Hereís something else interesting hiding in the diagram. Remember in the speed/meter relationship post when I said that the average scene highlight falls 3 stops above the metered exposure and the average shadow falls 4 1/3 stops below, but in camera with the effects of flare, they balance out making the metered exposure the mean?
The four red arrows are the same length. They are based on the distance between the highlight point and three stops down (which can be considered the metered exposure point) in the Subject representation. With the Optical Image, this relationship hasnít changed, but the distance between the metered exposure point and the shadow exposure is reduced almost to the same three stop difference. I have to admit it's not perfect in this example because the amount of flare they use is slightly lower than average. I believe they had to "fudge" some in order to make the rounded 7 stop range (instead of 7 1/3 stops) fit into the 1.05 negative density range.
Thing I point out to folks when studying any graphic representation of the B&W film and paper tone reproduction cycle is the gross distortion of values, even in the best of situations. The prints out of a typical 'creative' darkroom are likely going to demonstrate a tone reproduction cycle even more distorted than that presented. The point I make is that B&W film/paper is a creative medium and a beginner's naive attempts to 'reproduce reality' will be futile.
The other thing I point out is that the "S" and "J" portions of the film and paper account for this, and as Steve has shown, flare distorts things also. It is possible to make 'flare-insignificant' images with small format, multicoaed lenses in low contrast scenes. The tonal reproduction curve is still going to be buggered up because neither the film or paper are straight-lined.
This CI / NDR Chart was an internal reference used at Kodak under Dick Dickerson. For a grade 2 paper with a negative density range of 1.05, a 7 1/3 stop scene luminance range should be processed to a contrast index of 0.58. This is slightly different from the Kodak diagram because the use a different value for flare.
1.05 / (2.2 Ė 0.40) = 0.58
If flare isnít factored in, then the resulting CI for the same set of conditions would be:
1.05 / 2.2 = 0.48
A CI 0.48 falls on the chart at a scene luminance range of 8 2/3 stops. Thatís a 1 1/3 stop, or log 0.40, difference. The same difference between the scene luminance range and the camera exposure range.
For 7 1/3 stop luminance range
(2.20 - 0.40) * 0.58 = 1.05
2.20 - 0.58 = 1.276
For 8 2/3 stop luminance range
(2.60 - 0.40) * 0.58 = 1.276
(2.60 - 0.40) * 0.48 = 1.05
But wait. Here's contrast index values from the Xtol information data sheet that use different values.
The data sheet vs Kodak's CI/NDR Chart
N - 0.58, 0.58
+1 - 0.65, 0.70
+2 - 0.75, 0.88
+3 - 0.85, 1.17
Why the difference? One is for adjusting for different luminance ranges and one is for adjusting for under exposure. In other words, pushing for contrast vs pushing for speed. For pushing for speed, the scene luminance range remains constant, but the exposure is shifted to the left. This lowers the negative density range as the shadows drop further down into the film's toe. Increasing processing increases the the contrast of the film making up for the loss from under exposure. It also increases density in the toe. This effectively increases the film speed. The general rule of thumb is a stop increase in contrast increases the "speed" 1/3 stop. A film under exposed one stop and push processed results in only a 2/3 stop under exposure. In order to match the same density range as a normally exposed and processed film, the under exposed film is processed 2/3 of a stop contrast for each 1 stop under exposure.
The difference between pushing for contrast and pushing for speed is that the difference between the steps for pushing for contrast is 0.30, while the difference between pushing for speed is 0.20.
Contrast Indexes for pushing for contrast vs pushing for speed
1.05 / (2.20 - 0.40) = 0.58
1.05 / (2.20 - 0.40) = 0.58
1.05 / (1.9 - 0.40) = 0.70
1.05 / (2.0 - 0.40) = 0.656
1.05 / (1.60 - 0.40) = 0.88
1.05 / (1.80 - 0.40) = 0.75
1.05 / (1.30 - 0.40) = 1.17
1.05 / (1.60 - 0.40) = 0.875
All the answers are out there. They're just slightly hidden in plain sight.
A version of the Kodak diagram can be found in Way Beyond Monochrome. It can be found online: http://www.waybeyondmonochrome.com/W...ductionEd2.pdf
At first glance it appears to contain the same data as the Kodak diagram but with a Zone System emphasis. They both have a Subject line, an Optical Image line, a Negative line, and a Print line. Both have a 7 stop scene luminance range for the subject. Way Beyond Monochromeís negative is developed to CI 0.57 which is almost identical to Kodakís CI 0.56.
