# Alphabet Soup? Help

Discussion in 'B&W: Film, Paper, Chemistry' started by fhovie, Jun 27, 2005.

1. ### fhovieSubscriber

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Could someone please explain the meaning of all those abreviations used in sensitometry ... CI etc. How are these generated? I can guess Dmax, Dmin B+F etc but many terms are used that still confuse me. Gamma?? etc

2. ### JeremyMember

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CI = contrast index

That is what I have always thought it meant, but I'm not sure so this is a good question!

3. ### JorgeInactive

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SBR = Subject Brightness Range.
DR = Density Range
ES = Exposure scale of the paper.
Gamma = Method used by Ilford to define the two necessary points to claculate the slope of the straigh part of a H&D curve. Similar to CI which is what Kodak uses.
Log E = Natural logarithm of the exposure. If we expose in units of light which double then the progression is 1, 2 , 4 , 8, 16, 32, 64, 128, 256. It is simpler to write the log than the numbers.
Lux= Unit of light measurement, I leave it up to you to see your physics book..

These are all I can recall off the top of my head, let me know if you need more definitions and which are they.

4. ### Helen BMember

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"Log E = Natural logarithm of the exposure."

Is it normally the natural log or the base 10 log? I thought the latter - with one stop being 0.3, not 0.7?

Thanks,
Helen

5. ### JorgeInactive

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You know Helen I dont recall exactly, I remember many years ago sitting with a calculator and workig out the numbers, so it might be base 10. In any case it is simple to figure it out...no?

6. ### Claire SenftMember

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dmin least density
dmax maximum density
bf base density + fog for a particularl amount of development
fb+f as above
SP speed point, the amount of exposure to xreate minimum density

7. ### gainerSubscriber

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It is the common log, base 10, that is used. Gamma was not first used by Ilford. It was employed many years ago by Hurter and Driffield and other research scientists to represent the slope of the straight line portion of the characteristic curve. If you only used that portion, gamma and CI would be the same value. Some films have practically no straight line. In any case, we use more of the curve than was thought proper in H&D days.

8. ### Helen BMember

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The base 10 log is used for both the density and the exposure, with 0.3 being one stop. I wrote my previous post as a question because that seemed more conversational than a blunt contradiction. To be honest I find these internet discussions very difficult style-wise - I feel that I write in an overly harsh manner.

Best,
Helen

9. ### fhovieSubscriber

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How do you generate CI? if the SBR is 5 and the DR is 1.25 - what is the CI? and how do you get it - also - I know from my step wedge that my grade 2 paper can print a DR of 1.25 - What if I do a kalitype - I need a DR of 1.75? I am always reading about CI differences from AZO to Silver to Alt process. How do you calculate CI?

10. ### JorgeInactive

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No need to do so, anybody can make a mistake and it is no sin to correct it.

11. ### Helen BMember

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"How do you generate CI?"

The simple, approximate version of CI is the angle of a line between

- the point on the curve that is 0.1 density above FB+F, and

- the point on the curve with an exposure of 2.0 greater than the first point.

But this isn't exactly correct: the points are really on arcs of 0.2 and 2.2 scale units (x and y axis drawn to same scale) with a centre at the intersection of the horizontal FB+F line and the straight line intersecting the two points. Most easily found by using a little ruler with marks at 0.0, 0.2 and 2.2 units and placing it on the curve so that the 0.0 mark is on the FB+F line and the other two marks are on the curve; or in software by iteration based on the initial value given by the simple method.

Best,
Helen

12. ### JorgeInactive

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If you are doing the BTZS why dont you just go with Phil's average Gamma?
In any case you can approximate CI values with the formula.

CI=DR/SBR.

In real practice you need to make a "ruler" marked at log lenghts of 0.2 and 2.2 and keeping the 0 point at the b+f line you adjust the ruler until the curve touches both the 0.2 and 2.2 points. You can then calaculate the slope from these two points. Of course for this to work you have to havew the ruler as well as the coordinates for the curve with the same scale.

