Adjust tonal range in bitmaps
A
bitmap with a full tonal range must have an even number of pixels
in all areas. Three options are available for adjusting tonal range:
using Levels, using Curves for precise control, or using Auto Levels
for automatic adjustments.
The Levels feature corrects bitmaps with a high concentration
of pixels in the highlights, midtones, or shadows.
- Highlights
- Corrects an excess of light pixels, which makes the image
look washed out
- Midtones
- Corrects an excess of pixels in the midtones, which makes
the image bland
- Shadows
- Corrects
an excess of dark pixels, which hides much of the detail
The
Levels feature sets the darkest pixels as black and the lightest
pixels as white, and then redistributes the midtones proportionally.
This produces an image with the sharpest detail in all of its pixels.
Original with pixels concentrated in the highlights; after
adjusting with Levels
Use the Histogram in the Levels dialog box
to view the pixel distribution of a bitmap. The Histogram is a graphical
representation of the distribution of pixels in the highlights,
midtones, and shadows.
The Histogram helps you determine
the best method of correcting an image's tonal range. A high concentration
of pixels in the shadows or highlights indicates that you could
improve the image by applying the Levels or Curves feature.
The
horizontal axis represents color values from darkest (0) to brightest
(255). Read the horizontal axis from left to right: the darker pixels
are on the left, the midtone pixels are in the center, and the brighter
pixels are on the right.
The vertical axis represents the
number of pixels at each brightness level. Typically, you adjust
the highlights and shadows first. Adjusting the midtones second
lets you improve their brightness value without affecting the highlights and
shadows.
PDF (adobe.com)
