Why PPI is Pointless

I've just seen yet another article about PPI (and DPI) and how it relates to resolution that yet again completes misunderstands what PPI is about.

PPI & DPI stands for Pixels Per Inch and Dots Per Inch.

So, let's get this absolutely clear.

PPI HAS NOTHING TO DO WITH RESOLUTION

When I say resolution I mean how much detail an image has. That is determined entirely by the number of pixels in the image (ok there is a slightly more complicated version to do with file formats but I'm going to ignore that)

The more pixels (or megapixels) your image has the better the resolution.

It does not matter whether it is 72dpi or 300dpi.

These numbers only define the print size of an image, not it's resolution.

If you print a 2,400 x 3,000 pixel image as a 8x10 image you will have lots of detail

If you print a 240 x 300 pixel image as a 10 x 8 it won't have any detail at all.

The smaller file has insufficient resolution.

However, if you print either file as a 1 inch square, or as a small image on the web, both will look absolutely fine, because either has sufficient resolution for the job.

What PPI tells you is how many pixels will be printed per inch from your file. For a good quality print you need around 300 pixels per inch. So your ten inch print needs 3,000 pixels.

An ok resolution for the web is 72 pixels per inch, but here you've got no direct correlation between display size and image size anyway, because it all depends how big the viewers screen is.

How stupid is the misuse of the language.

Well a recent magazine request I had was for a 300dpi picture, So I could have sent them a 300 pixel picture if they only wanted to print it an inch high. When you see such sloppy language there is an expectation/assumption that the 300 dpi image will be suitable for an 8 x 10 (or larger) print, so what they should be asking for is 2,400x3,0000 pixel image (or approx 9mega pixels)

The trouble is the kind of usage is very common, not just in magazines, but for stock libraries and all sorts of other places that rally ought to know better. It's sloppy and a misuse of the terminology.

Worse it leads people to think they have high resolution images when they haven't.

Just as an example Canon cameras are often defaulted to images at 72 dpi, yet they've got 20+ mega pixel sensors. Of course the images are high res.

The only way you should consider ppi/dpi is as a scaling factor to work out just how big you could push an image to print..

To which the answer by the way is that a 10 mega pixel image can be pretty much reproduced at any size you like and nobody will notice. Why? Because the larger the print, typically the less dots per inch are printed, but the greater the viewing distance. So in the end it all balances out.

(as an aside you can ignore the macho bullshit about medium format resolution and all that crap when it comes to prints)

Go look at a billboard close up and you can see massive dots, but seen at the appropriate distance it looks fine. (dpi on a billboard will start at 70 and go down from there some as low as 12 dpi)

I suspect I haven't explained this any better than the article I was complaining about, so let me trey and summarise.

Pixels are a measure of resolution

PPI/DPI is only a number that divides your resolution to a particular print size.