When you create an image file yourself by using a scanner, you choose the resolution, expressed in dpi, through the scanner software. For example, suppose you scan a 4-inch by 6-inch photo at 100 dpi.

The scanner will break down each 1-inch section of the photo horizontally and vertically into 100 separate pieces and decide on a numeric value that best represents the color of each piece. The result is a total number of pixels of 4 x 100 x 6 x 100, or 240,000 pixels. Assuming each pixel requires 3 bytes of storage, the fill becomes approximately 720KB in size. The actual size varies slightly depending on the file format.

Now, suppose you scan the same photo at 200 dpi. The scanner breaks down each 1-inch section of the photo into 200 pieces, so that the result is 4 x 200 x 6x 200, or 960,000 pixels. Assuming again that one pixel required 3 bytes for storage (24 bits), the file will be approximately 2.9MB in size.

That’s a big difference. The higher the resolution in which you scan, the larger the file becomes, but the details of the scan also become finer. However, unless you are zooming in on the photo, you cannot tell a difference between 100 dpi and a higher resolution. That’s because most computer monitors display at 96 dpi, so any resolution higher than that does not improve the output.

Let’s look at an example. In Figure 24-36 you can see two copies of an image open in a graphics program. The same photo was scanned at 75 dpi (left) and 150 dpi (right). However, the difference between them is not significant when the two images are placed on a PowerPoint slide, as shown in Figure 24-37. The lower resolution image is at the top left, but there is no observable difference in the size at which they are being used.

FIGURE 24-36
At high magnification, the difference in dpi for a scan is apparent.

FIGURE 24-37
When the image is used at a normal size, there is virtually no difference between a high-dpi and low-dpi scan.

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