3. Compression

Compression is the process by which data is selectively removed from a file in order to reduce its total size.

The most common compression format is the JPEG (pronounced jay-peg). It works on the principle that small color changes in an image are almost imperceptible to the human eye: some pixels can be deleted without effecting the overall image quality to a significant degree. The keywords, obviously, being significant degree.

All forms of JPEG compression are "lossy," meaning that data is lost, and the image quality is degraded somewhat.

Digital cameras today offer several image file size settings as part of their basic menu, in order to allow more pictures to be stored on a single memory card. But it is important to note that a small, compressed image can never be enlarged to a high-resolution image.  The reverse is not true. 

Since the cost of memory cards has plummeted dramatically, it is strongly recommended that the maximum "quality" setting is chosen at the onset:  large, high-resolution images can always be reduced later for emailing.

In some cases, compression is imperceptible. JPEG compression ratio can be controlled, and the images below demonstrate the relationship between compression, image quality and file size.