OK, prepare to be temporarily confused (but not for long). A lot of terms are used to describe image resolution. The problem is that many people (and companies) use the wrong terms, which (understandably) leads to a great deal of confusion. Let’s take a quick look at the most common terms and their accurate meanings. Knowing how to describe the resolution of images and output devices will help you make the right decisions when purchasing or choosing gear to use.
Dots per Inch (dpi)
The most common term used to describe image resolution is dots per inch (dpi). Although you’ll hear it used for digital cameras and scanners, it is really only appropriate for printers. As a measurement of output resolution, dpi is fairly straightforward.
To determine dpi, it is necessary to count the number of dots that can fit in a 1″ × 1″ area. A higher dpi can mean smoother photographs or line art; for example, newspapers tend to use around 150 dpi, whereas magazines can use up to 600 dpi. Consumer printers easily print at 600 dpi or even higher, which can produce extremely good results (when using the right paper). An increase in dpi can produce even better-looking images. You’ll see (and hear about) dpi used a lot, but it solely refers to print and physical output.
Pixels per Inch (ppi)
When you view your images on a computer monitor, you are seeing pixels displayed on your screen. Computer monitors use the concept of logical inches. Originally, the Mac OS most commonly used 72 pixels per inch (ppi) to match the concept of the printing idea of 72 points per real inch of paper. The Windows OS has traditionally used 96 ppi.
As computer monitors and portable devices have evolved, they’ve advanced to support variable resolution settings. As such, the actual ppi for a screen can vary greatly depending on the physical size of the screen and the resolution being used by the computer’s graphics card. For example, modern laptops often use resolutions between 100 ppi and 140 ppi, and devices like an iPhone can jump all the way up to 326 ppi to make images crisper on the small screen.
Worry less about the ratio of pixels per inch on your screen and simply accept that the standard measurement of resolution in Photoshop (and most computer programs) is ppi. When talking about displayed graphics, its ppi, not dpi.
This post is from the book Understanding Adobe Photoshop CS6: The Essential Techniques for Imaging Professionals