The old builder’s adage “measure twice; cut once” holds just as true in digital imaging as it does in woodworking. If you don’t measure your images carefully in the correct units, you might end up with an image that is too small or too large for your purposes.
In the Units & Rulers preferences you can choose your units for rulers, column sizes, resolutions, as well as the point or pica sizes.
1. If you are a Macintosh user, go to the Photoshop menu and select Preferences -> Units & Rulers to open the Units & Rulers Preferences dialog box. If you are working on the Windows platform, choose Edit -> Preferences and select Units & Rulers. If you are in the dialog box from the previous task, select Units & Rulers from the drop-down menu at the top of the dialog box.
2. In the Units & Rulers preferences dialog box, under Units, you can select several units for Rulers: pixels, inches, cm (centimeters), mm (millimeters), points, picas, or percent.
3. Under Units , you can select the units for Type: pixels, points, or mm. A pixel is on grid unit on a computer screen. One point is equal to 1⁄72 inch and 25.4 millimeters (mm) is one inch.
4. Under Column Size, you can specify the Width and Gutter measurements for placing images into a desktop publishing program. These settings enable you to precisely place an image in a set number of columns.
5. The print and screen resolutions are set under New Document Preset Resolutions. When you are creating a new image, Photoshop presents the values you place here as editable settings before creating the image. These values can be set in pixels per inch or pixels per centimeter.
6. Under Point/Pica Size you find two options: PostScript and Traditional. PostScript sets picas at a value of 72 pixels per inch (ppi), whereas Traditional places the value at 72.27 ppi.
7. Another way to change units is in the Info palette, which also changes the preference setting. To change the units through this alternative method, select Windows -> Info to open the Info palette.
8. Click the crosshairs in the lower lefthand corner of the Info palette. Select the units you want from the drop-down menu with the available units will appear.