What do the grid divisions in a Curves editor represent?
Asked 12/7/2017
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In Curves editors such as Adobe Camera Raw, Photoshop, and Lightroom, the graph is divided into equal grid squares on the X and Y axes. Do those divisions have any absolute meaning, or are they just visual guides? For example, do they correspond to exposure stops, or are they simply tonal/RGB value intervals? I'm specifically asking what the grid means in a typical Curves tool and how the divisions are chosen.
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
8y ago
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The X and Y axis of the curve is the histogram range of [0..255]. See the gray scale tonal graduations shown there with it? X is input and Y is output, and the purpose of the curve line is to map an input RGB value to an output RGB value.
Photoshop shows four divisions, that I would call RGB 0, 64, 128, 191, 255. If you are currently showing image data, that image histograms [0..255] values are also superimposed there.
It absolutely is NOT stops. Stops are logarithmic, but the curve scale is linear RGB [0..255]. However, the histogram data itself is gamma encoded, data to the power of 1/2.2 (roughly square root).
So perhaps it does superficially seem to approximate stops, partially. Regarding stops, if linear data (which is Not shown), one stop down from 255 is 128, and 2 stops down is 64.
But in gamma 2.2 data (which is shown), one stop down from 255 is 186 (at 73%), and two stops down is 136 at 53%. We could call that close.
But three stops down is 99 at 39%, and four stops is 72 at 28%, neither very close to the graduations.
Originally by user38978. Source · Licensed CC BY-SA 4.0
user38978
8y ago
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Generated from our catalog & community — verify before relying on it.
In a typical Curves tool, the grid divisions represent tonal value intervals, not exposure stops.
The curve maps input tone values on the X axis to output tone values on the Y axis. In the common 8-bit-style display, that’s essentially a 0–255 scale, with the grid often dividing that range into equal chunks such as 0, 64, 128, 191, and 255. The histogram shown behind the curve is plotted in that same displayed tone space.
So the graticules do have meaning, but it’s an editing/display meaning: equal numeric tone steps. They are not scene-referred exposure units.
They are not “one square = one stop.” Stops are logarithmic, while the curve display is based on image tone values in the working/display space. Because image data shown in these editors is typically gamma-encoded rather than linear sensor data, the spacing may sometimes look vaguely related to exposure changes, but it is not a true stop scale.
So the divisions are chosen as convenient equal subdivisions of the tone range, mainly to help you judge and place curve adjustments consistently.
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