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Screen Resolution:

Graph of pixels on your screen:

Use a ruler to measure the pixel graph to see what the approximate screen resolution is in pixels per inch(ppi). This will be different depending on the screen resolution and the size of the monitor.
or
Divide the width of the monitor resolution by the width of the monitor in inches.

Example for a 14" monitor set at 600 x 800
Resolution = size in pixels/ size in inches
73ppi = 800 pixels / 11 inches

For more information:
Video Resolution - How Much to Scan? http://www.scantips.com/basics1c.html
Why 72 dpi? - It's a false concept? http://www.scantips.com/basics1a.html

Scan Resolution:

Scan: spi - samples per inch
When you are scanning the photo or object this tells you how many scan (dots) per inch the photo will be scanned at. I have scanned a ruler at 75, 150, 300. As you can see as one doubles the dpi the image on the web gets twice as large.

Even though spi and ppi are the correct terms. We will use spi, ppi, dpi as the same concept - image pixels per inch.

Ruler scanned at 75 dpi:

Ruler scanned at 150 dpi:

Ruler scanned at 300 dpi:

Resampling:

Resampling is when the computer recalculates the dots of the image to adjust the number of pixels per inch. Usually you ONLY use this when changing the image size for web. Usually NEVER for print!

I scanned the ruler at 1200, 600, and 300 and resampled to enlarge the image to 1200ppi. Notice the loss of detail in the two resampling images.

More information: What is Resampling? http://www.scantips.com/basics2e.html

Ruler scanned at 1200 dpi

 Ruler scanned at 600 dpi
resampled to 1200		 Ruler scanned at 300 dpi
resampled to 1200

Printer Resolution:

Printer: dpi - dots per inch

Printer dpi is entirely different, referring to ink dots instead of image pixels You need a 600 or 720 dpi printer to print an image at 150 or 200 dpi

Rule of thumb:

300 or 360 dpi printer – use 100 to 120 dpi image for older

180 to 300dpi image for newer

600 or 720 dpi printer – use 120 to 240 dpi image for laser printers

The middle range is good but go for the high end if you can.

Printer Resolution - How much to scan? http://www.scantips.com/basics03.html

Print Resolution:

Print: lpi - lines per inch

Used for prepress images

Scaled dpi = (lpi x 1.5) x (printed image width/ original photo width)

Magazines 133 to150 lpi – scale to 225 to 300 dpi

Newspapers 85 to100 lpi – scale to 150 to 200 dpi