May 2000

Understanding Digital Image Resolution

Resolution of digital images is one of the least understood topics in electronic presentations and publishing but is one of the most important to understand. Understanding what resolution is and how it affects the quality and file size of your images will help make sure your images contain enough picture detail for your output and that your computer screen shows run quickly and smoothly and Web pages load quickly.

Resolution is defined as a bunch of dots that are assigned or mapped to a given space. In digital images, these dots are referred to as pixels. On laser printers, these dots are called dots. For printing presses, these dots are called lines. Pixels are not the same thing as the dots or lines of print and should not be confused.

Think of your digital images as being made up of tiny squares of information like a checkerboard. These tiny squares are called pixels. Pixels contain color, grayscale or black and white information about each tiny square of the image. Resolution of digital or bitmapped images is the number of pixels per inch of the image. The more pixels per inch (the higher the resolution), the smaller the pixels, the more detail the picture will have and the larger the file size will be. Fewer pixels per inch (lower resolution), the larger the pixels, less detail in the picture and smaller file size. The goal is to have the correct number of pixels needed for the output. This will result in an appropriate file size and good picture quality.

Computer monitors, projection equipment, and slide imaging cameras all have different pixel measurements. Computer monitors and projection equipment usually have a pixel resolution of 640w x 480h, 800w x 600h, 1024w x 768h, or more. These numbers indicate how many pixels wide and high the screen on your monitor is measured. For 35mm slides, the pixel dimensions are usually either 2000w x 1333h or 4000w x 2666h.

When you incorporate digital images into these different outputs, the goal is to match the number of pixels in the image with the pixel dimension it will occupy on the screen or slide. For instance, if your output is 800w x 600h and you want the image to occupy one-half the width of the computer screen, the image must be 400 pixels wide.

JPEG (JPG) images are used on Web pages and work great when importing into a computer screen show. Since JPEG's are a compressed file format, the files are much smaller than an original format file such as TIFF. This helps speed a computer show up. However, always save your file in the original file format (TIFF) in case editing is needed later on. Editing a compressed file format (JPEG) will cause degradation of quality in the image since each resave compresses the file more, thus throwing away information that may be vital for a good quality image.

~ Lana Johnson

Editor's Note: For a visual presentation on image resolution, please see the "Tips for Effective Presentations" seminar video.

CIT Information is published by Communications and Information Technology - Computing section, Institute of Agriculture and Natural Resources, University of Nebraska-Lincoln. Newsletter articles may be copied and distributed for nonprofit, educational purposes only and the source must be acknowledged. Direct all correspondence to the editor, Pamela K. Peters (E-mail: pkpeters@unlnotes.unl.edu; Phone: 402/472-5630; FAX: 402/472-5639).

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