DPI has become the accepted and most commonly used term when referring to image resolution ( 300 dpi, 72 dpi, etc.) . DPI really refers to dots in printing and PPI (Pixels Per Inch) is a more accurate term for digital images whose "dots" are pixels. However since DPI has become the defacto accepted term and is used widely to mean Pixels Per Inch, we have come to accept DPI as interchangeble for PPI.
300ppi (or dpi) is the accepted standard in stock photography because it meets the needs of most high quality reproduction. Magazines, books and most other high quality publications are typically printed at a 133 or 150 Line Screen ( LPI for Lines Per Inch). The common rule of thumb for determining the dpi required for print is to double the LPI, so for a 150 Line Screen a 300 ppi image resolution would be appropriate. Newspapers typically use a lower 85 Line Screen so 170 ppi is all that would be needed.
So an 8 inch by 10 inch image with a 300ppi resolution would have a pixel dimension of 2400 px (8x300=2400) by 3000 px ( 10 x 300=3000). That same image could be reproduced for a newspaper requiring ony 170ppi at up to 14 inches x 17.6 inches ( 2400/170 = 14 and 3000/170 = 17.6 ).
A digital image's size is really dependent on the number of pixels it contains.
| W x H in pixels
| PPI ( or DPI)
| Line Screen
| Max. repro size in Inches
| Example end uses
|
| 2610 x 3900 |
300 | 150 |
8.7" x 13" |
magazine, book, calendar |
| 2610 x 3900 |
266 | 133 |
9.8" x 14.6" |
magazine, book, calendar |
| 2610 x 3900 |
170 | 85 |
15.3" x 22.9" |
newsprint |
| 2610 x 3900 |
| 10* |
261" x 390" (32.5 ft) |
billboard |
| 3930 x 6000 |
300 | 150 |
13.1" x 20" |
magazine, book, calendar, poster |
| * Line Screen used for this example is common but actual LPI may vary depending on printing method and viewing distance |
For more articles on Image Resolution and how it relates to print quality see:
UPDIG Guidelines - Resolution
Understanding Resolution at fotofinish.com
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