March 20, 2003

Scanner Basics: Printing Digital Photos & Scans (Part 3 of 3)

The information here pertains to both color and grayscale images that will be printed on an inkjet printer. It does not apply to printing line art 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 rule, and for the purposes of this Tutor Tip, use the term PPI when referring to image resolution and the term DPI when referring to printing resolution. How can you remember this? Monitors display pixels, and printers produce dots.

Pixel Resolution

Digital photos and scans are all bitmap graphic types, no matter what the image format. 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 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 are 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:

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 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 needs 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 ? of the output resolution. That would 240 ppi for your printer's 720 dpi setting.

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.

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 on which you based your calculations.

I Don't Know What the Output Will Be!

Whenever you increase or decrease the pixel dimensions of an image you lose some quality, but it's less destructive to reduce pixel dimensions rather than increase pixel dimensions. If you're scanning for archival or you don't know what the intended output will be, it's better to scan at a higher resolution and reduce the image later. However, it is of no advantage to scan higher than your scanner's maximum optical resolution.

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 (the maximum your camera is able to capture). Today's moderately priced cameras have an average maximum resolution of 1200 x 1600 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 1600 pixels we can get a normal quality print at 8 by 10 inches, a high quality print at 5 by 7 inches, or a photo quality print at 4 by 5 inches maximum. 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 one-half to one-third 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