Determining Resolution, DPI, PPI, Etc.
A confusing topic!
Everyone asks for 300dpi but what does that really mean? Why does the image get bigger when you lower the resolution or vice versa? What's the difference between megabytes and megapixels and why should you care?
Basics of Image Resolution - Our simple guide to determining Pixels Per Inch or PPI.
Image Resolution and View Distance Calculations - Complex, but includes a nice calculator for determining ppi by viewing distance.
Image Interpolation Explained
Answers the Question: What print resolution works for what viewing distance?
Viewing Distance: The Misunderstood Concept - A good, understandable expanation of why an image printed on a billboard may not require more pixels than one printed in a magazine.
You Say You Want A Resolution? - Some good info on image resolution.