Digital Image Resizing Primer

http://www.bobatkins.com/photography/digital/digital_image_resizing.html

One aspect of digital images which seems to cause a lot of confusion to beginners is the matter of image size. There are three basic measures of image size

• Pixel count - e.g 3000x2000 pixels
• Physical size - e.g. 4" x 6"
• Resolution  - e.g. 72 pixels per inch (ppi)

The confusion seems to arise because people aren't sure of how these are related. They wonder if ppi affects the size of the image you see on a monitor screen. We know that displays are measured in pixels (e.g. a 1024x1280 display), and that screens are a given width (e.g. 15"), it seems logical that the pixels/inch setting should affect the size of the displayed image. Logical maybe, but wrong. Although monitors do have a measurable figure for ppi (pixels per inch), the ppi information in an image is NOT used for monitor display in web browsers.

Digital image files are "tagged" with other information. For example the width and height in pixels is in there, as is the resolution (ppi). Some image files also contain all sorts of information about the image, such as exposure data, focus data, flash data - and this is stored in what is referred to as the EXIF header. EXIF stands for Exchangeable Image File Format, and is a standard for storing interchange information in image files, especially those using JPEG compression. So how is this information used, and what uses it?

Read the Full Primer here:
http://www.bobatkins.com/photography/digital/digital_image_resizing.html