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CompTIA A+ Certification All-in-One Exam Guide

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How to Choose a Scanner

You must consider four primary variables when choosing a scanner:

resolution, color depth, grayscale depth, and scan speed. You can and will

adjust the first three during the scanning process, although probably only

down from their maximum. The scan speed relates to all four of the other

variables, and the maximum speed is hard-coded into the scanner.

Configurable Variables Scanners convert the scanned image into a grid of

pixels (often referred to as dots). The maximum number of pixels determines

how well you can capture an image and how the image will look when scaled

up in size. Most folks use the term resolution to define the grid size. As you

might imagine, the higher-resolution images capture more fine detail.

Older scanners can create images of only 600 × 600 dots per inch (dpi),

while newer models commonly achieve four times that density, and high-end

machines do much more. Manufacturers cite two sets of numbers for a

scanner’s resolution: the resolution it achieves mechanically—called the

optical resolution—and the enhanced resolution it can achieve with

assistance from some onboard software.

The enhanced resolution numbers are useless. I recommend at least 2400 ×

2400 dpi optical resolution or better, although you can get by with a lower

resolution for purely Web-destined images.

The color depth of a scan defines the number of bits of information the

scanner can use to describe each individual pixel. This number determines

color, shade, hue, and so forth, so color depth makes a dramatic difference in

how easily you can adjust the color and tone in your photo editor. With

binary numbers, each extra bit of information doubles the color detail in the

scan. The most common color depth options you will run across in scanners

today are 24-bit and 48-bit. A 24-bit scan, for example, can save up to 256

shades for each of the red, green, and blue subpixels that make up an

individual pixel. This gives you a total of 16,777,216 color variations in the

scanned image, which explains why some scanners refer to this as “millions

of colors” in their settings. A 48-bit scan, in contrast, can save up to 65,536

shades per subpixel, giving you a scan that holds a massive

281,474,976,710,656 color variations. All this extra color does come with a

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