Microsoft Office Tutorials and References
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Resolution on Graphics You Scan Yourself
Resolution on Graphics You Scan Yourself
When you create an image fi le yourself by using a scanner, you choose the resolution,
expressed in dpi, through the scanner software. For example, suppose you scan a 4”
photo at 100 dpi. The scanner will break down each 1” 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
100, or 240,000
pixels. Assuming each pixel requires 3 bytes of storage, the fi ll becomes approximately
720KB in size. The actual size varies slightly depending on the fi le format.
Now, suppose you scan the same photo at 200 dpi. The scanner breaks down each 1” sec-
tion of the photo into 200 pieces so that the result is 4
200, or 960,000 pixels.
Assuming again that 1 pixel requires 3 bytes for storage (24 bits), the fi le will be approxi-
mately 2.9MB in size. That’s a big difference.
The higher the resolution in which you scan, the larger the fi le becomes, but the details of
the scan also become fi ner. 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 11.11 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 signifi cant when the two images are placed on
a PowerPoint slide, as shown in Figure 11.12. 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 11.11
At high magnii cation, the difference in dpi for a scan is apparent.
75 dpi
150 dpi
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