DPI & PPI explained
There seems to be a lot of confusion about what PPI means (apart from the fact that it means Pixels Per Inch of course). This article is for beginners in computer graphics, digital photography and printing. It does not cover scanners but the info in this article is useful if you use a scanner.
Dots Per Inch usually means the maximum dots a printer can print per inch. Roughly speaking the more DPI the higher quality the print will be. DPI is for printers, PPI is for printed images. But I don't think there is an official definition of the difference.
PPI and DPI are sometimes but not always the same, I'll assume for simplicity in this article that they are.
However, until you print an image the PPI number is meaningless.
Let me repeat that :
Until you print an image the PPI number is meaningless.
Imagine, for simplicity's sake, that the image below, when printed on your printer is one inch (or 2.54 cm) square:
If you count the pixels (blocks, dots) you'll find that there are 10 across the width of an image. If this was printed at the size of 1 inch it would be a 10 PPI image. Here is a 50 PPI version of the same image:
You need not count the pixels, believe me the square above is a 50 by 50 pixel image, and if printed so that it covered exactly 1 square inch it would be a 50 PPI image. Now lets look at a 150 PPI image:
PPI is simply how many pixels are printed per inch of paper. You may not be able to see the pixels (because your eyes or your printer is not that high a quality). The above images are approximations but you get the idea.
I don't care what the "image information" on your camera says, or the what PPI reading on your paint program says, only when you print can you really say what the PPI is. And the same image will have different PPI when printed at different sizes.
The rest of this article is for the really nit picky...!
Now images on your screen do have a PPI. Imagine taking a ruler and a very good magnifying glass and counting the pixels in an inch on your screen. That would be the PPI at that moment on that screen. But wait, if you use a program like Paint Shop Pro or Photoshop you can zoom into these images. And when you get your magnifying glass and ruler out again and count the pixels in a single horizontal inch you'll find it has changed. PPI is a number which chops and changes as you zoom into and out of your image on the screen!
Even the above explanation is not entirely correct, because not only does you image have, say, 1280 pixels horizontally, but your screen may have 1600 pixels horizontally. So here the confusion is deep and wonderful. What do you mean then by PPI on the screen? How many pixels of the image are shown in an inch, or how many are possible to see in an inch, on your screen?
It gets very messy at this level, suffice to say that PPI won't help you here!