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Subject: Re: Exposure to the right and tone placement



Hello Group,

This thread has gone on for some time. Many people have put forth various theories. Many people have quoted many authorities. I would like to put forward for your consideration some hard data.

I have a Leaf Valeo 22 digital back. I also can look at the RAW output from it using the Leaf Capture 8.4.3 application. This program displays a histogram with f-stops across the bottom of the graph and output values (8bit or Adobe 16bit) down the side. It overlays the histogram with a “develop” curve. This is an adjustable curve that allows the end user to map the input exposure range of the scene to the output values in the edit space file. The RAW input is expressed in f-stops ranging from +3 stops above the midtone to –8 stops below the midtone. The f-stop scale is a log scale while the ouput ranging from 0 to 255 (for 8 bit) is linear. You select the output color space that sets the gama you wish to map to. The develop curve is an “S” Shaped curve to mimic the typical film curve. This compresses the highlights and shadows. On the “S” curve you have five points that you can adjust both up and down (output values) or left and right (input brightness). The five points are called “Highlight”, “One Quarter”,
“Midtone”, “Three Quarter”, and “Shadow”. After you set these points you can bring up a second window that displays a table with two columns that display the actual RAW input values and output values associated with each of these five points.

To build the following table of data I set each of the five points at exactly one f-stop intervals and then recorded the RAW input values associated with each stop.

The Valeo values run up to a maximum of 16,384 which indicates that this backs A/D is 14 bits.

f-stop RAW value
+3 16,384
+2 8,192
+1 4,096
+0 2,048
-1 1,024
-2 512
-3 256
-4 125
-5 64
-6 32
-7 16
-8 8

Now folks, that looks to me like in this Valeo 22 that the RAW output of the chip decreases by one half for each decrease in photon input of one f-stop.

My camera back behaves exactly like Bruce Fraser said it would.

Rags, are you confusing RAW output of the chip with the output of the algorithm that maps the RAW data to the final edit space (Adobe RGB 98 in this case)?

There is an old saying…In God we trust. All others have to show us the data.

I hope Tom Knoll would add this kind of display to ACR. It really makes all of this easy to understand. A graph is worth several megabytes of words.

BTW...I have designed several CCD airborne cameras for the Canadian Center for Remote Sensing while working as an Electronic Engineer for MacDonald Detwiler and Associates.

Thanks,

Ray Maxwell

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