Managing Image Files

One of the most important terms in digital imaging is dpi. It stands for "dots per inch," and is sometimes used interchangeably with ppi, or "pixels per inch."

It refers to how many individual dots (pixels) there are in every inch of an image. The more dots per inch, the higher the resolution. The average computer screen displays 72dpi. An entry-level inkjet printer can print 300dpi. A decent inkjet and many laser printers produce 600dpi or 1200dpi, while photo printers can print 2400dpi or higher. The more dots, the higher the resolution. What the image is being used for determines what resolution is needed.

The following images help to illustrate the relationship between size and resolution:

These three images are all the same size, but they have very different resolution from one another. Image A is 72dpi; it has 72 dots per inch. Image B is roughly 48dpi, and image C is only around 24dpi. As you can see, for an image to appear sharp and well-defined it needs to have a resolution appropriate for its size. If we were to take image A and increase its size to be three times larger, to 9 inches square, its resolution would drop accordingly and the image would no longer be clear, appearing instead much like image C does in our example.

Since a computer screen generally displays somewhere between 72dpi and 96dpi, there is no point uploading an image of higher resolution. Every dot of an image translates into digital information, so there is a direct correlation between the size and resolution of an image and how large the file will be when the image is saved to disk.

File size increases dramatically as resolution increases. Even with compression, high resolution image files can easily exceed 20mb of space. A file that large would take hours to transmit over a 56k modem.

High-resolution images waste library space and will be deleted!

Most scanners allow you to choose what resolution you will be scanning at. Many image editing programs allow you to change the size while the computer works to keep the resolution the same. The name for this can be "Keep Proportions," "Lock Proportions," "Recalculate Image," or something else depending on the program you are using.

adapted from http://de.uoregon.edu/support/graphics/resize.php