Today’s Question: I recently saw a tutorial from a photographer who recommended cropping images at a resolution of 72 ppi when the intent is to share to a digital display. But is this the correct number regardless of the display, such as with different monitors or digital projectors?
Tim’s Quick Answer: No, 72 pixels per inch (ppi) is not the correct resolution for digital displays. In fact, the ppi resolution is only relevant when you are preparing an image for print, while for digital sharing only the actual pixel dimensions matter.
More Detail: One of the most persistent misunderstandings about resolution for photographic images relates to the pixel per inch (ppi) resolution. For digital images this information is essentially meaningless.
For an image that will be displayed digitally, all that really matters when it comes to resolution are the pixel dimensions of the display upon which the image will be presented. For example, if an image is intended to be displayed full screen on a display or digital projector with a resolution of 1920×1080 pixels (often referred to as “1080p” resolution), the image should be resized to pixel dimensions close to 1920×1080 pixels.
In the context of a display the pixel per inch resolution doesn’t impact the resolution needed for the image prepared for such a display. You simply want to size the image based on the pixel dimensions of the display. The setting for the output resolution is simply another metadata value in this context.
The only time the pixel per inch resolution is really helpful is when printing, and even in this context it is only a convenience in terms of describing the resolution needed for optimal quality. For example, with offset press printing the effective resolution is generally around 300 pixels per inch. With that information you can resize an image easily, because you know how many pixels are required based on the intended output size. For example, an 8″x10″ print would require 2,400 pixels by 3,000 pixels based on 300 pixels per inch.
With a digital display the pixel per inch resolution isn’t as meaningful, in large part because different displays will have widely variable pixel per inch resolutions. For example, my 27-inch 4K display has a resolution of 163 ppi. The same size display at 1080p resolution would have a resolution of about 82 ppi. A digital projector at 1080p resolution projected onto a 10-foot-wide screen would have a resolution of 9 ppi. The point is that the pixel per inch resolution isn’t as uniformly helpful in the context of a digital display the way it is with print, so you can simply resize the image based on optimal pixel dimensions and effectively ignore the pixel per inch resolution.
