In PS 5.0 the only choices under Histogram are Luminosity and the three individual channels. Funny, in about 15 years of using Photoshop I hardly ever look at a histogram. : )
From the Photoshop 5.0 Help Menu:
A histogram graphs the number of pixels at each brightness level in an image. It can show you whether an image contains enough detail to make a good correction. It 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 one has detail concentrated in the highlights. An image with full tonal range has a high number of pixels in all these areas. Identifying tonal range helps determine appropriate tonal corrections.
A quick way to adjust an image that̢۪s too dark is to move the white point to where the pixels begin, using the histogram in the Levels dialog box.
To determine whether a scan (sic) captured enough detail for high-quality output, check the RGB pixel values of your highlight and shadow areas in the Info palette—generally, 240 and 10, respectively. The scan must also be free of large gaps within these ranges.
To check the histogram for an image:
1 Open the image you want to examine, and choose Image > Histogram.
Note: To include pixel data from spot channels and alpha channels, hold Alt (Windows) or Option (Mac OS) when you choose Image > Histogram.
Histogram of gray levels in entire image
The x axis of the histogram represents the color values from darkest (0) at the far left to brightest (255) at the far right; the y axis represents the total number of pixels with a given value.
Note: If part of the image is selected, the histogram represents only the selected pixels. In addition, the histogram for an adjustment layer depends on the visibility of the layers beneath it.
The numerical values at the lower left of the Histogram dialog box display statistical information about the color values of the pixels:
â€Â¢ Mean is the average brightness value.
â€Â¢ Standard deviation (Std Dev) represents how widely the values vary.
â€Â¢ Median shows the middle value in the range of color values.
â€Â¢ Pixels represents the total number of pixels used to calculate the histogram.
â€Â¢ Cache Level shows the setting for the image cache. If the Use Cache for Histograms option is selected in the Memory & Image Cache Preferences dialog box, the histogram is based on a representative sampling of pixels in the image (based on the magnification), rather than on all of the pixels.
2 For RGB, CMYK, and indexed-color images, choose an option from the Channel menu. You can plot either the color values in the individual channels or the luminance of the composite channel (with the Luminosity option).
3 To view information about a specific point on the histogram, place the pointer there. To view information about a range of values, drag in the histogram to highlight the range.
The values at the lower right of the dialog box change to display the gray level (Level) of the point (from 0 to 255), the total number of pixels at that level (Count), and the percentage of pixels at or below that level (Percentile).
Message edited by author 2004-08-06 18:39:26. |
