Image Dots per Inch

With an image obtained from a scanner, the more samples (pixels) per inch than the more detail captured. It’s fairly obvious that if you scan a document at 1000 pixels per inch you will get a lot more detail than if you scan it at 10 pixels per inch. The scanner stores the number of pixels per inch in the image file, which calls this value dots per inch (dpi) instead of pixels per inch (ppi). This stored dpi value determines the size of the printed image.

With an image created by a graphics program the image is generated by creating pixels on screen and not by sampling a document. The author specifies a dpi value, stored in the image file, that determines the size of the printed image.

If you know the image was obtained from a scanner then the stored dpi value tells you the size of the scanned document, determines the size of the printed image, and gives you information about the quality of the image because the dpi value is the number of samples (pixels) per inch. If the image was created in a graphics program the stored dpi value determines the size of the printed image, but tells you absolutely nothing about the quality of the image because no sampling was involved. Even more - when an image is displayed on screen all that matters are pixels, and nothing but pixels, so dpi values, no matter how they were obtained, have no impact on screen display. Images that have identical pixel data, but different dpi values, will display exactly the same on screen.

“....if you plan for the scan to be viewed on a web page, you don’t need more than 72 dpi (because a computer screen only has 72 pixels per inch). ”

— Fireworks MX 2004 book

One often sees statements such as “You only need save images for the Web at 72 dpi” or “Increase the image dpi for better display quality”. Books and articles have been making statements like this for so long that it’s difficult to get over to people the nonsense that it is. The fixation on 72dpi seems to be based on the completely wrong assumption that PC screens actually have 72 physical pixels per ruler inch, and even if they did, there would be no logical reason to choose 72 over any other number. As for “Increase the image dpi for better display quality” it makes, again, the completely wrong assumption that the stored image dpi affects the quality of a screen image.

Perhaps the images shown below will convince you. Better still, try it yourself using the excellent Irfanview program with which you can change the dpi value stored in an image file to almost anything you like, and you will see that it won’t make the slightest difference to a displayed screen image.

0 dpi

72 dpi

5000 dpi

You will have to take my word for fact that the 0 dpi image really is set to zero dpi. If you save this png image and open it in your favorite graphics program then that program has to decide how to handle it because, remembering that the dpi value is only for determining print size, it implies infinite print dimensions. Your program will change the zero value to what it considers an appropriate value.