Compression and File Size
One of the most useful image formats is JPEG (short for Joint Photographic Experts Group), which uses a .jpg file extension. JPEG compresses files to create a smaller size file. On a digital camera a "quality" setting sets how much compression is used when saving the file. The levels are often refered to as fine, normal and basic. These settings are associated with a compression ratio.
Set you camera to the highest resolution setting i.e. 2400 x 1800 The higher the level of compression you choose, the smaller the size of the file. A typical compression ratio is 8 to 1. This means that a 3-megabyte (MB) file saved as a compressed JPEG file will be reduced to 375 kilobytes (KB). The drawback to this compression is that every time the file is saved again, a small amount of image quality is lost. This method is similar to making a photocopy of a photocopy of a photocopy. Each successive copy is slightly reduced in quality from the original. If the image is going to be edited and saved numerous times, then another file format such as Tagged Image File Format (TIFF) should be used for the interim saves of the file. The TIFF file format is lossless, it does not lose image quality in exchange for a smaller file size, but it also does not compress files, so files are larger and require more space to store.
The file sizes and number of images stored on memory are an average only. As is the case with JPEG it's difficult to predict the size of an image because it will vary a fair amount depending on the content of the image (especially the amount of detail captured). For example, take a photograph of a fairly empty wall and you'll get a small JPEG, take a photograph of a crowd of people with a lot of detail and you'll get a larger image. Different camera manufacturers also use slightly different compression ratios and different terms for describing them (see the table extreme bottom for conversion of manufacturers terms).
NB: All sizes will crop slightly in one dimension.