This article appeared in the May/June 2002 issue of PT. To purchase this issue and receive this and other valuable articles in this issue, CLICK HERE: Ship within the U.S. | Ship outside the U.S.

Digital images are composed of blocks called pixels. Pixels and resolution are fundamental concepts relevant to every part of the digital imaging chain. When you scan an image, you talk about scan resolution and scanner dpi. If you use a digital camera, you refer to CCDs with 2 or 3 million pixels. Displaying an image on a monitor involves screen resolution, and when printing we use Epson inkjet printers with 1440 or 2880 dpi. The whole digital imaging chain captures, stores, views and prints the image in terms of pixels. Let’s look at how pixel resolution affects image quality, image size, print size and virtually everything else we do in digital imaging.

Scanner resolution
Since this article deals with pixels and resolution, let’s define the terminology. Units for resolution are dots-per-inch (dpi), although some prefer pixels-per-inch (ppi). (In this context, I use dpi and ppi interchangeably.) Printers use another type of resolution—lines-per-inch (lpi). High resolution, such as 300 dpi, refers to a high pixel density of 300 dots or pixels-per-inch. Low resolution, such as 100 dpi, refers to 100 dots-per-inch.

Digital images usually are captured by scanning a transparency or using a digital camera. With a scanner, the transparency is put into the scanner and samples are taken at small regular intervals. Depending on what the scanner “sees,” it generates a pixel. When the entire transparency is completely scanned, the pixels are assembled to make a digital image. The scanner can scan the transparency very finely, recording every little detail, or it can cover the entire transparency much quicker by taking averages over much larger areas. Smaller stops take longer, but the fine detail in the original is retained. If the scanner averages over a larger area, it creates a pixel with the average intensity and color from the area, but the individual hair detail, windowpane or single leaf structure is lost. The dpi setting of the scanner dictates how samples are made.

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