I'm not going to disagree, but I would point out that the graph doesn't say anything about how contrast index is determined - it just says that it changes with changes in development - which isn't the issue, because it does.
What else can change it?
What else? Nothing. A different contrast index means a different characteristic curve. This can only be obtained by different development. For a given developer, dilution, agitation and temperature, only time can give a different curve.I'm not going to disagree, but I would point out that the graph doesn't say anything about how contrast index is determined - it just says that it changes with changes in development - which isn't the issue, because it does.
What else can change it?
What else? Nothing. A different contrast index means a different characteristic curve. This can only be obtained by different development. For a given developer, dilution, agitation and temperature, only time can give a different curve.
What should the correct CI be when developed for the same time at speed 800 compared to 400. Can this be worked out?
If the CI remains the same because the development time is the same then what will be the changes in a 800 negative compared to the identical negative exposed at 400 and will these changes be noticeable in the prints from the 400 neg and 800 neg respectively?
With T-Max 400, under-exposing by shooting at EI of 800 and not changing the development time will give you a negative that will be easier to print and give you better highlights than under-exposing by shooting at EI of 800 and increasing the development time (a "push" development).Under exposing by shooting at ISO 800, merely looses detail and increases the work necessary to make a good print.
Nope, there's nothing about exposure that would alter contrast index. The characteristic curve covers a wide range of exposure anyway. When calculating contrast index, you place one point of a straight line on the film base plus fog density level. The second point is 0,2 units away from the first, but must fall on the characteristic curve. The third point is on the aforementioned line, but also on the characteristic curve and 2 units away from the second point. You don't have a choice where to put these points, the rules dictate where they would fall. The slope of this straight line is the contrast
This is all true - except when you give the film less exposure (EI of 800), the resulting curve moves and changes its shape (a bit). So the three points you take the measurements from are at different actual positions (not relative positions) than the three points used for the measurement at an EI of 400.
In the majority of photographic applications, the level of exposure is such that much of the toe of the D-log e curve is involved. The use of CONTRAST INDEX [rather than GAMMA] is then likely to be advantageous. "Contrast Index"; C. J. Niederpruem, C. N. Nelson et al 1966.
Thanks Bill (and others).MattKing,
The definition of CI does allow this experiment...
You can take two rolls of film, fog one roll of film slightly and then expose, develop and read sensitometry exposures on both films... developed in the same tank for the same time and get different CI.
Another thought... In reality you are right to think the overall contrast of two pictures taken with two different exposures can be different, and the lesser exposed picture will have less contrast typically..... it’s just not called CI
You can carry on with your explanation of the differences how “that” contrast measure is less significant with TMAX-400 than it is for other films such as those with long toes, because TMAX-400 has a sharp toe that makes the straight line seem to go all the way to zero.
This whole topic is a lot simpler than it may look. In the end we want to compare contrast, and contrast is defined as the slope of the curve "density over exposure". Accurate measurement of slope in a situation with detector noise is very difficult, therefore numerically stable estimates for slope were defined. Examples for such slope estimates are shown in the document posted by ic-racer. When these books were written, even pocket calculators were out of reach for most people, so some of these explanations sound a lot more complicated at first glance than they should. It doesn't really matter, which estimate you use, as long as you use the same type of estimate in comparisons.I may not have grasped all that has been said at this point but I will persevere.
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