Image Resolution and Output size
Graphic files that will be output for print often contain art that is resolution dependent. The print quality
of this type of art, called raster art, is dependent on the resolution of the art and the final output size of the document. These graphic files may also contain resolution independent art, which is called vector art. Graphic artists must carefully consider the type of art used in their document to achieve the proper print quality.
High quality printing requires higher image resolution than necessary for display on a computer monitor. Raster art is created using horizontal and vertical rows of pixels to represent the image data. The number of pixels in a row that is 1-inch high or wide is called pixels per inch (ppi). The resolution of an image is determined by dividing the ppi of one physical dimension by the physical size of the corresponding dimension. For example, an image with 288 pixels in each row and 4 inches wide would have a resolution of 72 ppi (288 ÷ 4 = 72). However, the same 4 inch wide image with 1200 pixels in each row would have a resolution of 300 ppi. While these 2 images have the same physical dimensions, example 2 would produce a much better quality printed image. Thus print quality images require higher resolution and have much larger file sizes than images used for web sites.
General rules for print image quality
The good rule of thumb for images used in printed output is to use a default of 300 ppi at the final output size. This is sufficient resolution for traditional printing at 150 line screen (also called screen ruling or screen frequency) or below. A more exact method is to use an image resolution of 2 times the line screen. 300 ppi would also be sufficient for output on a digital press at the same final size.