Jean Yves Chasle

I'm glad we eventually agree. For a professional, the path I follow can be hard to understand at first sight. It's actually more than curiosity. But I'm afraid this thread is now slipping away from its original subject. If you want to, why don't we go on with this discussion by private mail ? I'm always interested in sharing experiences and points of view, especially about that subject.

My apologies if I have offended anybody, it was certainly not my purpose. Nevertheless, I don't think that the lines I have written deserve so much criticism. I'm merely juggling with published curves related to each other by a Jones diagram. This kind of diagrams has been and is used by many people in order to observe relationships between characteristic curves on a sheet of paper. The software approach I have talked about is merely making the diagram dynamic. I'm really sorry for having been so much misunderstood when I was talking so lightly about experimenting and processing, I was just talking about curves. Now I do believe that understanding relationships between curves on a computer program is better than nothing, when you cannot get your hands on the "real thing". I just had a question about the use of LAD that I think is now answered, thank to you and Dirk DeJonghe. I sincerely hope that a debate about the validity of synthetic imagery is not about to start, and I don't want to argue the case for it. That's not my point.

Yes, unfortunately I couldn't get my hands on the Adams' Bible, but I do use "Zone VI Workshop" by Fred Picker who describes the Adams' Zone System quite well I think. Apart from the time development control (mainly used in black and white processing), he explains in details how one has to control the shadows and highlight placement onto the characteristic curves, much more than the mid-range gray itself, in order to obtain a satisfying tone distribution in the final image. As in painting, an artistic approach cannot be summed up by placing a single tone on the curves. But don't get me wrong, I am not trying to obtain actual images. I'm involved in the development of an educational software, and so my purpose is merely to represent a simplified workflow in which a laboratory, getting a piece of negative film exposed following strictly lightmeters recommendations, calibrates a printer with the LAD, then tweaks the trimmer values to obtain a "correctly exposed" positive film. By "correctly exposed", I mean a 16% gray exposed on the negative (wherever it's been chosen to be placed on the curves) looks like a 16% gray to the audience, no less no more. All this is purely theoretical, there is absolutely no craft involved. It's a starting point to let people (and myself) play with, and eventually depart from to gain more specific knowledge about the real craft. In this aspect, everything that you and Simon Wyss has written is a huge help to me, I'm extremely grateful to both of you. I would just need to be sure that the three diagrams I have showed are roughly representative of what I have just described, a very simplified workflow used by laboratories, lacking any kind of artistic considerations. If this approach complies enough with reality, I can start experimenting with the negative latitude, placement of the zones, consider the impact of color shift, grain, saturation, even processing (pushing, flashing, bleach bypass, ...).

Thank you VERY much indeed, this is truly enlightening. On average, a value of 30 seems to be a serious leap, close to half a stop, from 25 which happens to be an inferior limit to the green value. Is this partly because most DP would normaly over-expose their 18% gray to time for a less grained / more saturated image, or is there any technical reason that wouldn't show on characteristic curves ?

Dirk, Simon, thank you so much for your detailed answers. I have programmed a Jones diagram which I hope will show better than words what I have understood. The Jones diagram itself is based on explanations and sensitometric curves from "La Sensitométrie" by Jean-Louis Fournier. Top-left curves are the characteristic curves of a KODAK VISION Color Print Film 2393 along with the three current densities read on the vertical axis. The bottom-right curves are the characteristic curves of a KODAK VISION2 500T Color Negative Film 5218 along with the three current densities read on the vertical axis. The bottom-left segments represent the printer trimmer settings. The printer trimmer setting values are displayed under the diagram, at the bottom of the screen copy. Adding one to the values slides the segments by 0.025 LogH to the right, subtracting one slides them by 0.025 LogH to the left, changing the shape of the top-right curves, the sensitometric curves of the final image. Figure 1. If I have got it right, the laboratory always starts by reporting the negative LAD 0.80-1.20-1.60 on the positive LAD 1.09-1.06-1.03 with trimmers set to 25-25-25, independently from the actual negative curves. These settings are represented on the screen copy below. The three LogH corresponding to the three negative LAD densities are of no use here. Figure 2. At a certain LogH, the green negative density can be kept around 1.20 with a positive density 1.05 almost equal to the positive green LAD 1.06. Obviously, at the same LogH, the blue and red negative densities are different from the negative LAD, no wonder that the densities 1.49-1.05-1.25 read on the positive film are different from the positive LAD 1.09-1.06-1.03 (except for the green). Figure 3. Now to simplify matters, suppose that the recorded scene contains a 16% gray card, illuminated with tungsten lights at the exact same temperature than the negative film stock balance color (3200 K), and exposed at the same LogH as in figure 2. In order to get this 16% gray printed as close to the positive LAD as possible, so it looks to the audience like a 16% gray under a projection light (5600 K), the laboratory has to change the printer trimmer settings, as on the following screen copy. The positive densities 1.10-1.05-0.99 we obtain are close to the positive LAD 1.09-1.06-1.03. Now a 16% gray filmed on the stage should be seen by the audience as a 16% gray. Is this procedure roughly the way laboratories work or is there something I have misunderstood ?

Hello everyone, There's something about Laboratory Aim Densities I cannot figure out by myself, I hope someone will be kind enough to explain the obvious. I'm studying how LADs are used in the film development process. In an educational brochure, Kodak shows negative film stock characteristic curves where the LADs (0.8 red, 1.2 green, 1.6 blue) clearly don't share the same LogH. I have done the same projection with different negative film stocks, the results are the same: LADs never share the same LogH. Well, how do these densities can represent a 16 % reflectance neutral gray patch if they don't align vertically at the same LogH ? Of course, the printer can be set up so that the negative LADs are printed on the positive film stock as a neutral gray, but then no "real" gray card filmed on the stage will be printed as neutral gray on the positive film stock, colors will be shifted. Won't they ?... Jean-Yves Chasle Reference: Kodak's brochure H-61