a little Densitometery for Dummies...

Thanks,

Get ye some standard graph paper and scale the vertical axis for density in 0.10 intervals. For the horizontal axis have a spacing interval of one stop (zone) equal to 0.30 density units. From the same graph paper make a ruler scaled identically as the vertical axis. Make an index mark at the edge of origin of the scale and mark it "A". Go to the 0.20 value and make another mark labelled "B" at the edge. At a value of 2.20 make a mark "C". (I'm relating this from memory but the last value is 2.20 IIRC. Check this in the Kodak or Ilford literature or perhaps someone else could confirm?)

Once you plot your characteristic curve of film test densities vs. exposure, determine the level of fbf and draw a horizontal line through the (0.00 net density) FBF value parallel to the horizontal axis. Take the CI index ruler you've made and position point A on that horizontal line. Slide the ruler along that horizontal line pivoting the ruler until points B & C both lay on the characteristic curve you've drawn. When you have A on the horizontal line and both B&C on the curve at the same time, the slope between points B&C will equal the Contrast Index. Make a mark on the curve where B&C intersect it and measure the rise over run to get the slope or CI. The rise is the vertical distance (difference in density) from point B to C and the run is the horizontal distance between the two points.

As an example, assume B plots on the curve at .30 gross density (at Zone II 1/6 exposure) and C plots at 1.40 gross density (at Zone VII 1/2), and that fbf is 0.15 density. The rise is then C-B or 1.40 - 0.30 = 1.10 density units. The curve will be such that the horizontal distance between B&C is about 1.15 density units (remember the horizontal scale is based on the 0.30 unit interval on the vertical axis). The Contrast Index for this curve would therefore be 1.10/1.15 = .97 approximately.

If instead the numbers were B = 0.25 density (at Zone I 5/6) and C = 1.05 (at Zone VIII) with fbf = 0.15 this curve would have a CI of (0.1.05-0.25)/6 1/6 exposure zones = (0.1.05-0.25)/1.85 = .43 as shown in the example picture which will hopefully post below.

It's pretty easy to actually draw this on graph paper and determine CI. It's probably much harder to explain or read.

Joe

2. Originally Posted by Jorge
Hate to sound like a broken record but the best way
to figure this out is to read Phil Davis' book "BTZS".
I would not recommend Davis' book. Sensitometry is,
as has been mentioned in other posts this thread, likely
incomprehensibly presented. I've not read the book but have
studied a few of his articles. I've eight volumes of D-Max,
a quarterly put out by The View Camera Store.

I've a densitometer and off and on do speed tests, and
zone density measurements which can be read and/or
plotted. It's an eight hour a day five days a week
job for some. Dan

3. Uh...let me see if I understand this, on one hand you say:

I would not recommend Davis' book

And then you turn around and say:

I've not read the book but have
studied a few of his articles

I really dont mind when people say: "I have read the book and found it useless" but really, saying you have read a few articles and thus do not recommend the book is incredible!

4. That is not ALL that I said. I also mentioned others this
thread who have issued guarded warnings and I think them
credible; I've read some of his work. Those quoted
portions of my post are all "on one hand". Dan

5. Originally Posted by dancqu
That is not ALL that I said. I also mentioned others this
thread who have issued guarded warnings and I think them
credible; I've read some of his work. Those quoted
portions of my post are all "on one hand". Dan

What others? I dont see anybody else here saying it is not good. Yes, the BTZS is a dry book but I would think that anybody that is going to recommend it or not, at least knows what they are talking about and has read/used the method.

I have no personal interest in the BTZS, I dont sell it, I dont give workshops on it, but I know it works and I can recommend it for those starting and wanting to use a closed loop system to better control their materials. The difference is that I have actually read the book, I have used it and have compared it to all the other books I have on the topic such as AA, Fred Picker, Todd and Zakia..etc...

Your example is like me trying to advice 35 mm shooters and telling them..." you know, I dont have a Canon camera, but I have read the instruction booklet and I think it is not good".....If all you have read about the BTZS is what you read in the DI letters, trust me, you know nothing about the BTZS.

6. Originally Posted by dancqu
I would not recommend Davis' book. Sensitometry is,
as has been mentioned in other posts this thread, likely
incomprehensibly presented. I've not read the book but have
studied a few of his articles. I've eight volumes of D-Max,
a quarterly put out by The View Camera Store.

