Today’s Question: [In response to the answer from February 15th regarding sharing images for high-resolution displays] What about PPI [pixel per inch resolution]? 1000 pixels measures differently at say 72 PPI than 110 PPI, and our re-sizing applications do ask for both resolution and pixel dimensions.
Tim’s Quick Answer: The pixel per inch (PPI) resolution is only truly applicable when printing an image. For digital sharing only the actual pixel dimensions matter, and the PPI resolution can essentially be ignored.
More Detail: When sharing a photo, you want to be sure that the size of the image is large enough to appear with good quality using the intended output method. Ultimately, all that means is that you need to have enough pixels in the image. The pixel-per-inch (PPI) resolution is effectively irrelevant.
The PPI measurement for resolution is really just a shorthand. Instead of saying that an image is “3,000 by 2,400 pixels” you can say the image is “10-inches by 8-inches at 300 pixels per inch”. The latter is just a method of describing the pixel dimensions in a way that is easier to translate to the final output.
However, the PPI resolution for an image really only applies when the image is being printed. If the image is being shared digitally, all that matters are the pixel dimensions, not the metadata setting that defines the PPI resolution.
For example, if a monitor display features a resolution that is 2,000 pixels across and you resize an image to be 1,000 pixels across, the image will by default be displayed at a size that is half the width of the monitor display. The number of pixels in conjunction with the physical pixel dimensions of the monitor display determines how the size at which the image will be presented.
Obviously, the software being used to display the image could alter the presentation of the image, such as by displaying the image larger or smaller than the actual pixel dimensions call for. But when preparing an image for digital sharing you only need to consider the actual pixel dimensions, not the PPI resolution value stored in metadata.
In fact, even when printing an image, the PPI resolution value is really just a number in metadata. The print quality depends upon having enough pixels in the source image to produce the intended output size based on how the output is being rendered by the printer. If you have enough pixels in the source image for the intended print size, a good quality print will be the result regardless of what the PPI resolution in metadata was set to.
Again, the PPI value really just provides a way to talk about the output size in more real-world terms. Producing output of good quality simply requires enough pixels for that output.
Note, by the way, that today’s question was a follow-up to an answer in a previous edition of the Ask Tim Grey eNewsletter, which you can find here: