Selecting halftone screen attributesHalftone screen attributes include the screen frequency and dot shape for each screen used in the printing process. For color separations, you must also specify an angle for each of the color screens. Setting the screens at different angles ensures that the dots placed by the four screens blend to look like continuous color and do not produce moiré patterns. Halftone screens consist of dots that control how much ink is deposited at a specific location on-press. Varying their size and density creates the illusion of variations of gray or continuous color. For a process color image, four halftone screens are used: cyan, magenta, yellow, and black--one for each ink used in the printing process. In traditional print production, a halftone is produced by placing a halftone screen between a piece of film and the image and then exposing the film. In Photoshop, you specify the halftone screen attributes just before producing the film or paper output. For best results, your output device (a PostScript imagesetter, for example) should be set to the correct density limit, and your processor should be properly calibrated; otherwise, results can be unpredictable. Before creating your halftone screens, check with your print shop for preferred frequency, angle, and dot settings. (Use the default angle settings unless your print shop specifies changes.) To define the screen attributes:
Choosing Custom from the Shape menu displays the Custom Spot Function dialog box. You can define your own dot shapes by entering PostScript commands--useful for printing with nonstandard halftone algorithms. For information about using PostScript language commands, see the PostScript Language Reference published by Addison-Wesley, or consult the imagesetter's manufacturer. For optimal output on a PostScript printer, the image resolution should be 1.5 to 2 times the halftone screen frequency. If the resolution is more than 2.5 times the screen frequency, an alert message appears. (See About image size and resolution.) If you are printing line art or printing to a non-PostScript printer, see your printer documentation for the appropriate image resolutions to use. To save halftone screen settings: In the Halftone Screens dialog box, click Save. Choose a location for the saved settings, enter a filename, and click Save. To save the new settings as the default, hold down Alt (Windows) or Option (Mac OS), and click the --> Default button. To load halftone screen settings: In the Halftone Screens dialog box, click Load. Locate and select the settings, and click Load. To return to the original default settings, hold down Alt (Windows) or Option (Mac OS), and click <--Default. |