Nikon 4 user manual Setting the Midpoint, Midpoint Slider, Sampling the Image for Midpoint, Gamma

Image Adjustment

Setting the Midpoint

The point midway between the maximum and minimum input values is known as the “mid- point,” which represents a neutral shade corresponding to an output level of 128. The midpoint is controlled by a midpoint slider at the bottom of the curve edit display. Moving the midpoint slider to the left increases the brightness of mid-tones without “washing out” highlights. Moving the slider to the right darkens mid-tones without obliterating detail in shadows.

The midpoint can be set using the midpoint slider or by directly sampling the image using the midpoint eyedropper tool.

The Midpoint Slider

The midpoint slider is located at the bottom of the curve edit display.

After clicking the slider once to activate it, move it to the left to increase the bright- ness of mid-tones in the selected channel, or move it to the right to decrease brightness. Alternatively, you can enter a value for gam- ma directly in the text box under the slider. The effects of changes to the midpoint can be seen in the active image.

Midpoint slider

Sampling the Image for Midpoint

The midpoint can also be set by direct sampling. Regardless of the channel currently selected in the channel menu, sampling simultaneously defines the midpoint for each of the red, green, and blue channels while leaving the midpoint for the master curve unchanged.

To sample the midpoint, click the Midpoint button . When the cursor is moved over the active image, it will take the shape of an eyedropper. Move the cursor over the image to find the pixel that you want to use to set the midpoint, keeping an eye on the pixel level display in the Information palette to identify pixels with a neutral input value (look for pixels that you think should be a neutral gray color in the final image). With the cursor over the target pixel, click the mouse button. The input value for the selected pixel will be used as the reference value for the midpoint. The cursor will return to its original shape, and the image will auto- matically be altered to reflect the new value for midpoint.

Gamma

Gamma (also written “γ”) is a fundamental property of video systems which determines the intensity of the output signal relative to the input. When calculating gamma, the maximum possible input intensity is assigned a value of one, and the minimum possible intensity (no input) is assigned a value of zero. Out- put 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 moving 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 moving the midpoint slider to the right, lowering mid-tone output values and darkening the image. The default 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 0.05 and 6.00.

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