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Understanding Image Resolution Sizes
Image resolution is often the bane of desktop publishers. It is the issue that turns images that look great on screen into jagged, posterized graphics on the printed page.
The first thing a desktop publisher has to realize is that the ‘What you see is what you get’ concept in desktop publishing (or WYSISWYG as it is commonly referred) only goes so far. Yes, you see the visual on screen pretty much as it will appear in your document, but what you don’t see is the quality of the graphic in your printed document. This is because your monitor is optimized for 72 to 75 dp (dots per inch); due to the science of optics (which is far too complicated for us to get in here), that is the optimum resolution for a graphic to appear clear to the human eye. When the eye views a printed piece, on the other hand, a 75 dpi graphic looks fuzzy and jagged. A printed graphic needs to be of a much higher resolution.
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Higher Resolution is Preferred for Images and Graphics
How much higher? In order to determine that, a quick primer on the difference between dots-per-inch and another industry term, lines-per-inch (lpi) is in order. Again, these are complicated concepts, but for the desktop publisher it suffices to say that dpi is the measure he or she will be dealing with, lpi (which is also referred to as line frequency or screen frequency) is a concept that an offset printer will be concerned with. A standard magazine or corporate newsletter will usually use 133 lpi, a high quality print may use up to 300 lpi, while a newspaper (on the lower end of the scale) may only use only 85 lpi. The standard rule of thumb on graphic resolution is to make sure that an image’s dpi is double (or at the very least 1.5 times) the lpi that will be used to print the document. Thus, if the magazine you are designing is going to be offset printed using 133 lpi, graphics should be from 200 to 266 dpi.
Keep in mind that the effective dpi of a graphic changes as you blow it up or scale it down in your document. For example, if you scan a 5 by 8 inch photo at 300 dpi, drop it in your document, and then blow it up to double size using your page layout software, it is now effectively a 150 dpi graphic, below the optimum range you should be shooting for. The reverse holds true. If you reduce it to half size, it is now really a 600 dpi graphic, which is well above the required resolution (unless you are printing a high quality lithographed print). A graphic with too high of a resolution will not degrade the quality of the printed document, but it will make your file unnecessarily large.
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In an ideal situation, desktop publishers would have access to graphics that were of the optimum resolution using the above rule. As anyone who has worked in the industry knows, however, we live in a less than optimum world. Sooner or later every desktop publisher runs into a situation where a client hands him or her a CD full of 75 dpi files, telling them that they want them blown up big in the printed document. What to do then? Check out our next article in the process, which looks at dealing with low resolution graphics that need to be turned into high resolution graphics.