Correcting Images by Adjusting the Histogram

Use the Histogram Adjustment command to adjust the contrast and color balance of your image. The Histogram Adjustment dialog graphs one color channel at a time, unlike the Histogram Window which can display all color channels at once. You may want to first display the Histogram Window (choose View > Palettes > Histogram), then use autoproofing on the Histogram Adjustment dialog so you can see changes graphed in the Histogram Window.

Important: This command applies to greyscale and 16 million (24-bit) color images only. To increase the color depth of an image, see Increasing the Color Depth of an Image.

Note: When you place the cursor over the histogram, a ToolTip displays the following information: Input is the value on the horizontal axis, Count is the number of pixels that have that value, Integral is the percentage of pixels in the image with that value, and Output is the adjusted value.

The graph in the Histogram Adjustment dialog displays how many pixels are at each value of the selected channel. The vertical axis represents the number of pixels and ranges from zero pixels to the highest number of pixels in the graph. The horizontal axis represents the value from zero to 255 of the selected channel.

When you change the histogram, the graph displays the adjusted histogram as red and the original histogram as grey. To turn off the red overlay, clear the Overlay result histogram check box.

For detailed information on the graph, see Understanding Histogram Values and Guidelines for Analyzing the Histogram.

In the Histogram Adjustment dialog, you can edit the luminance (lightness) and the red, green, and blue color channels. The Histogram Window (opened by choosing View > Palettes > Histogram) also graphs the hue, saturation, and greyscale values, but these cannot be edited by adjusting the histogram.

To use the Histogram Adjustment command:

To limit the adjustment to a specific area, make a selection in the image. See Using the Selection Tools.
Choose Adjust > Brightness and Contrast > Histogram Adjustment to open the Histogram Adjustment dialog.

Click to magnify the histogram or click to reduce the view of the histogram. Click 1:1 to display the histogram at its original size.

For the Edit option, choose a channel:

Luminance Lightness values of the image. If you are correcting contrast, start with this channel.

Color Pick a color channel from the drop-down list: Red, Green, or Blue.

In the Presets drop-down list, choose Default. The default values represent no adjustment to the image.
On the left side of the histogram, look for a gap between the left edge and the point where the graph starts to rise. If there is a gap, it shows that the darkest pixels in the image are not completely black. Click and drag the Low slider (the black triangle) to the point where the graph rises. The top Low edit box displays the low position (from 0 to 255). The bottom edit box displays the percentage of pixels that are between zero and the low value; their contrast will be lost. As a general rule, keep the percentage below 0.1%.
On the right side of the histogram, look for a gap between the right edge of the window and the point where the graph decreases to zero pixels. If there is a gap, click and drag the High slider (the white triangle) to that point on the graph. This changes the lightest pixels in the image to white. As a general rule, keep the High percentage change (the bottom edit box) to below 0.1%.
If the overall image is too dark or too light, adjust the gamma. (Gamma is a standard measure of change in an image’s contrast.) If the image is too dark, drag the Gamma slider (the grey triangle) to the right to increase the gamma (the number changes in the Gamma edit box). If the image is too light, drag the Gamma slider to the left to decrease the gamma.
If the graph has peaks on the left and right sides and contains low points in the center, the midtones need to be compressed. Shadows and highlights need to be expanded to reveal the information they contain. This often occurs in flash photography when a subject is too close to the camera; the subject appears brightly illuminated but the background of the photograph is very dark. To compress the midtones, click and drag the Midtones slider up.
If the graph peaks in the center and has a low number of pixels at the left and right sides, expand the midtones by dragging the Midtones slider down.
If you are improving your image, skip this step. If you are creating artistic effects, use the Output Max and Min sliders on the left side of the histogram to make further adjustments. The Max slider is the white circle within a grey square; the Min slider is the black circle within a grey square. To darken the image’s whites, click and drag the Max slider down. To lighten the image’s blacks, click and drag the Min slider up.

Note: Even though you are dragging sliders along the vertical axis, the Max and Min values actually affect the horizontal axis. They indicate the maximum and minimum value (from 0 to 255) in the image. All pixels outside those values are converted so that they fall within the maximum and minimum.

To adjust another color component, choose it in the Edit drop-down list and follow these steps again.
Click OK.

Correcting Images by Adjusting the Histogram

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