Understanding PPI and DPI

Unfortunately, most people accidentally mis-use the terms PPI and DPI. Believe it or not, they DON'T mean the same thing!

DPI refers to the printing process, and the dots per inch that a printer is capable of laying down on paper. MOST inkjet printers for example can resolve THOUSANDS of DPI, from a 300 PPI image...

PPI refers to the image's physical dimensions, and THAT is where you get 300 PPI as the max number of pixels that a printer can use, and 72 PPI as the default web viewing size, even though nowadays computers are cramming more than 72 pixels into each inch...

Now, concerning PPI and "lab requirements"...

First of all, if someone is using DPI to ask for 300 DPI or something, they're an idiot, feel free to correct them and say "you mean PPI, by the way... DPI is printer ink dots, not image resolution." You'll be the life of their party, LOL!

Okay seriously: Honestly these days the PPI number ALONE does not matter if you're using most of the up-to-date online labs. As long as the image itself is the original size you captured it at, they take care of all that for you. (10 megapixels, 20 megapixels...) The only reason you might need to worry about this is if you're using a local lab with oldschool printing practices. (Basically they're just lazy and print the image at whatever PPI you get them, instead of adjusting the PPI so that the print comes out the right size.)


Alright, still confused? Let's break it down. So, we've got three measurements. ONE that controls how large an image ACTUALLY is, and two (plus the first one) that control how large it gets printed.

1.) The sheer number of pixels, width x height. (3000x2000 equals 6 megapixels. Do the math. ;-)
2.) The PPI resolution, pixels per inch.
3.) The inch resolution, which DEPENDS ON the above two measurements, obviously.


SO, let's say you wanna make a 4x6 print at 300 PPI. That's a 1200 x 1800 pixel image.

What if you wanna make an 8x12 image at 150 PPI? Whoa, do the math, it's still a 1200 x 1800 image! All you changed was the PPI and the inch dimensions.

So, when your local, oldschool (lazy) lab asks for image files at 300 PPI, then yes the chances are that they REQUIRE the image to have correct inch and PPI dimensions, regardless of whether or not the actual pixel size is sufficient for a quality print. So, here's what you do- Either crop the image with the exact inch and PPI dimensions and Photoshop will automatically re-size the image for you, OR just go into Image > Image Size, ...and directly input the inch and PPI dimensions. You'll see the third factor, the TRUE pixel resolution, automatically change accordingly. Unless you have the "Resample Image" box UN-checked, in which case it only allows you to tweak the inch and PPI dimensions, and changing one automatically affects the other. Kinda like shooting in aperture or shutter priority- your exposure brightness can remain exactly the same, while the aperture and shutter speed adjust accordingly.

I hope this clears things up a little! The visual aids below ought to help...

Take care,
=Matt=