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# Thread: ISO speed determination constants - question for Stephen, Bill etc

1. Originally Posted by Stephen Benskin
As the paper says, W speed "is of particular interest in the mathematical theory of the shape of the sensitometric curve and its relation to speed."
Isn't that what this thread is about? My understanding is that one reason W-speed was not adopted because of lack of computing power. That is no longer the case.

2. Originally Posted by Bill Burk
My take-away from the W speed article that these different speed ratings are compared based on how well they correlate to the "Picture Tests".

Then the writer makes an outlandish statement that I will challenge: "It is, of course, too time-consuming to use the print-judgment method."
Of course they were interested in developing a universal standard. For one's own work an EI based on what gives one the best prints is the way to go.

3. ## Pt 1

What makes the fractional gradient speed method significant and still relevant is its connection to the print judgment speed test. Photography is a subjective medium and the only way to determine what process works or not is to evaluated the result from the process. The first excellent print test was just such a psychophysical test. Photographs were made of a range of subjects over a period of time. Detailed information for each scene and exposure was recorded. Sensitometric tests were run at each step. All of this is thoroughly covered in the Loyd Jones paper The Evaluation of Negative Film Speeds in Terms of Print Quality.

A series of prints were made from each negative and these prints were judged by a panel to determine the quality of the prints. The negatives were then evaluated to determine the correlation. The conclusion was print quality increased as negative exposure progressively increase, but only to a point. Then the level of quality leveled off.

The initial point where the prints were deemed excellent is call the first excellent print point. The results from the test indicates there is no ideal or correct exposure with black and white negatives. Only that a negative shouldn’t be exposed below a certain point. In practice, there are advantages to using the minimum necessary exposure. Exposure times are increased. Sharpness is maximized and grain is reduced. When the tests were conducted in the late 30s, the average negative size would be considered large format today and didn’t require a high degree of enlargement. Consequently, the range of “over exposure” without a change in print quality would be larger than with 35mm film. This still leaves a bit of room to chose from. What is key about determining the first excellent print point is that it defines the minimum, the base to work from.

As Nelson and Simonds writes, “it is too time consuming to use the print judgment method except for basic studies. A sensitometric method is much more rapid.” Once the basic study was finished, it was necessary to find a sensitometric alternative that would yield consistent results with was many different types of film agreeing closely to the print judgment speed tests. The legitimacy of any method is contingent upon this point.

4. I love that chart. My challenge to the method, while I accept as true that the "First Excellent Print" is the best place to begin.

I wonder how hard the darkroom workers had to work to make those prints.

That is why I choose, my own preference, to pick a point slightly to the right. So I overexpose a little past "First Excellent" deliberately. That makes my work in the darkroom easier.

5. ## Pt 2

On the surface this would appear to be an easy task. Just give the film more exposure than the first excellent print exposure. In reality there’s more to it. In Jones and Nelson’s paper A Study of Various Sensitometric Criteria of Negative Film Speeds, they evaluate a number of different methods. They write, “after a careful examination of the sensitometric characteristics of the negative materials used, such a correlation was found. This correlation may be explained most conveniently by referring to Fig 1 (below). The negative from which the first excellent print was obtained was found to use a portion of this characteristic lying between the points M and O. Let the slope of the used portion of the curve be designated by G bar and the slope at point M by Gmin, this being the limiting gradient at the extreme shadow end of the object tonal scale.”

Jones introduced the concept of the use of a minimum gradient in a 1935 paper, Minimum Useful Gradient as a Criterion of Photographic Speed. The paper makes no conclusions as to what the gradient would be. I believe this is what lead to the first excellent print test.

Nelson summarizes the different film speed methods, “The inertia speed criterion was found to give poor correlation. Speeds measured at a density of 0.20 above fog were also unsatisfactory. Speeds measured at a density of 0.10 above fog were better than inertia or 0.20 density speeds, but did not correlate with the print judgment speed as closely as was desired for a primary standard method.”

A good example of unsatisfactory correlation is the comparison of where the different methods would fall on curves of different shapes and with different gradients. In the example, S represents the print judgment speed. The tiny arrow represents the fractional gradient speed. S` represents the speed at 0.10 density above Fb+f, and the where the straight line extending down from the film curve intersect the line representing Fb+f is the inertia speed point.

