making the right choice
Using the proper graphic file format and resolution for the job can mean the difference between a professional-looking document and one that looks blurry or is missing graphics. Graphic file formats for the Internet and offset printing are totally different animals. Do not interchange them!
internet file formats
Low-resolution raster graphics are used on the Internet. These graphics are made up of thousands of pixels (squares of colour). Internet browsers will read JPG and GIF graphics, which are best scanned or sized at 72 PPI (pixels per inch). Because of the limits of screen resolution, anything greater will result in larger file sizes and longer download times than necessary. All Internet graphics are limited to a special palette of 256 colours. Scan your photos using RGB colours to the JPG file format. JPG file sizes are very small and compatible with nearly every graphical browser. This format is best suited for photographs and any image that contains a complex mixture of colours. The GIF format is best suited for images with a limited number of distinct colours and graphics that have sharp, distinct edges (most logos, menus and buttons). A special GIF89a file format gives you the option to make the background transparent so you don’t get a white rectangle behind the graphic.
Common raster image file extensions include .BMP, .TIF, .JPG, .GIF, and .PNG.
printing file formats
Graphics for offset printing require much higher resolution than for websites. If you use a low-resolution graphic (i.e., a logo copied from a website) on an offset printed job, a fuzzy “bitmapped” image—or no image—will result. Offset printed graphics can be one of two types: Vector-based or high-resolution raster. Raster images (which are colour or grayscale digital photos and scans) must be at least 300 PPI (pixels per inch) and in the TIF (Tagged Image File) or EPS (Encapsulated PostScript) file format. Your scans of black and white line art (images that do not contain any shades of gray) must be at least 1200 PPI. Be careful not to enlarge your raster graphics, because the pixels will also enlarge and become more noticeable.
Vector-based graphics are made of mathematically defined lines and curves. Because they are not made of pixels, these unique files can be scaled to any size without losing their crisp, smooth edges. Use professional drawing programs such as Adobe Illustrator, Macromedia Freehand, or Corel Draw to create these types of graphics for printing, saving them in the EPS format. Vector graphics are made up of paths, rather than individual pixels. These paths can be used to represent lines and shapes within the image. Most vector image formats can also include colours, gradients, and image effects. Since vector graphics store image data as paths, they can be enlarged without losing quality, which makes them a good choice for logos and other types of drawings.
Page layout files are documents that may contain both text and image data. They also include formatting information, which defines the page size, margins, and how content is organized on the page. Page layout documents are often used for creating printable publications, such as newspapers, magazines, and brochures.
Common page layout file extensions include .PDF, .INDD, and .QXP.
offset printing colour systems
Colour files for offset printing must be specified with PMS or CMYK inks. Do not use RGB colours unless you are planning to print only to a low-end colour desktop printer.
For a more complete list of file extensions and descriptions, please check out www.fileinfo.com/filetypes/common