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factfile3.12 Resolution


Before you start to scan you need to understand something about image resolution.

Resolution determines the quality of a scan or digital photo: the higher the number of dots per inch (dpi), the finer the quality, but the larger the file size. An A4 scan at a high resolution and at 100% of the original size can use so much RAM (Random Access Memory) that it takes a long time for the software to apply any formatting and may even cause your computer to crash. Try scanning large originals at a scale of 50% or even less.

Interpolation is a process used to enhance the resolution of an image by creating extra pixels in between the ones actually scanned in. Interpolated resolution gives a misleading measure of actual resolution, making it seem higher.


Optical resolution gives a true measure of a scanner's capability, being the amount of information its optics can sample as the CCD moves across the image, typically 300 x 600 to 600 x 1200. When considering a scanner's specifications this is the figure to take into account.

Remember these three points:

1. Many printers have an optimum resolution of 300 to 400 DPI (dots per inch). Check yours. There's no point in scanning work at a higher resolution than your printer can output.

2. The higher the resolution, the bigger the file size.

3. For web graphics or multimedia presentations 72 PPI (pixels per inch) is adequate for viewing on most monitors.

A typical 17-inch monitor might have a horizontal screen area of approximately 12 inches. If your screen resolution is set to 1024 x 768, then 1024 pixels = 12 inches, or 1 inch = about 85 pixels. Round the figure up to 100 for a rough calculation. If the scan or other digital image is for output to a monitor screen at the same size as the original, the resolution needs to be about 85 DPI. For a screen resolution of 800 x 600, the resolution only needs to be 67 DPI. 72 DPI is a good compromise.

Remember DPI for print (dots) and PPI for screen (pixels).

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