Why web graphics don't work in print...

Several Reasons. Most web graphics are low resolution - usually 72 dpi (dots per inch). Color separation is often a problem... just because you see the colors you want doesn't mean the graphic will print separations as needed. Web graphics use an Indexed color or an RGB color space instead of the Spot Color or CMYK color space needed to print. File formats are sometimes not supported in high resolution printing and resizing the graphics can cause it to become blurry or pixelated. They are raster images (as opposed to vector images) that are resolution-dependant. In general, graphics created for the web don't have the quality needed to reproduce well in print.

File Formats...

First let's define the two types of graphics that are generated from a computer. All photographs and graphics created in Image Editing programs (such as Photoshop or Corel Paint) generate a file that is a "raster" image. That means that the quality of the image is dependent on the resolution and size that it will be printed at - "resolution dependent." (See the section on resolution for a discussion of how all that works.) Resizing of the graphic will change the quality. Programs such as Illustrator, Corel or Freehand can create vector art files (as well as being able to export raster images). Vector art is not dependent on resolution and can be used at any size and resolution without loss of quality.
Now what about all those different formats in the "save as" dialogue box? The format that you select here helps determine how your picture will look when output. Vector Images are saved in EPS (Encapsulated Postscript) and WMF (Windows MetaFile) format for placement into other programs. Note that if you are working on a Windows system without a postscript printer, your printer cannot accurately use the EPS format for printing. Both formats are editable and can generally be imported or opened in a vector art program since they are self-contained pieces of coding. This is the preferred format for logos since they are easily editable and can be used at any size. Raster art can be saved in a variety of formats... including the EPS format listed above, but we are just going to talk about the most common formats here.
Let's start with the formats that are common to both the Macintosh and Windows systems. With the onset of digital photography and online pictures - JPEG (.jpg) is probably the most common. JPEGs are files that have used a compression method to keep the size of the file down. For print media, use a maximum or high setting. For web (depending on size and load time) use the medium to high setting. These are continuous tone images (such as photographs) and do not support any kind of transparency. JPEGS are usually not the preferred format for your print professional - generally they will prefer a TIFF format (Tagged-Image file Format - .tif). This is an easily cross-platformed image format and it is supported by virtually all programs. TIFFs can be used as RGB, CMYK or Grayscale color modes. Line art is usually saved as a TIFF also. Photoshop also has the ability to save it's layered files in a TIFF format, allowing you to keep one version of the file instead of a layered file for working and a flattened image to place. EPS files (as discussed above) can also be used on both platforms, but Mac files use a PICT or TIF preview and Windows files use a TIFF or BMP preview - so when saving Raster images as an .eps file remember to save the proper preview for the platform it will be used on.
Formats suitable for use on a specific system include the BMP (Bitmap format) for Windows and PICT for Macintosh. We prefer not to use these formats because of the limitations on cross-platforming and the quality.
To sum it all up - a TIFF image is your safest bet if you do not know what platform your pictures will end up on for Raster images. Each format has it's drawbacks, but we have found TIFF files to be the least troublesome. If you have any questions on what format to save a file in, contact us or your printer.
A Special Note on PDF files
When saving a complete job as a PDF you must remember a few basic rules. Always include all fonts in your job. Keep compression to a minimum - saving with a maximum quality setting. If at all possible, view your PDF file on a computer that does not have access to images and fonts you used in the file to make sure that all pages, fonts and images are appearing. Most printers now accept files in PDF format for output - but the responsibility for the set-up will be yours, so always check with your printer to ensure you are using the settings compatible with their system.

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