Resolution Printing Knowledge
Resolution

For printed images, the resolution is based on the size of the dots that make up the tonal values of the images. It is usually expressed in dpi (dots per inch). The higher the dpi, the higher the resolution and the higher the quality. 

In scanning, optical resolution refers to the amount of detail captured by the scanner. The optical resolution is expressed as two numbers, such as 600 x 1200 ppi (pixels per inch). The first number is the quantity of pixels per inch that the scanner captures in the horizontal direction. The second number is the quantity of steps passed over as the scanner head moves in a vertical direction. The actual resolution of the image is only the number of pixels per inch, not the number of steps. Some scanner resolutions are referred to as interpolated or enhanced resolution.

Generally, images should be scanned at a resolution of twice the line screen (lines per inch or lpi) that you plan to use for printing, unless you plan on enlarging or reducing the image.

Scanning Resolution  =  2  x  Screen Frequency (lpi)

 

If you are going to be enlarging the image, scan it at a higher resolution and use the following formula:

Scanning Resolution  = 

  Largest Sized Final Image  

  x  2  x  

 Screen Frequency (lpi)

Largest Sized Original Image

 

After you have scanned the image using the proper resolution, go to the "Image Size" dialog box and enlarge your image, with the "Resample Image" check box deselected. Your resolution will then be at the proper setting.

The only exception to this rule is when black-and-white line art images are being scanned. They should be scanned at a resolution which is set as high as possible for the clearest possible reproduction. 1200 dpi is usually the standard setting.

If you don't know which line screen will be used, ask your printer. When asking the printer about the resolution, let them know which kind of paper will be used. Generally, newspapers are printed at 85 lpi, and magazines are printed using either 133 or 150 lpi.

Scanning at a higher resolution than you need only consumes more memory than necessary and produces a larger sized file that is harder to work with. If you scan at too low a resolution, your image may be blurry or you may see the individual jagged pixel elements in the image.


Scanned at 300 dpi at 100%
They look the same

Scanned at 72 dpi at 100%
Looks like the 300 dpi photo

300 dpi enlarged

72 dpi enlarged to show pixelating

 


 

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