A smartphone, tablet, or laptop with a sharp display can make the world of difference when browsing your buddies Facebook photos. If you’re on the hunt for a really sharp display on your mobile device, you’ll see terms like “retina,” which describes sharpness through a measurement called pixels-per-inch (ppi).
For large displays on computer monitor and TVs, a display’s sharpness has typically been characterized by how many pixels it contained, or "resolution." When it comes to resolution, the higher the number, the better. Instead of just stating the number of pixels, TV, and computer monitors often state how many vertical columns and horizontal rows of pixels are displayed. For instance, a standard-definition TV displays 640 vertical pixel columns and 480 horizontal pixel rows. This column and width count is called resolution and is typically written as 640x480. High-definition TVs are sharper because they offer even higher resolution, either 1280x720 or 1920x1080.
But resolution isn't as accurate when it comes to mobile devices because it doesn't take into account display size. Enter pixels-per-inch. These numbers quantify why a smaller display can feel sharper than a larger display.
Take the HTC Thunderbolt, it offers a 4.27-inch display whose resolution is 800x480. This works out to 215 ppi. The HTC Titan II features a massive 4.7-inch display, but it’s 800x480 resolution translates to 199 ppi. So even though the screen is larger, its 199 ppi makes it feel less sharp.
The HTC Rezound has a smaller screen size (4.3-inch), but a resolution of 1280x720. This calculates to 342 ppi--much sharper than 199 ppi on the Titan II, despite its smaller display.
If screen resolution matters, keep an eye on those ppi numbers next time you're shopping for a new device.