Color is not the only method of varying an image. An image may also be varied by increasing or decreasing the number of pixels the computer image uses to represent the original object, its resolution.

The term ‘dpi’, or dots per inch, is used to measure the number of pixels in the computer image relative to each linear inch, horizontally or vertically, of the original object. Therefore, for an object scanned at 100 dpi, a horizontal or vertical line one inch long is represented by 100 pixels in the computer image. Every square inch of the object is represented by 100 pixels horizontally by 100 pixels vertically, hence making a square containing 10,000 pixels in the computer image.

The same square inch captured at 300 dpi would measure 300 pixels by 300 pixels, with a total of 90,000 pixels. The greater the number of pixels, the higher the resolution of the computer image, the larger the image itself, and the larger the resulting file.

These three images are graphic examples of some of the issues having to do with resolution. The image on the left was scanned from the original at 300 dpi. Thus, one inch of the original picture is represented by 300 pixels. The image in the center was scanned at 100 dpi; one inch of the original picture is represented here by 100 pixels. It is one-third the size of the image on the left. The image on the right was created by extending the pixel size of the 100 dpi image to 300 dpi, using Adobe Photoshop, with a rather blurry result.

You can see how important it is to consider the actual size of your materials relative to the size you want them to appear on the computer screen. It is also better to scan your materials at a higher resolution than you think you will need and then reduce the size or dpi of the image if you need to; if you attempt to go from a lower resolution to a higher one, your result will look something like the image on the right. At the same time, it is important not to overdo resolution; for every image there will come a point where higher resolution will not make the image any more clear, but it will continue to add to the size of the file.

For the Holyoke-Reaves project, the author initially scanned the images in at 300 dpi, then saved these in JPEGs as the master images. She then modified these, creating small images at 100 dpi and larger images at 200 dpi and 300 dpi, depending on the detail of the image.

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