BTZS 0,5 ZS 0,57 Why?
Ok, here the issue because I was reading another thread and got very, very confused. When I look at the BTZS way of working an average scene of SBR 7 the avg. gradient is developed to a contrst of 0,5 and in most other publications I read to 0,57.
When pushing or pulling the values are very different as well.
What am I missing?
All the various forms of average gradient, including CI, are about determining the slope of the curve, Rise / Run. The value for the average gradient depends on a combination of three major variables: Log subject luminance range, paper LER, and flare. The average subject luminance range is 2.20 logs. Grade 2 paper printed using a diffusion enlarger has around a 1.05 LER. Average flare for a statistically average scene is considered to be 0.34 for large format and 0.40 for smaller formats. Flare reduces the apparent subject luminance range - 2.20 - 0.40 = 1.80.
Originally Posted by AndreasT
1.05 / 1.80 = 0.58
This is what Kodak considers normal processing for a average scene.
When it comes to compensating for longer or shorter luminance ranges there are two basic ways to deal with flare. One is using the same value of flare and the other is to use a variable flare model. The amount of flare does decrease with shorter luminance ranges and increases with longer luminance ranges, but as flare also can vary greatly within a given luminance range, there's a question whether the extra effort of the variable method is worth it.
Here is a comparison of developmental models differing only in the application of flare.
Almost all methodologies are going to give you a workable normal. Scene luminance ranges fall within a bell curve which means the statistically average scenes occur in the majority of situations. What distinguishes a method as effective is how successfully it applies to more extreme conditions. One of the difficulties with extreme conditions is in determining operator error from failed methodology.
Last edited by Stephen Benskin; 02-11-2013 at 08:17 PM. Click to view previous post history.
When you find yourself getting confused here's the ONE thing you have to remember -- "plastics."
Great desertratt or should I call you Benjaman?
Andreas, you might be misinterpreting Davis. Where did you see the 0.50 in BTZS?
Frequently the ZS variables are 1.20 NDR and 2.10 subject luminance range which is 1.20 / 2.10 = 0.57. While this version of the ZS average gradient is practically identical to Kodak's (and the standard model), the justification for it doesn't reflect what is actually occurring.
Here's a comparison of developmental models from Way Beyond Monochrome, the Zone System, no flare, and my Practical Flare model.
Last edited by Stephen Benskin; 02-11-2013 at 08:33 PM. Click to view previous post history.
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OK basically he say a normal scene has a contrast of 2,10 right. He uses a log of 1,05 for paper. His equation is G= DR/SBR, so G=1,05/2,10 which equals 0,5.
I mentioned this to Fred Newman and he wrote me G=0,57 is too contrasty.
Give me a page. Actually, this is one of the reasons why I find systems so problematic. The very nature of a system is to simplify the process in order to make it more manageable. Things will naturally tend to get lost in the process. On page 95 of the fourth edition, Davis discusses what he calls the effective DR. From page 95, "because you will want to produce negatives that print comfortably on your chosen paper, use the SI value you calculated in you paper test to locate IDmax (I hate his abbreviations), but do so only after making one further adjustment. This final adjustment is necessary because your curve data are based on a no-flare test condition, but you'll be applying the data to camera exposures that invariably involve significant amounts of flare."
Davis then applies a compensation to the effective paper LER instead of simply adjusting the LSLR, which is what actually happens. If someone skims past these two paragraphs on page 95 that explains how flare changes the aims, then their results will be off. Why not just have the equation NDR / (LSLR - Flare)? It's impossible to miss flare as a variable when it's part of the equation.
Did Fred say why? Don't accept unsupported statements.
Here's a four quadrant reproduction curve with a LSLR of 2.20, flare of 2.0, and CI of 0.56. The NDR and LER match.
Last edited by Stephen Benskin; 02-11-2013 at 09:47 PM. Click to view previous post history.
I really have to try the quadrants/"windmill" representation.
Working with them and writing the program made the theory tangible. It really clarified things.
Originally Posted by Michael R 1974