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What is important to understand about image resolution measurements?



Digital Image File Resolution

Digital image file resolution is one subject that confuses a lot of people but it is not really so difficult to understand. Pictures should always be supplied at an effective resolution of 300 pixels per inch (PPI) for best results when printing on professional printing presses. Just what does that really mean?

DPI (Dots per inch):

DPI is the number of individual pixels located within one inch that can be produced by any printer or output device. This is probably the most commonly and most misused measure of resolution. Laser printers are usually capable of 300, 600, or 1200 DPI while Imagesetters and Platesetters used in professional color printing are usually either 2400 or 2570 DPI devices.

PPI (Pixels per inch):

PPI usually refers to the number of pixels per inch present in a digital photograph or other bitmap image. A digital photograph or other bitmap image is made up of tiny squares of color. These tiny squares or pixels (short for picture elements) act exactly as a tile mosaic to build up a image. The smaller the pixels are, the greater their density (more per square inch) and this results in greater image detail. Think of the mosaic: the smaller the tiles the more intricate the design can be. Professional printing uses a standard 300 PPI minimum effective resolution.

Note: PPI may also be used to refer to how a computer monitor displays an image. The amount of pixels is directly related to the size of the monitor as well as the number of pixels in the horizontal and vertical directions. This is the display resolution, not the image resolution. Do not confuse the two meanings. Macintosh computers traditionally displayed screen images at 72 PPI and Windows PCs at 96 PPI. Most modern computers today displays approximately 96 PPI.

LPI (Lines per inch):

LPI is a measure of the number of lines of dots per inch which are used in a traditional halftone grid to reproduce continuous tone images or photographs. The higher the line screen number the sharper the overall image on paper. Newspapers will often use 85 to 100 LPI (in part because of the relatively poor newsprint paper quality) while color printers such as BestPrintingOnline may use up to 175 LPI. This results in much sharper image detail. This method of describing resolution is used in output systems which use traditional halftone screens. Certain printers and some color presses such as the HP Indigo may use other methods than halftone screens. The most common of these alternate screening methods is often called stochastic screening and usually renders a series of randomly placed dots instead of the typical regular pattern of conventional screening.

Please see the Image Resolution page in our Help Center section for a detailed explanation of image resolution and how to get the best results when using your digital photographs for print reproduction. This page explains just what resolution is, what it looks like when printed (high and low resolution) and how it is affected when pictures are placed within a document and scaled up or down. When we are through you will understand effective resolution and will be able to figure scale and place pictures just like a pro.

You will also find a handy reference table there that shows the proper reproduction size for a variety of common digital photo sizes and their corresponding approximate CMYK Tiff file size.

Back to File Prep FAQs.