Adobe Photoshop CS3 - Manual (Page 504)

PHOTOSHOP CS3
User Guide
497
Trapping to correct misalignment
A. Misregistration with no trap B. Misregistration with trap
Trapping is intended to correct the misalignment of solid colors. In general, you don’t need traps for continuous-tone
images such as photographs. Excessive trapping may produce an outline effect. These problems may not be visible
on-screen and might show up only in print. Photoshop uses standard rules for trapping:
• All colors spread under black.
• Lighter colors spread under darker colors.
• Yellow spreads under cyan, magenta, and black.
• Pure cyan and pure magenta spread under each other equally.
1 Save a version of the file in RGB mode, in case you want to reconvert the image later. Then choose Image > Mode >
CMYK Color to convert the image to CMYK mode.
2 Choose Image > Trap.
3 For Width, enter the trapping value provided by your print shop. Then select a unit of measurement, and click OK.
Consult your print shop to determine how much misregistration to expect.
Determine scan resolution for printing
You can use a number of techniques to determine the resolution at which to scan a photograph. If you are scanning
photos to be printed, and you know the exact size and the printing screen frequency, you can use the following
techniques to determine the scanning resolution. Often it is easiest to scan at your scanner's maximum optical
resolution and then resize the image in Photoshop later.
Estimate scan resolution
You can determine the resolution for your scan using the original and final image dimensions and the resolution of
your output device. Scan resolution translates into image resolution when you open the scanned image in Photoshop.
1 Do one of the following:
• For laser printers and imagesetters, multiply the printer’s screen frequency by 2. To determine your printer’s screen
frequency, check your printer documentation or consult your service provider.
• For inkjet printers, check your printer documentation for the optimal resolution. Many dye sublimation printers
and devices that print directly onto photographic paper have an optimal resolution of 300 to 400 dpi.
A B