What in the World is "DPI"?
Jul 23 '01
The Bottom Line DPI is a measure of the amount of detail in an image, independent of the image's physical size. Read on for more info on dealing with this measurement.
There are two sets of numbers to consider when scanning an image, the physical size of the image (width and height), and how detailed the image data is ("dots per inch"). The physical size of the image is no problem, say, a 5x7-inch photo. The dots per inch, or DPI, can be a little more tricky to deal with. This number tells how much detail is packed into the image file. A 600 DPI 5x7 image will show a lot more detail than a 96 DPI 5x7 image. The higher the DPI, the more detail you get, and the larger the resulting image file.
To get the actual pixel resolution of an image, simply multiply the DPI by the image size. A 300 DPI 5x7-inch image is 1500x2100 pixels. A 96 DPI 5x7-inch image is 480x672 pixels.
The confusion arises when attempting to match DPI to a display device, such as a printer or your computer monitor. Most printers are 300 to 2880 DPI, while most monitors are 72 to 100 DPI.
When you send an image to the printer, the software usually resizes the image to match the printer's DPI. This is good, since a 5x7-inch scanned photo will usually print at 5x7 inches on your printer, no matter what DPI you scanned it in at. The effect of the DPI on a print is the amount of detail in the printed photo. If you scanned it at 96 DPI, you will see a low-detail, blocky, pixellated print, as the printer can print at much higher detail and will show every "dot" in the photo as a little block of color. If you scanned it at 300 DPI or above, you will have a smooth, very detailed print. Scanning photographs beyond 300 DPI is usually not very useful, unless you are going to blow it up. The higher the DPI, the more information that is being packed into the image file, and the larger the resulting image file.
When you view an image on your monitor, it is typically displayed dot-for-dot, regardless of the DPI. Most software does not scale the image to display the correct size on you monitor. (The exception to this would be desktop-publishing programs, which must pay attention to DPI.) If you display a 300 DPI 5x7-inch image on a 96 DPI monitor, then the image will appear more than three times its size (about 15x22 inches). If you display 96 DPI on a 96 DPI monitor, then a 5x7-inch photo will appear around 5x7 inches. Monitor DPI is usually not a precise measurement; any difference depends on your monitor's particular settings.
So, if you are going to scan a photo for e-mail or for a web page, scan it at 96 DPI, or the average monitor resolution (so that's where that funny number came from). If you are going to scan it to print out, scan it at 300 DPI or a resolution that is an even division of your printer's resolution. Doing this will allow the print software to scale the image evenly for output to the printer. For example, Epson inkjets print at a multiple of 360 DPI. Scanning at 180 or 360 DPI for these printers will reduce fuzziness while maximizing image detail. Scanning at 1440 or 2880 DPI is overkill, and won't gain you very much, except a very large file.
There is one caveat: some scanners don't perform well below 150 DPI. You will get a picture that is full of "the jaggies". When you scan below the hardware resolution of a scanner, each sensor in the scanhead takes a narrow sample of the photo, leaving out the pieces between the sensors that are being used. If you scan at 96 DPI on a 1200 DPI scanner, the scanner is only using every 12th sensor on the array. This is like trying to look through a picket fence or a piece of pegboard. To avoid this effect, first scan at 150 DPI or higher, then reduce the photo to 96 DPI through software. The software that came with your scanner should be able to do this sort of scaling.
If you want to blow up an image, yet keep a good amount of detail, then you should scan at a higher DPI. Say you want to print a 8x11-inch photo from a 5x7-inch print, and you want the detail of 300 DPI. Get out the calculator. You want to scan at 11/7ths (or 8/5ths) of 300 DPI, which is about 470 DPI. After scanning the image, you will want to change the DPI without rescaling the image. This can be tricky, as different programs will do this differently. Most will rescale the image data when you change the DPI. The trick is to get the DPI and the image size to change at the same time. In this case, you want to change a 470 DPI 5x7-inch image into a 300 DPI 8x11 inch image. For example, Adobe Photoshop allows you to change the DPI without rescaling the image by unchecking a checkbox in the "Image Size" dialog. When you type in a new DPI, the image size measurements change automatically.
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Member: Greg Searle
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Computer graphics designer and software developer with interests in technology, home improvement, and family life.
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