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Welcome to our Tips and Tricks section of the newsletter. Each month we will feature information on something we hope will be found useful! This month we discuss some tips on getting the best print results from your digital images.
PPI vs. DPI
The first important point to understand is the differences between PPI (pixels per inch) and DPI (dots per inch). Many software programs and scanner interfaces use these two terms interchangeably but that's not exactly accurate. As a general rule, and for the purposes of this article, the term PPI should be used when referring to image resolution, and the term DPI should be used when referring to printing resolution. How can you remember this? Monitors display pixels, and printers produce dots. So, as you follow this discussion, whether your software calls it DPI or PPI, when I talk about the resolution of your image, I will be using the terminology PPI.
Pixel Resolution
Digital photos and scans are all bitmap graphic types, no matter what format the image is saved to. That means they are made up of a grid of individual, tiny pixels with each pixel representing a single color in the image. If you zoom in on one of these images (see example) in your imaging software you'll be able to see the individual squares of color.
What this means in relation to printing is that these types of images are resolution-dependent. In other words, the quality of the print and the size of the printed image is limited by the number of pixels in the image. You can't increase one value without effectively decreasing the other.
Inkjet Printer DPI
Today's inkjet printers have three standard output settings:
normal: 300 x 300 or 320 x 320 dpi
high quality: 600 x 600 or 720 x 720 dpi, 1440 x 720
photo quality: 1200 x 1200, 1440 x 1440 dpi, 2880 x 1440 and up
You might also have a draft or economy setting, but you should never use this setting for printing images. It's primarily used for printing text and rough drafts.
A popular myth is that it's necessary to scan an image at the same resolution that you will be using to print. In the case of color and grayscale images, this only results in excessive file sizes. 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 (300 dpi), your image needed to have at least 150 ppi. When using the higher quality printer settings (720 dpi and up), you can bring the PPI down to about 1/3 of the output resolution. That would be 240 ppi for your printer's 720 dpi setting.
Since writing this article photo printers have gotten much better--the dots are smaller and more compact--but still, you rarely need your image resolution to be higher than 240-300 ppi for inkjet printing.
Calculating Image Size and PPI
So how do you calculate the image size you need? It's simpler than you might think. First, decide what size you want your printed image to be, then multiply the height and width in inches by the PPI you need for the print resolution you'll be using. Here's a reference chart for some common print sizes:
Generally, printing photographic images requires more pixels per inch. For photo printing, you will get better results using the higher end of the scale. For scanned illustrations, cartoons, or paintings, you can use a lower PPI and not see a noticeable change in the printed quality.
Article from the Graphics Software Guide at graphicssoft.about.com
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