Why do we print images at 300dpi?
DPI (dots per inch) and PPI (pixels per inch) may seem as though they're the same thing, but one relates to the printing process, and the other to screen resolution. With a smart phone in every pocket and handbag, most people are familiar with PPI because of their laptop and phone screen resolution. But the term DPI relates very specifically to print - PPI literally describes the number of physical dots of ink on a printed document. The terms are often used interchangeably, but they have different considerations.
Let's begin with Pixels Per Inch (PPI), as that's the more familiar term. A pixel is a 'picture element', and is the smallest thing on a screen that an eye can determine. If you zoom in on a picture on a screen, you will soon notice the tiny squares that the screen is made up of. If the PPI is too low, the image will appear to be pixelated, and look slightly hazy. But the differences between PPI and DPI are greater - a computer screen is actually made up of three overlapping colourways, which most people recognise as RGB - that is Red, Green and Blue. However printed images are made up of CMYK, that is Cyan, Magenta, Yellow and Black. Any image used on a computer screen actually needs to be converted from RGB into CMYK before it can be used in the printing process.
When you look at an image on a screen, your PPI has already been determined by your hardware - most images are based on 72ppi, because that's the resolution that computers, phones and tablets adequately display them at. And for that reason, more smartphones will take photographs at 72ppi, because that's the resolution at which they will be typically displayed.
However, images intended for print need to be 300dpi to print adequately. The size and value of the image at 72 ppi is completely different to 300 dpi. If I take a photograph taken on a decent camera at 300 dpi, and place it onto an A4 sheet of paper, it can be displayed at the full width of the page and still be over 300 dpi. If I reduce the same image to 72 dpi (the resolution of most screen images), in order to get it back to 300 dpi, I need to reduce the size of the image drastically , as you can see here...
If you know that you're going to print your image on a bill-board, and the typical viewer will be standing some distance from the image, the blurring of a lower DPI image won't be quite so crucial, but on something that will be handled close-up, it will be apparent to the naked eye.
The larger the DPI of an image to be printed, the smoother the image colours will become to the naked eye.. There are some interesting exceptions though - lots of newspapers are printed at 85dpi, but we're used to seeing them at a lower quality, and our brain does an excellent job of interpreting them normally.
If you intend to print a job, please save your printer the pain of asking you to re-submit all your photographs - take the photos on a good camera, and save them from the off at 300dpi - they'll be very grateful!