Looking closely at most commercial printing, dot patterns or screens can be seen in graphics and photographic images. Black and white (greyscale) photos provide the best viewing of these screens made up of “halftone dots”. The varying size of tiny solid dots creates the tones needed for ink to represent different shades of grey.
A color image is displayed with a set of four different color screens overlapping each other. These four colors (“process colors”) are Cyan, Magenta, Yellow and Black (CMYK). Their screens are rotated at specific angles to balance the screen dot postions. This reproduces photographs and graphics in their various hues and tones. Looking at a Sunday comics section in a local paper is a great example for seeing process color screens in action.
These screens are printed with a specific number of lines per inch (lpi). Most commercial printing is 150 lpi. Newspapers generally use around 100 lpi.
Pixels Per Inch for Images
Resolution for images in layouts is different than the lines per inch used in printing. Images for layouts are set up in pixels per inch (ppi). A traditional rule is that layout images are set up as twice the value of the lines per inch for printing. Therefore, if 150 lines per inch is the printing specification, then 300 ppi images should be used. The reason the ppi is double the lpi is that line screen dots are created from pixel values at each corresponding location on the image. More accurate color and sharpness results when more pixel information is supplied.
An layout image can be enlarged and still provide enough information for each line screen dot during printing. It’s best not to enlarge an image beyond 150% the original size since the line screen will start interpreting a blurry original image. Each image will have unique details to consider.
Some argue that the 2:1 rule of image to line screen is excessive and advocate a 1.5:1 rule. In the example above, that would allow a 225 ppi image to be used for 150 lpi printing.
Fixed Resolution for Web Images
Web images are 72 pixels per inch because computer monitors are 72 pixel per inch displays. Web images are set up in RGB format which is presented as various tones of red, green, and blue light. CMYK printing uses physical inks placed on paper. CMYK images will not show up properly in web browsers. Use a photo imaging program to convert a CMYK image to a web RGB image at 72 ppi.
Using Web Images for Printing – a Big Caution
A common misconception is that any image on the web can be used for printing. This can lead to undesirable results. A 72 ppi image cannot be interpreted into 150 line screen without resulting in fuzzy, blurry images. Look at the image’s file size (listed on screen in a stock photo site, or by examining it within an image editing program) to see if it can be used. A good rule to use in determining if a web image can be used for print is this: can it be resampled (squeezing it down so there is more pixels per inch) in an imaging program to make it 1/3 to 1/4 its original size and still be desirable to use? If so, then an image off the web could be used (make sure you have permission to use the image!).