Digital images are stored as a grid of pixels, however a digitally stored image has no inherent physical dimensions.
It’s absolutely true. An image that is 1,000 px X 1,000 px has no measurement because there is no resolution associated with it. The resolution of an image is an indicator for how to interpret the pixels and is generally expressed as PPI (Pixels Per Inch) or DPI (Dots Per Inch). You can think of DPI like a map key showing you how long a mile is for example.
Welcome to Image Math
1000px X 1000px @ 100dpi = 10” X 10”
Resolutions are generally specified by the output device. Web graphics only need to be 72dpi, whereas print graphics should be 300dpi.
1000px X 1000px @ 300dpi = 3.3 X 3.3” printed
1000px X 1000px @ 72dpi = 13.8” on screen!
So, where did this 300dpi requirement come from?
DPI requirements for offset printing are based on a printer’s LPI or Lines Per Inch, which is a measurement of the density of the halftone screens output by the RIP (Raster Image Processor). The RIP is what takes the print data to color-separate and convert it into halftone screens on printing plates.
Low linescreens are used for porous papers like newsprint or single color printing, while higher LPIs are used for high-end work. Most commercial printers run at 150LPI for 4 color work. Some run as low as 133, and some at 180+! The LPI chosen by a printing company has a direct impact on the difficulty to print as well as the perceived quality of finished work, as it determines the amount of detail the press will be able to reproduce. Lower linescreens are have more ‘give’ on the press by allowing for significant dot-gain (ink bleed into paper causing dots to enlarge) and are thus easier to run.
The industry standard formula for determining the recommended DPI for raster images is:
OUTPUT LPI * 2 = Recommended DPI or 150lpi * 2 = 300dpi
This allows each printing “dot” in a halftone screen to represent 4 pixels (2x2) giving finer resolution to the final reproduction.
If you really want to geek out, let’s talk about the resolution of printing plate halftones - not LPI but the pixels it takes to make dots on printing plates. Imagesetters use lasers to image film for making plates and platesetters image the plates directly. Generally there are 2 industry standard resolutions depending on where the equipment was made, or is installed. Metric systems run at 2540dpi, Imperial at 2400dpi. Why is knowing this little detail significant? Vector art and fonts are imaged at these super-high resolutions giving them ultra-crisp lines.
So, in a perfect world, Grayscale and color images should be 300dpi and lineart (bitmap) should be 1200dpi. but even this is a gray area... in offset printing, it is generally acceptable to go as low as 1.5*lpi, or:
150lpi * 1.5 = 225dpi
Since you won't always know the LPI your printer will use, in the end it’s simply easier to hold to the 300dpi rule where you know you’re safe. More resolution just means better quality!