Lightroom: 300 DPI vs 72 DPI

1. DPI stands for “Dots Per Inch”. DPI only applies to output devices, that’s for monitors and printers…
2. PPI stands for “Pixels Per Inch”. PPI only applies to input sensors, that’s for camera sensors…

If somebody talks about PPI when talking about screens or printers, they don’t know what they are talking about! Many printers do not deal in DPI but in LPI “Lines Per Inch”, those are mostly CMYK commercial printers, printing presses, magazines, newspapers…

What’s the difference between a 300 DPI and a 72 DPI? None, zero, zilch! Almost… It’s only a double flag that is stored at location: 0Ah for the horizontal resolution and at 0Ch for the vertical resolution.

1. If you have an original photo that’s 5184 pixels by 3456 pixels, the full size of a Canon Tags: Lightroom - 18 megapixels sensor, that you export as a jpeg of 518 pixels by 345 pixels at 100%, meaning no compression, with a DPI of 300, the file size will be 1.8 megabytes.
2. If you have an original photo that’s 5184 pixels by 3456 pixels that you export as a jpeg of 518 pixels by 345 pixels at 100% with a DPI of 72, the file size will be 1.8 megabytes.

Oops! What happened? Why are they both 1.8 Megabytes? One photo is set to be 300 DPI and the other is set to be 72 DPI. If you use a program like bsdiff to extract the differences, it’s only 4 bytes. The DPI, that’s it! No other difference, otherwise both jpegs are identical.

The DPI and the LPI are only indicators of what density you should output the photo. The important word is should. Most printers will obey the DPI suggestion, but on many printer drivers you can override the DPI/LPI settings. Also you can use an “hex editor” to change the DPI from 72 to 300 and save the file.

Back to what’s the difference between the 72 DPI and the 300 DPI photos? The output of the 72 DPI jpeg image will be 4.166 wider and taller than the 300 DPI jpeg image. But the 72 DPI will look much spottier, and if you are close enough you will see the individual pixels.