The myths about DPI or “dots per inch” are everywhere on the internet. And like any “urban legend”, the more often they get repeated, the more they are accepted as fact. So let’s do some myth-busting and get to the truth.
Myth #1: A high-resolution image is 300 DPI.
False. DPI is a term borrowed from printing, and this myth arose because 300 DPI is the magic number. A photo printed at 300 DPI and held at a normal reading distance of 12” will look sharp because at that resolution and distance, the eye cannot see the individual dots making up the image. A high-quality print of an image requires enough pixels in the file so that it can be printed at 300 DPI. An 8″x10″ print, for example, needs 2400×3000 pixels (inchesxDPI). The number of pixels in a “high resolution” 300 DPI image, therefore, depends on the size it will be printed—so the DPI is meaningless until you want to make a print.
Myth #2: Website images must be 72 DPI.
False. This myth got started with early computer screens that had a pixel density of 72 PPI or “pixels per inch”. The idea was that text would be the same size on the screen as on paper (in typography, there are 72 points per inch). Screens for many computers and mobile devices now have pixel densities of 300 PPI or more. How an online image is displayed depends today on several factors: the number of pixels in the image, the dimensions (size and PPI) of the screen, and the instructions sent to the browser by the website—the DPI of the image is not part of the equation and so makes no difference.
Myth #3: Increase the DPI for more resolution.
False. The origin of this myth lies in the JPEG image format, which includes a value for resolution. The default is 72 DPI, and it can be changed using photo editing software, like Photoshop™ Elements. Changing the DPI does not by itself alter the number of pixels in an image, which is what determines the resolution. But the photo editor can be used to calculate the number of pixels needed in the image to print at a given DPI and resize it accordingly. When an image is increased in size, however, the added pixels are filled in by the computer based on the immediately adjacent pixels—so the number of pixels is increased, but the resolution is not actually improved.
Myth #4: A lower DPI reduces the file size.
False. The basis for this myth, too, is the JPEG image format. The file size of a digital image depends on the format, the number of pixels, and the amount of any compression (in a JPEG file, for example). And because reducing the DPI does not by itself decrease the number of pixels in an image, the file size is unchanged—so only by resizing the image can you change the file size.
Myth #5: DPI changes the image dimensions.
False. This myth is a result of the way photo editing software, like Photoshop™ Elements, shows the size of an image when you use the Resize tool. The dimensions are given in pixels, as well as in inches (or cm) for the DPI value you input. Digital images don’t have a physical size, however, so the dimensions are calculated from the number of pixels divided by the DPI. Changing the DPI value changes the dimensions of the image—but only if printed at that DPI.
The Truth: DPI Doesn’t Matter
Let’s compare two images. The first was saved at 300 DPI; the second, at 72 DPI.
They look the same, don’t they? Although the images have different DPI values, each one has the same number of pixels, the same dimensions, and the same file size. Whether viewed on a computer screen or printed, the images will always be identical. Go ahead and prove it for yourself. Click on each image to open it in a new window and switch between to compare how they look.
Put simply, the DPI doesn’t matter! In fact, computers and printers just ignore the DPI value in the image. Only the number of pixels determines the resolution and how an image will be displayed or printed.