Checking scan quality and tonal range

A histogram illustrates how pixels in an image are distributed by graphing the number of pixels at each color intensity level. Histograms can show you whether the image contains enough detail in the shadows (shown in the left part of the histogram), midtones (shown in the middle), and highlights (shown in the right part). You can use this information to make more precise corrections.

The histogram also gives a quick picture of the tonal range of the image, or the image key type. A low-key image has detail concentrated in the shadows; a high-key image has detail concentrated in the highlights; and an average-key image has detail concentrated in the midtones. An image with full tonal range has a high number of pixels in all areas.


Original with insufficient detail, and sufficient detail

Original, and corrected average-key image

Original, and corrected high-key image

Original, and corrected low-key image

To display the histogram for an image:

1. Do one of the following:
• To display the histogram data for the entire image, choose Select > Deselect to make sure nothing is selected.
• To display the histogram data for a portion of your image, make a selection in the document window. See Selecting pixels.
2. Choose Image > Histogram.

The horizontal axis of the histogram represents the intensity values, or levels, from darkest (0) at the far left to brightest (255) at the far right; the vertical axis represents the total number of pixels with a given value.

3. For RGB images, choose an option from the Channel menu. You can plot either the luminance of the composite channel (Luminosity) or the intensity values of an individual channel.
4. To view information about a specific point on the histogram, place the pointer there. To view information about a range of values, drag and hold in the histogram to highlight the range.

Statistical information about the intensity values of the pixels appears below the histogram:

Mean

Represents the average intensity value.

Standard deviation (Std Dev)

Represents how widely intensity values vary.

Median

Shows the middle value in the range of intensity values.

Pixels

Represents the total number of pixels used to calculate the histogram.

Level

Displays the intensity level of the area underneath the pointer.

Count

Shows the total number of pixels at the intensity level underneath the pointer.

Percentile

Displays the percentage of pixels at and below the level underneath the pointer. This value is expressed as a percentage of all the pixels in the image, from 0% at the far left to 100% at the far right.

Cache Level

Shows the setting for the image cache. If Use Cache for Histograms is selected in the Preferences dialog box, Photoshop Elements displays histograms more quickly but less accurately. To display more accurate histograms, either deselect this option, or hold down the Shift key while choosing Image > Histogram.