30 September, 2008

PPI is the Opposite of DPI when it comes to Photo Quality

As a webmaster, I often have to field tech questions unrelated to my job in the office. Usually this is no big deal; I generally give the answer and then move on. But the other day, a question was posed to me that really threw me for a loop.

I was asked why, when increasing the Pixels/Inch (ppi) in photoshop, a photo became bigger.
If you stop and think about it for a minute, this is a really good question. DPI (dots per inch) is used quite often in print circles; it literally refers to how many individual dots are printed per inch. With a DPI, more dots are squeezed into each inch, and the picture is therefore sharper; with a low DPI, there is more space in between each dot, and the picture is therefore of a less quality.
So when you increase the PPI (which one might assume is the same as DPI, except with pixels), you should be increasing the quality of the picture by making the dimensions smaller, right?
Wrong. If you try this for yourself in Photoshop, you'll find that the photo increases in size when you increase the PPI, thereby decreasing the quality—which is exactly the opposite of what you might at first expect.
It takes a bit of extra thought to understand what the logic is behind this. The key to comprehending this paradox is that while printed materials can vary the spacing between dots, computer screens cannot vary the spacing between pixels. So if you vary the PPI, you are increasing or decreasing the number of pixels in your image, not increasing or decreasing the space between the dots as with DPI.
So what does this mean? In a nutshell, this means that when you increase the PPI of an image, what happens is Photoshop adds in additional pixels, horrendously ruining the quality of the photo while simultaneously making the pixel dimensions increase.
The moral of the story: DPI≠PPI! If you're a print person who is just getting into web stuff, don't make the mistake of thinking that just because DPI count and quality are proportional that must mean that PPI count and quality are also proportional—on the contrary, they are inversely proportional!
And the figuring of this out is how I avoided looking like a fool in front of my non-tech-savvy coworkers. I hope this blog entry will help you to also not look like a fool. (But if it doesn't, it sure as hell isn't my fault.)