The visual quality of pictures is mainly the result of their resolution. Resolution is measured in dots per inch (dpi) or pixels per inch (ppi). They’re basically the same thing, though the latter strictly only applies to images use in a digital format.
A Graphic Designer in this day and age may find themselves designing for a wide range of media, from web pages through to adverts for newspapers and magazines. Each of these have very different technical requirements. Printing an image from a web page in a magazine can result in a poorly rendered picture, while using an image that has been saved for a magazine on a web page could leave users waiting for long periods while the image downloads.
Pictures on the Web
Web pages are displayed on monitors that have a resolution of 72dpi. Despite this relatively low resolution, images appear well defined when viewed on screen. If a viewer studies an image more closely, however, the dots are quite obvious and it becomes even more obvious when a 72dpi image is printed out. On paper, images from a web page seem very poorly defined. These images are often referred to as ‘Low-res’.
Images for Print
There are many different types of printed materials and the necessary resolution of images varies quite widely between them.
One simple rule to remember is that an image intended for use in a print job should never be less than 150dpi, more than twice the resolution of images used on the web.
Print outs from most desktop printers will not show any benefit from images of more than 150dpi, though these will generally only be used for proofing to clients. Images intended for reproduction in newspapers will usually be saved at a resolution of 150dpi to 175dpi. Newspapers are produced to tight deadlines and lower resolutions make for smaller file sizes, which can make the output of printing plates quicker.
Magazines are generally printed on better quality paper and the quality of images is usually significantly better than in newspapers. Generally, image resolutions for magazines range from 200dpi to 300dpi, though as magazines aren’t usually produced to such demanding deadlines, it is advisable to save at 300dpi, despite the larger file size
Ask for Technical Specifications
The points above are a good guide to suitable resolutions for different media, but the best advice is always to ask for a publisher’s technical specifications and then save images in line with their advice.
Graphic Designers should remember that it may be possible to change an image’s resolution to suit requirements, as long as the number of pixels isn’t increased. If a publisher requests an image of 3 inches width at 150dpi, a 72dpi image measuring 7 inches wide could be converted without any loss of quality.
Thia is because the publisher requires an image that is 450 pixels wide (3 x 150 = 450 pixels) and the Low-res image is 504 pixels wide (7 x 72dpi = 504). If that image is converted to 150dpi, it will measure a little over 3 ¼ inches, which is still more than the publisher requires.