Using the colour adjustment tools
A histogram illustrates how pixels in an image are distributed by graphing the number of pixels at each colour intensity level. This can show you whether the image contains enough detail in the shadows (shown in the left part of the histogram), mid-tones (shown in the middle), and highlights (shown in the right part) to make a good correction.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 mid-tones. An image with full tonal range has a high number of pixels in all areas. Identifying the tonal range helps determine appropriate tonal corrections.
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 To display histogram data
for a portion of the image, first select that portion. By default, the
histogram displays the tonal range of the entire image.
2 Choose Image > Histogram.
To include data from spot
channels and alpha channels, hold down Alt (Windows) or Option (Mac OS)
when you choose Image > Histogram; then choose the desired channel from
the pop-up menu.
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.