Decades ago computer screens commonly had 72 ppi (pixels per inch), as opposed to the roughly 200 to 500 ppi screens we now use. So there was a mistaken belief that jpeg files saved at 72 ppi would display better on screens. Perhaps people believed the pixels in the image would align better with the pixels on the screen if the ppi of the image matched the ppi of the screen. If you display a 1,000 pixel wide image on a screen, at full size, it will use 1,000 pixels of width on the screen. This is true whether the image was saved with 1 ppi, 72 ppi, or a billion ppi.
Images and screens don’t have a dpi – printers do. The image’s pixels per inch determine how many pixels of the image will be used to fill each inch of the paper. For example, your 1,000 pixel image, printed at 200 ppi, will be 5” wide. The software that drives the printer might let you specify the dpi. This sets how many dots of ink will be used to print each inch of image, regardless of the size of the printed image. And in this day and age, the dpi for photographic printers is a lot higher than 72. An image printed using only 72 dots of ink for each inch of paper would look pretty bad.
The next time you see a request to submit your images at 72 dpi, tell them you don't believe in Santa Claus.
- David Garnick
David Garnick is the Founder and Editor of Bokeh Bokeh Photo, and directs the San Francisco Bay Month of Photography. He specializes in still life and landscape photography that references visual styles of the past. David collaborates with museums to create compelling imagery for exhibitions.