Digital images can be saved in any of a number of file formats. They are all equally valid, but your intended use of the image has a significant bearing on which you should adopt. The first concept to understand is compression. Files have to be compressed in order to fit a suitable number of images on to a camera’s memory card. Or, you might need to compress the image files on your computer in order to e-mail them to a friend.
When files are compressed, you can use a ‘lossy’ method, in which detail is irretrievably removed in order to make the file size smaller; or you can use a ‘loss-less’ method, in which all data is preserved. Lossy methods can deliver very small files: the most common example of this technique is the JPEG (standing for Joint Photographic Experts Group), with the file extension JPG.
Many digital cameras use the JPEG format, which is also employed by Kodak in their Picture CD system. Using modest compression, very little data (i.e. image detail) is lost, but the effect can be cumulative. If you repeatedly open and save an image in JPEG format, the degradation will increase.
Original Image by Jeff Stroub
The TIFF format uses a compression regime called LZW (Lempel-Ziv-Welch, after its creators) that results in no data being lost. At the risk of being simplistic, you can describe this method as squeezing the data up more tightly rather than throwing some of it away, as is the case with JPEG.
A file compressed in the TIFF format is generally larger than a corresponding JPEG file.
Most image editors have their own format or often called the native format that preserves the information (such as image layers and other characteristics) that the particular application supports. Photoshop, for example, uses the .PSD format for its native files.
When you are using a digital camera, file compression becomes particularly important. Without it, your camera’s memory card would very quickly fill up; even a large, 128MB memory card holds only a handful of the 17MB image files created by many digital cameras in uncompressed form.
However, not all images need to be stored at high resolution, so a range of compressions is offered. This gives users the opportunity to select a compression to meet their needs. Important images, for example, could be stored at high resolution, and more casual shots at a lower one; in this way you can optimize the quality and number of images gathered before you download them to your computer.