What did Jones conclude? "From the standpoint of tone reproduction theory there seems to be no justification for the adoption of any value of density as a significant criterion of the speed of a photographic negative material. The primary function of the negative material is to record brightness (luminance) differences existing in the scene. Density, per se, has no significance as an indication of the ability of the photographic material to perform this function. The value of negative density by which any particular object brightness (luminance) is rendered, as, for instance, the deepest shadow, is of no consequence except insofar as it may have some bearing on the exposure time required to make a print from the negative."

"Tone reproduction theory indicates that there is only one characteristic of the negative curve that is significant in expressing the capacity of the material to reproduce brightness (luminance) differences, and it is upon the way in which brightness (luminance) differences are reproduced that the quality of the final positive must depend. This characteristic of the D-log E relationship is the gradient or slope, since this determines the magnitude of the density difference by which brightness (luminance) differences in the object will be rendered in the negative and eventually in the positive made therefrom."

As Nelson noted about the fractional gradient method in the introduction to Simple Methods, "This criteria was adopted because it gives speed values which correlate more closely with speeds obtained by picture tests than do the speeds obtained by any other known sensitometric speed criterion."

6. Stephen - I've gone through the Nelson/Simonds and Delta-X papers and I have some concerns and questions which I will try to post later tonight when I have some time. In particular the print judgement speeds in the figure with the six hypothetical curves surprised me at first - which leads me to more questions concerning 0.3G. And the papers really raised some fundamental questions for me.

As you know based on my many previous posts on the subject, I think Jones had it right when it comes to the underlying concept. Local contrast is what matters, not a density. Which is what led me to start this thread. How that principle evolved into practice though, I'm still not sure I "agree", which may ultimately help to bridge the gap between ISO and ZS-style personal EIs.

For now I'll leave you with one quick question. How did an extension of the straight line down to fog density end up being called Inertia Speed? What is the basis for that construction? Is it essentially a speed point such that there is zero toe/compression relative to the maximum gradient in any development scenario?

Michael

7. Originally Posted by Michael R 1974
For now I'll leave you with one quick question. How did an extension of the straight line down to fog density end up being called Inertia Speed? What is the basis for that construction? Is it essentially a speed point such that there is zero toe/compression relative to the maximum gradient in any development scenario?
Inertia Speed comes out of Hurter and Driffield's work. The line extending down is basically Gamma. Hurter and Driffield felt that the truest reproduction happened on the straight-line portion of the curve. This was based entirely on the sensitometric concepts. They never did psychohysical testing. That's what makes the first excellent print test so important. It began to tie the technical aspects of the photographic process in with the psychological.

I can't give you specifics on the Inertia Speed at the moment as I am at work, plus I've never played with the method. The use of inertia speed was discontinued sometime around 1918. Curve shape had too big of an effect on inertia speeds, and like with Gamma, it was difficult to accurately define the straight-line portion with some curve shapes.

8. Originally Posted by Michael R 1974
As you know based on my many previous posts on the subject, I think Jones had it right when it comes to the underlying concept. Local contrast is what matters, not a density. Which is what led me to start this thread. How that principle evolved into practice though, I'm still not sure I "agree", which may ultimately help to bridge the gap between ISO and ZS-style personal EIs.
Jones was the first ASA chairman of the Subcommittee on Photography. Not to start any conspiracies, but he probably exerted a great deal of influence during his tenure on which methods were adopted. But the answer to most questions about the adequacy of the speed point, the speed point is a place to calculate the speed. It doesn't necessarily imply where the shadow exposure is supposed will fall. Fractional gradient method defines the mininum useful gradient. The fractional gradient film speeds are 1/2 of what they would be using the current ISO method. What many of the popular methods tend to miss is the speed constant which actually determines where the exposure is supposed to fall. (more on this later)

9. I have been trying to make a mental picture about the Delta-X Criterion. Where I agree is that when pushing to expand contrast keeping the speed near the normal EI one moves up on the curve which results in a steeper gradient (one could shorten the development as a result) and one gets away from the toe.
What bothers me is when pulling. Looking at curves left of the 0.1 speed point the curve has largely the same shape as right of the speed point. So shortining the exposure would work out mathematically.
But in my point of view in the shadows we have dark places next to light places, in the dark places we have even more dark places which I feel may have too thin a density.

10. Inertia Speed = 34 / i where i is the the point where an extended line from the film's characteristic curve intersects at film based plus fog density.

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