Operation Manual

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The Tool Chest — Curves
Gamma
Gamma
Gamma (also written “γ”) is a fundamental property of video systems
which determines the in ten si ty of the output signal relative to the input.
When calculating gamma, the maximum possible input intensity is as-
signed a value of one, and the minimum possible intensity (no input)
is assigned a value of zero. Output is calculated by raising input to a
power that is the inverse of the gamma value (output = input
(1/γ)
). In
practical terms, raising the gamma value has the same effect as mov-
ing the midpoint slider to the left, raising mid-tone output values and
brightening the image while leaving the maximum and minimum values
untouched. Lowering the gamma value has the same effect as mov-
ing the midpoint slider to the right, lowering mid-tone output values
and darkening the image. The de fault value for gamma is one, which
produces a linear curve in which input and output values are the same.
Gamma can be set to any value between 0.45 and 3.00.
The Gray-Point Slider
The gray-point slider is located at the bottom of the curve
edit display.
Step 4—Setting the Gray Point
Move the slider to the left to increase the brightness of
mid-tones in the selected channel, or move it to the right
to decrease brightness. Al ter na tive ly, you can enter a value
for gamma directly in the text box under the slider. The ef-
fects of changes to the gray point can be seen in the active
image.
Gray-point
slider
Gamma value
The point midway between the maximum and minimum
input values is known as the “gray point” (or “midpoint”).
This point represents a neutral shade corresponding to an
output level of 128. The gray point is controlled by a slider at
the bottom of the curve edit display. Moving the midpoint
slider to the left increases the brightness of mid-tones with-
out “washing out” high lights. Moving the slider to the right
darkens mid-tones without obliterating detail in shad ows.
The midpoint can be set using the gray-point slider or by
directly sampling the image using the gray-point eyedropper
tool.