Why 300 DPI is the photo lab reference

At a normal reading distance, the eye stops telling individual dots apart somewhere around 300 of them per inch, which is why photo labs and magazine printers settled on that figure decades ago and never really moved off it. It is not a hard limit built into the file format or the printer, just the point past which spending more pixels on the same print stops paying off. A print held in the hand, like a 10x15 or a photo book page, is exactly the case where the extra resolution is worth using, because the eye is close enough to notice.

DPI on paper is not the same thing as resolution on a screen

A screen shows a fixed number of pixels regardless of how big the display physically is, so a 1920x1080 image looks identical in pixel terms on a 21 inch monitor and on a 40 inch television. Paper works the other way around: the pixel count stays fixed, but the print size decides how tightly those pixels get packed, which is exactly what DPI measures. Stretch the same file over a bigger sheet and the DPI drops, because the same number of pixels now has to cover more physical surface.

Viewing distance changes what counts as sharp

A billboard printed at 20 or 30 DPI looks perfectly crisp because nobody stands close enough to see the individual dots, while the same low DPI on a postcard held at arm's length would look visibly soft. This is why large format prints such as roll-up banners or bus stop posters get away with far less resolution than a photo album page: the format itself guarantees the viewing distance, and the print only needs to look sharp from there.

Reading the standard format table

The table below takes the resolution entered above and works out the DPI it would land at on formats people print most often, from a small 10x15 print to a 50x70 poster. A given file can look excellent on a small print and merely acceptable once blown up to A2, without anything about the file itself changing: only the surface it gets spread over does.