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Correct Scan Resolutions

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.

Key Points

PPI refers to image resolution
DPI refers to printer resolution
Inkjets need half the PPI of the printer's DPI setting
Print size and print quality have an inverse relationship
Print size and print quality are dependent on pixel resolution

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
Photo Quality: 1200 x 1200, 1440 x 720, 1440 x 1440 dpi

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. A good general rule for inkjet printing is that you need half 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 needs to have at least 150 ppi

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:

Printer Quality
	Normal	High	Photo
Print @	300 - 320 dpi	600 - 720 dpi	1200 - 1440 dpi
Scan @	150 ppi	300 ppi	600 ppi
Printed Size
2" x 3"	300 x 450 pixels	600 x 900 pixels	1200 x 1800 pixels
4" x 6"	600 x 900 pixels	1200 x 1800 pixels	2400 x 3600 pixels
5" x 7"	750 x 1050 pixels	1500 x 2100 pixels	3000 x 4200 pixels
8" x 10"	1200 x 1500 pixels	2400 x 3000 pixels	4800 x 6000 pixels

The chart above is mainly based on photographic images. If you have a scanned illustration, cartoon, or painting, you can usually go a little lower with the PPI and not see a noticeable change in the printed quality

Why is my scan so HUGE?

Keep in mind that when you scan an image at anything higher than 72-100 ppi, it's going to display much larger on your monitor than the original picture or page. That's because your monitor can only display a fixed number of pixels per inch, usually 96. When you place the image into a page layout program, most software will be able to read the PPI information and will place your image into the page at the proper size. Some of the low-end printing software may not, however, and you'll have a very large image placed into your page. When that happens, you'll have to size the image in your layout program to the printed size you based your calculations on.

PPI and Digital Cameras

When you're printing images from a digital camera, you have to calculate things a little differently. With a digital camera, you have a fixed number of pixels which is the maximum your camera is able to capture. Today's moderately priced cameras have an average maximum resolution of 1200 x 1500 pixels. In this case, we have the pixel dimensions, so we need to work in reverse to figure out the best quality setting and printed size. Using the formula above we can calculate that with 1200 x 1500 pixels we can either get a high quality print at 4 by 5 inches, or a photo quality print at 2 by 2.5 inches. That's not a very big image for a photo-quality print, so if you thought you'd be producing photo-quality 8 by 10 prints with your $300 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.

To review:

PPI refers to image resolution
DPI refers to printer resolution
Inkjets need half the PPI of the printer's DPI setting
Print size and print quality have an inverse relationship
Print size and print quality are dependent on pixel resolution