13. ### fhovieSubscriber

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So AZO (1.7 DR) CI would be higher than Grade 2 (1.25 DR) Silver CI - with an SBR of 5: DR/SBR 1.7/5 (.34) vs 1.2/5 (.24)

CI does seem rather complicated to me - I would think that DR would be plenty useful enough - I know what my paper ES is and the trick (I think) is to match what I want the SBR to translate to in DR so I can get the shadows and the sparkle on my favorite paper without highlight blowout or muddy shadows. What is the advantage of knowing CI?

15. ### JorgeInactive

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You are mixing terms, Azo has an exposure scale not a DR, for purposes of fitting the negative to the paper, we equate the negative DR to the paper ES. So to answer your question, yes you find out the paper ES and make the negative DR equal to this. The CI or gamma or whatever you want to use, is so that we all talk the same language and can use your data in our personal work. If you say I use TMY to a CI of .87 to print on azo grade 2, I know exactly what to do in case I want to print in azo.

Many times as you say, people specify a DR for certain processes, but if we are to give all the information then Gamma or CI are more accurate.

From what I have seen of your posts, you seem to have your process very well controlled, so I would say just keep on doing what you are doing...

16. ### Claire SenftMember

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Jorge, I think you meant to say average gradient not average gamma.

17. ### AggieMember

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Ya'll make my blonde roots hurt. I do not want to revert back to kinematics to figure out how to take a purty picture!

18. ### JorgeInactive

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Claire, I meant Gamma and CI as explained in the literature. Phil uses average gradient as his choice for calculating the slope which is very similar to Kodak's CI. BTW, I made a mistake, Ilford uses average gradient instead of gamma as I stated in the previous post.

19. ### fhovieSubscriber

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Well - they used to be blond - my kids (and CI) turned many of them grey! -

I hope to never see a density of 2.2 unless I do it as a test - so in the normal evaluation of exposing a scene and printing, CI is not very useful (I think) I just go after ES and DR and all should be good - (Both 1.25 for my Grade 2 papers)
Now - in evaluating developers (Which I am often doing) If I shoot 12 frames of a grey card metered one below zone 1 -each fram one stop up - and finishing with one above zone 10 and develop it - I should have a pretty good idea of how a developing process effects film speed and contrast expansion and compression. I should see zone 1 at B+F +.1 - which is exactly what I need to know to shoot a purty picture. It is a good thing I was able to bag 30 rolls of TMY cheap! The last batch of developer should be good for 200 rolls - I will know my process by the end of the week - I just needed good starting points.

20. ### Claire SenftMember

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Jorge, my mistake then. I thought Mr. Davis was using the term average gradient, not average gamma. Thank you for correcting me.

21. ### JorgeInactive

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You are correct, Phil uses average gradient, but I was talking about plain gamma and CI. Nothing to correct here....

22. ### dancquMember

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It's base 10. That base to the .3 power = 2, to the .6
power = 4, to the .9 power = 8, and on, doubling the value
with each additional .3.

More often than not exposure is not in the log format. I also
see EVs, ZONEs, and STOPs. I think 1, 2, 4 , 8, 16, 32, and on
would also due.

Perhaps the log format is used because of it's mathematical
manipulativeness. Dan

23. ### gainerSubscriber

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The purpose of all this technicality is to get the scene brightness to register on the film so that proper development will produce a density range that fits the printing material. It's nice when it happens by plan instead of accident, but the possibility of accident is why we have different paper grades.

24. ### Helen BMember

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"More often than not exposure is not in the log format. I also
see EVs, ZONEs, and STOPs."

If you think about it, they're logs too. Base 2.

Best,
Helen

25. ### fhovieSubscriber

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That would be true for exposure but isn't tranmission density in base 10?

I have received a sign from above - it appeared on my 361t - LAMP FAILURE!
I guess that means I have to go shoot real pictures again .... (the new one is on order ... \$80 from XRITE -ouch-)

26. ### Helen BMember

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Frank,

I thought that we'd already agreed that Log E and density were logs to base 10 - that was all sorted out on page 1. I was referring to EVs, zones and stops. They are all base 2 logs of relative exposure.

Best,
Helen