Adobe Photoshop CS4 - Manual (Page 218)

211 USING PHOTOSHOP CS4
Retouching and transforming
Last updated 1/10/2010
Examples of barrel distortion (left) and pincushion distortion (right)
Vignetting is a defect where the edges, especially the corners, of an image are darker than the center. Chromatic
aberration appears as a color fringe along the edges of objects caused by the lens focusing on different colors of light
in different planes.
Some lenses exhibit these defects depending on the focal length or the f-stop used. You can set the Lens Correction
filter to use settings based on the camera, lens, and focal length used to make the image.
Correct lens distortion and adjust perspective
The Lens Correction filter fixes common lens flaws such as barrel and pincushion distortion, vignetting, and
chromatic aberration. The filter works with 8-bits and 16-bits-per-channel images only.
You can also use the filter to rotate an image or fix image perspective caused by vertical or horizontal camera tilt. The
filter’s image grid makes these adjustments easier and more accurate than using the Transform command.
Correct image perspective and lens flaws
1 Choose Filter > Distort > Lens Correction.
2 Set the grid and image zoom. As you work you may want to adjust the grid lines to help you judge the amount of
correction to make. See Adjust the Lens Correction preview and grid, below.
3 (Optional) Choose a preset list of settings from the Settings menu. Lens Default uses settings that you previously
saved for the camera, lens, focal length, and f-stop combination used to make the image. Previous Conversion uses
the settings used in your last lens correction. Any group of custom settings you saved are listed at the bottom of the
menu. See Set camera and lens defaults, below.
4 Set any of the following options to correct your image.
Remove Distortion Corrects lens barrel or pincushion distortion. Move the slider to straighten horizontal and vertical
lines that bend either away from or toward the center of the image. You can also use the Remove Distortion tool to
make this correction. Drag toward the center of the image to correct for barrel distortion and toward the edge of the
image to correct for pincushion distortion. Adjust the Edge option to specify how you want to handle any resulting
blank image edges.
Chromatic Aberration Corrects color fringing. Zoom in on the image preview to get a closer view of the fringing as you
make the correction.
Fix Red/Cyan Fringe Compensates for red/cyan color fringing by adjusting the size of the red channel relative to the
green channel.
Fix Blue/Yellow Fringe Compensates for blue/yellow color fringing by adjusting the size of the blue channel relative to
the green channel.
Vignette Corrects images that have darkened edges caused by lens faults or improper lens shading.