DPI versus PPI Resolution


Today’s Question: In your answer about film scanners, you made reference to “pixels per inch” resolution. The scanner manufacturers list the resolution in “dots per inch”. Are these two terms interchangeable?

Tim’s Quick Answer: In my view the term “pixels per inch” (ppi) is more correct in the context of digitizing film images because the scanner is creating pixels based on the original, and the ppi resolution indicates the density of information being gathered from the original.

More Detail: In some respects I think it would be fair to say that the terms “pixels per inch” (ppi) and “dots per inch” (dpi) are interchangeable. Of course, some purists on the subject would say I am absolutely wrong about that!

In general, ppi resolution is used to refer to a digital image in the context of output size. This resolution is really only applicable when printing a photo, because when an image is shared digitally all that really matters is how many pixels there are. For example, if a monitor has a horizontal resolution of 1,920 pixels, you would need an image to be 1,920 pixels wide to fit the full width of the display. It doesn’t matter what the ppi resolution is set to for the image, because each pixel in the image would simply be represented by one pixel on the monitor display.

When printing the ppi resolution becomes important, because it impacts the quality of the print. In actual fact it is still the number of pixels that is important, but the ppi resolution provides an indication of whether you have enough pixels. For example, a typical photo inkjet printer requires at least 360 ppi resolution for optimal quality, which means an image that is to be printed ten inches wide would need to have at least 3,600 pixels in width.

The “dots” in the dpi resolution refers to dots on the paper when printing. That effectively translates to pixels in the context of the print, but the term “dots” is generally used instead. This is despite the fact that with many printing processes a single “dot” in the image is not produced by a single ink droplet. For example, many photo inkjet printers that render image data at 360 ppi typically place up to 1,440 or 2,880 (or more) ink droplets per inch.

So, in a very general sense dpi resolution relates to physical mediums, which is why I suspect the term came into common use for scanners. The ppi resolution is generally used when referring to a digital image. However, in my view the terms can be very reasonably considered interchangeable in most cases.