I've a densitometer and off and on do speed tests, and
zone density measurements which can be read and/or
plotted. It's an eight hour a day five days a week
job for some. Dan

I highly recommend Phil Davis' Beyond the Zone System. It is, without any question, the *most* comprehensible and clearest presentation I have ever read about sensitometry.

Since you admit that you have not read Beyond the Zone System, my suggestion would be that you get the book and read it before offering any more opinions about it.

Sandy King

7. Sandy King In Quotes

"I highly recommend Phil Davis' Beyond the Zone System.
It is, without any question, the *most* comprehensible and

Having not read the book I can only wonder if sensitometry
and it's association with exposure and development is it's only
concern. I've a very vague impression that Beyond the Zone
System, BTZS, the book, might be called The BTZS System.
I think the OP this thread, photomc, would like to wade in
rather than dive in.

"Since you admit that you have not read Beyond the Zone System,
my suggestion would be that you get the book and read it before
offering any more opinions about it."

I'll not infer any more from my readings of the di newsletter. Dan

8. Originally Posted by photomc
As the new owner of a densitometer, attempting to get a better understanding of how it is used to determine the contrast index (CI) of a negative. Can someone point me to a resource that might help explain how the CI is determined or perhaps offer a little Densitometery for Dummies...

Thanks,
What is it you're trying to do?

9. Originally Posted by dancqu
Sandy King In Quotes

Having not read the book I can only wonder if sensitometry
and it's association with exposure and development is it's only
concern. I've a very vague impression that Beyond the Zone
System, BTZS, the book, might be called The BTZS System.
I think the OP this thread, photomc, would like to wade in
rather than dive in.
Beyond the Zone System contains among other things a very clear and detailed discussion of the Zone System itself. What is not so widely understood is that the data provided by Davis' BTZS system of testing and plotting can be used with either incident metering or with reflective metering using zone system principles. In my opinion the incident system is a bit more precise but the reflective metering with a spot meter may be the best system for certain kinds of subjects.

I don't claim that Beyond the Zone System is an easy ready. In fact, it takes a lot of concentration to absorb the material. Someone once jokingly remarked to me that Davis should have put some erotica in there every five or six pages as in incentive to keep the reader moving ahead.

There may be easier systems for learning exposure/development/printing controls but IMO there is none more precise and comprhensive than BTZS.

Sandy King

10. We are working with very precise and exact materials with photography, even scientific materials if you will. So, doesn't it stand to reason one must approach this in a technical and scientific way if one wants to get the most out of their materials? If not, then go to Walgreens or whatever. It never ceases to amaze me how one can come all the way to working with LF and then stop so short of reaching their full potential in working with their materials. I am also amazed at how when BTZS is mentioned the reactions it seems to bring out. Yeah it's very dry....yeah it is boring....yeah watching the Waltons pit cherries is more exciting, but when the smoke clears it gives us the photographer a boatload of information that will enable us (if applied) to make the best exposures we ever have made. Why then, do people stop short at this point? We've bought big cameras, built darkrooms, bought big expensive film, big expensive lenses and then put the brakes on when we hear BTZS?

The unique situation with photography is the melding of technical and artistic. There isn't any way around it folks, photography does involve things of a technical nature and one does their craft a huge disservice by not going the distance with the technical. It's really wierd but when one wades through the very easy testing and gets the numbers down it goes quickly into the background and then the artistic takes front seat. It is a challenge to learn but it is learnable. I am not one that is at all accademic and learning this type of thing is hard, very hard for me. I won't be able to sit down and talk heavy numbers with Sandy and Jorge and some of the others but I can go out and make exposures and now print the best prints I have ever been able to make and in far less time. And another thing that fascinates me is the site for BTZS ....no one hardly ever is on the forum. Weeks go by between posts! Why do you suppose this is? It's too dry? Too much work? Too technical? No, it's because once people get through the testing they are out making photographs and no real need for stuff like....."What's a good film"? or "Got my first batch of AZO today and can't wait to try it out" or "What's a good paper to print crappy pictures of my kids with?" or "What's better...PMK, PYrocat or ABC?" and one of my all time favorites..."What's a good fixer?" What's a good fixer???????? Paaaleeezzzz.

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