With all due respect, where they differ is with the resulting negative density range. Way Beyond Monochrome has a NDR of 1.20 for a 7 stop scene with the film processed to CI 0.57. Kodak has a NDR of 1.05 for a 7 stop scene with the film processed to CI 0.56. How is it possible to have different resulting negative density ranges from almost identical conditions?
While the Way Beyond Monochrome diagram shows the effects from flare in the Optical Image, just like the Kodak diagram, it doesnít factor it into the equation for the resulting negative. This is confirmed in another part of the book, Testing Film Speed and Development, http://www.waybeyondmonochrome.com/W...Evaluation.pdf.
ďIn addition, we also sets the normal log exposure range to 2.10, since we need 7 subject brightness zones to expose the 7 paper zones above, and each zone is equivalent to 0.3 log exposure. The normal average gradient can be calculated as 1.20 / 2.10 = 0.57. ď
The desired negative density range is divided here by the scene luminance range, and not the camera exposure range. Both the Way Beyond Monochrome diagram and the Kodak diagram show the existence of flare in their models. Itís impossible to get different negative density values from the same set of conditions. In order to acquire a desired negative density of 1.20 under the conditions displayed, for the Way Beyond Monochrome negative needs to be developed to a CI of 0.67.
1.20 / (2.1 Ė 0.30) = 0.67
Here is a comparison between the Kodak tone diagram and one from Photographic Materials and Processes. Like Way Beyond Monochrome, it uses Zones. Both use a seven stop scene luminance range, average flare, and process the negative to a CI 0.56.
This is a comparison between tone reproduction diagrams. The one on the left is from Way Beyond Monochrome and the right is from Photographic Materials and Processes.
In both the WBM and Materials diagram, flare in Quadrant I has pushed the shadow exposure higher up on the film curve, Quadrant II. The major difference is the Materials diagram shows how flare compresses the darker tones, between Quadrant I and Quadrant II, while WBM shows no change.
Both examples also show how the Zone designations are associated with the original subject and not to any specific negative density based on the exposure range from the speed point. This is most noticeable in the Materials diagram with the Zone designations remaining constant even though the exposure range for the shadows is compressed.
Even though there isnít anything specifically about flare in The Negative, itís still there if you know where to look. Appendix 2, Film Test Data, has a number of film curves from which itís possible to tease out some information. While the hash marks make it impossible to determine precise density ranges, the concept isnít lost.
There is a seven stop range from Zone I to Zone VIII. The negative density range at Zone VIII from a density of 0.10 over Fb+f to where it crossed the film curve is approximately 1.20. The gradient of the curve is then 1.20 / 2.10 = 0.57.
In this example, there is no compression of the lower Zones and each Zone is placed along the log-H axis at equal 0.30 (one stop) intervals. Incorporating a one stop flare factor will reduce the exposure range from 2.10 logs to 1.80 logs. The filmís contrast gradient hasnít changed, but one value represents a negative density range resulting in a unrealistic non-flare situation, and the other represents a more realistic situation as all optical systems have flare. It also conforms to the middle of the paper LER range for a grade 2 paper, as well as information from Kodak and other publications.
The diagram on the left is from ISO Ė 6 Black and white pictorial still camera negative film / process systems Ė Determination of ISO speed. While it doesnít say in the standard, the reason why the specific contrast parameters are used is because they are part of an equation that links the fixed density method of speed determination with the fractional gradient method. The diagram on the right shows this relationship.
Itís possible to find the fractional gradient speed from any contrast simply by plugging the negative density range at 1.30 log-H units from the speed point into the Delta-X equation. The reason why a density range of 0.80 was chosen is because of the relationship between the speed point / metered exposure ratio and the average scene luminance range (See What is the Relationship Between Film Speed and the Exposure Meter thread).
When the ISO contrast parameters are met, itís not necessary to do the actual calculation to find the value for Delta X. Because the equation isnít required to determine the ISO Delta X value, itís not included in the standard.
The contrast parameters of the ISO standard are about creating a good correlation between the fixed density method and the fractional gradient method. It does not represent and has nothing to do what some call ISO normal development.
I have to admit, this one is a little more hidden.
This thread gets me thinking something else might be hiding in plain sight.
Could aligning "Sunny 16" to metered readings have had something to do with speed point and/or speed point / metered exposure ratio decisions?