Some links on Resolution

As mentioned in my post today on resizing and resolution, I wanted to find some simpler explanations on understanding resolution.  If my last post left concerning PPI left your head spinning a little, take a look at these pages:
Start here for a quick overview:
Understanding PPI/DPI


This is a little more in depth;
From Digital Home Thoughts:
First off, what is PPI you might ask? It stands for "pixels per inch". PPI is a physical measurement of how many dots are printed on a piece of paper. Ignore what you know about DPI and screen resolution - PPI is a measurement of the physical world, where digital pixels get transformed into physical dots on the page. The basic concept is this: the more pixels you can have per square inch on your page, the better the quality of your print will be.

The Painting Analogy...
Here's an analogy that might help this concept make sense: let's say you have one gallon of paint. If you have a wall that's 10 feet high and ten feet wide, you can give that wall several coats of paint and it will look great - full, lush, rich color. But if you had to paint a wall that was 100 feet high and 100 feet wide, you'd have to spread your paint very thin to cover it all - the net result would be a poor quality paint job. The concept is exactly the same when it comes to printing your digital images. If you take a low resolution (640 x 480) image without many pixels (paint) and print it at a 3" x 2" wallet size (small wall), the quality will be acceptable. But if you took that same 640 x 480 image and tried to do an 8" x 10" print (a big wall), the quality would be poor because there's not enough pixels (paint) to cover it properly.

How Many Pixels Per Inch Do You Need?
You can't have a PPI value until you know what size of paper you're printing it on - all you have is the resolution (total number of pixels). PPI is an unknown until you pick a physical print size.

If you have a program such as Picasa (or Photoshop/Elements), it will give you the PPI value for each print size that you choose.  300 PPI is ideal - anything above that is overkill. I was surprised to find, however, that I could go all the way down to 100 ppi on my 8.5" x 11" sample print image and it still looked quite good at 8.5 x 11 when viewed from a normal distance (holding the image at chest-height). At 50 PPI I could see significant image degradation. This will vary from image to image, and perhaps from printer to printer but 100 PPI is the "danger zone" that you don't want to go below. Some might argue that 150 PPI is the line never to cross - it depends on the eyes of the person looking at the image, and how closely they inspect it for flaws).