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Printers - DPI Printing Guide
DPI vs PPI
There is a lot of confusion between DPI (dots per inch) and PPI (pixels per inch). Traditional printing methods use patterns of dots to render photographic images on a printed page. Dots per inch (DPI) refer to printed dots and the space between them. Printed dots have space between them to make white, or no space between them to make black. Color photographs are printed using four inks, Cyan, Magenta, Yellow, and Black (CMYK), and four separate dot patterns, one for each ink.
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DPI is probably the most familiar and most misused measure of resolution. It is the measure of how many dots of ink or toner a printer can place within an inch (or centimeter). Most printers print the same number of dots horizontally and vertically, though some may have differing numbers. Basically, 600 dpi printers print 600 tiny little dots across one inch and 600 dots vertically for one inch.
Even though "dots per inch" (DPI) and "pixels per inch" (PPI) are used interchangeably by many, they are not the same thing. Pixels per inch (PPI) refer to the square pixels in a digital image, which are in contact with the adjacent pixels. There is no space between the pixels. The PPI is the display resolution - not the image resolution.
A digital image is what it is. It is however many pixels wide by however many pixels tall. If you divide each dimension by 300, you will have the size of the image at 300ppi. Now think about 300 pixels in an inch of space. Each pixel could be black, white, or any other color, but they are all next to each other with no spaces between them.
When a digital image is prepared for reproduction on a printing press, pixels are converted to dots. Dots have spaces between them. 300 pixels become 150 dots and spaces, so 300ppi becomes roughly 150dpi. 150dpi (or 300ppi) is the accepted standard for printing photographic quality images because the average person cannot see the "dots" at a few inches away.
How DPI Relates to Inkjet Printers
DPI does not correspond directly with PPI because a printer may put down several dots to reproduce one pixel. This is because printers use a limited number of colored inks to reproduce an image consisting of millions of colors. The higher a printer's DPI, the smoother your printed image will appear, provided you have a suitable amount of image resolution (PPI).
Basically the term DPI refers to the resolution of the printing device, where PPI refers to the resolution of the image itself. How can you remember this? Monitors display pixels, and printers produce dots.
Today's photo-quality ink jet printers have DPI resolution in the thousands (1200 to 4800dpi). They will give you acceptable quality photo prints of images with 140-200ppi resolution, and high quality prints of images with 200-300ppi resolution. Typically inkjet printers have three standard output settings:
At one time, a good general rule for inkjet printing was that you needed half to one-third of the PPI of the printer's DPI setting that you intend to use. So if you're using your printer's "normal" setting (300dpi), your image needed to have at least 150ppi. When using the higher quality printer settings (720dpi and up), you can bring the PPI down to about 1/3 of the output resolution. That would be 240ppi for your printer's 720dpi setting.
Summary: You rarely need your image resolution to be higher than 240-300ppi for inkjet printing.
How PPI Relates to Digital Cameras
When you're printing images from a digital camera, you have a fixed number of pixels which is the maximum your camera is able to capture. Today's lower-priced cameras have an average maximum resolution of 2048 x 1536 pixels (3 Megapixels).
The following chart shows the maximum print size for digital cameras based on the Megapixels of the camera. [A megapixel is 1 million pixels. It's an area measurement like square feet.] Most books and magazines require 300ppi for photo quality. For example, the chart shows that you can make a 5" x 7" photo quality print (300dpi) from a 3 Megapixel camera.
Megapixels vs. Maximum Print Size Chart
**At 150ppi, printed images will have visible pixels and details will look "fuzzy".
Using this table we can calculate that with 2048 x 1536 pixels we can either get a normal quality print at 8" x 10", a photo quality print at 5" x 7", or the highest photo quality print at 4" x 5" maximum. That's not a very big image for a photo quality print, so if you thought you'd be producing photo quality 8" x 10" prints with your $200 digital camera and inkjet printer, you're probably going to be somewhat disappointed. As you can see, there is a choice to be made as to whether you want to sacrifice printed size or image quality. If you plan to print many 8" x 10" (or larger) photo quality prints, you should have at least an 8 Megapixel camera.
Print size and print quality are dependent on pixel resolution. Notice that as the print size doubles, the megapixels required increases geometrically. You can make nice 8" x 10" prints with a 6 or 8 Megapixel camera, but to make a true photo quality 16" x 20" print, you need between 24 and 30 Megapixels. Don't be fooled by manufacturers' claims that say you can make 16" x 20" prints from an 8 Megapixel camera. While you certainly can make a print that size, it will not be true photo quality and the image will not be as sharp.