Over the years I’ve heard, and read in articles, these terms being used interchangeably. They are not the same thing. Please don’t perpetuate the error. Read on and learn the difference between DPI and PPI.

DPI

Stands for Dots Per Inch and goes back to the printing and publishing world. This is where the absolute of "… and you gotta have 300 DPI to make a decent print" fallacy comes from. (Beside the fact that when referring to digital images you should be using PPI!)

In our digital photography world, DPI is a term used to refer to film and print scanners, and inkjet printer, resolution. In scanners, it’s how many dots (actually pixels) are rendered in the final image file per inch of original. 35mm film scanned at 2000 DPI gives you a 2000×3000 pixel file. A 4×6 print scanned at 600 DPI results in a 2400×3600 pixel image.

In inkjet printers DPI is the number of dots of ink laid down per inch of printing paper. You may see different numbers for the horizontal and vertical numbers. Don’t worry. I’ve read a few articles that claim the human eye can’t distinguish difference in anything past 2000 DPI.

PPI

Used to refer to digital files going to print as 4×6s or 8×10s, Pixels Per Inch is no more than a reference number. It tells you how many pixels of your image are being used per inch of print. Again, simple math.

How many do you need? What’s the perfect PPI? Only you can decide. It’s rather subjective. Most people will try to keep between 200 and 300 PPI. In larger sized prints (16×20 and bigger) you can easily get away with 150 PPI and they look fine.

Ever see a billboard up close? They get printed at 10-20 PPI, but we view them from such a distance that it looks great.

Now here is something to wrap your head around. Imagine a 6 mp image at 2000×3000 pixels is printing to a 10×15. That’s 200 PPI. Your inkjet printer prints at 2400 DPI. That means each pixel is represented with 12 dots of ink. Is that enough fine detail for your liking?

Make some test prints for yourself and find your "level of tolerance" for your camera. You may find 150-200 PPI perfectly acceptable. My D70 has produced nice 16×20 prints and that’s my level. Everyone has a different perception and you need only find yours instead of accepting someone else’s.

Just do me one favor. Use the terms DPI and PPI properly. Thanks!