Clearing Up The Confusion

I have been amazed recently regarding the number of people getting all confused about the PPI (pixels per inch) setting of a digital image file. It is absolutely incorrect to use physical dimensions such as inches or centimeters to describe the resolution of a digital image file. Digital images are nothing more than an array of pixels (numeric values sepcifying color values at specific points in a rectangular grid); they have no intrinsic size or weight. The resolution of a digital image is correctly designated by the number of pixels it contains horizontally and vertically. The more pixels the file has, the greater the resolution of the image. The PPI tag in an image file is nothing more than a suggestion indicating what the recommended dimensions should be if the file is printed. It has nothing to do with any intrinsic size or quality of the image. Assuming a digital image has any intrinsic physical size is just as wrong as assuming it has a weight. (How much does the number 42 weigh?) The image itself has neither; only the physical medium on which it is printed can have a physical size or weight.

You can take an 800x1000 pixel image and print it at 100 PPI to get an 8x10 print, and you can take that same image file and print it at 200 PPI to get a 4x5 print. Both prints will have exactly the same resolution, even though they are different sizes physically. The 8x10 print has no more and no less image information than the 4x5 print, because both prints were made from the same 800,000 pixels. There is no difference in file size whether you set the PPI tag to 100 or 200, either. The amount of information in the image file is exactly the same either way. Here is an example: