What is compression? During compression, data that is duplicated or which has no value is eliminated or saved in a shorter form, greatly reducing a file's size. When the image is then edited or displayed, the compression process is reversed. There are two forms of compression, lossless and lossy, and digital photography uses both forms. Lossless compression Lossless compression uncompresses an image so its quality matches the original source. Although lossless compression sounds ideal, it doesn't provide much compression. A leading lossless compression scheme is LZW (Lempel-Ziv-Welch). This is used in GIF and TIFF files and achieves compression ratios of 50 to 90% Lossy compression Although it's possible to compress images without loosing some quality, it's not practical in many cases. Therefore, all popular digital cameras use a lossy compression (rhymes with bossy) that degrades images to some degree and the more they're compressed, the more degraded they become. In many situations, such as posting images on the Web, the image degradation isn't obvious. However, enlarged prints reveal it quite clearly. Although lossy compression does not uncompress images to the same quality as the original source image, the image remains visually lossless and appears normal if not enlarged too much. The trick is to remove data that isn't obvious to the viewer. For example, if large areas of the sky are the same shade of blue, only the value for one blue pixel needs to be saved along with the locations of the other identical pixels in the image. The leading lossy compression scheme is JPEG (Joint Photographic Experts Group) used in JFIF files (JPEG File Interchange Format). This scheme allows you to select the degree of compression. Compression Ratios between 10:1 and 40:1 are common. Digital camera file formats When images are stored in your camera, they are stored in one of the available formats. Over the years, a number of different formats have been developed to store photographs and other bit- mapped images. Each format has its own unique characteristics which determine when and where you might choose it over the others. Whichever format you choose you are not permanently locked into that format. There are programs that will convert it to any of the other commonly used formats. Raw image file formats Since compression degrades images to some degree, some cameras allow you to use an uncompressed format to preserve as much of the captured image quality as possible. In some cases, these raw formats are proprietary and not supported by many other programs. For example, Canon uses the CCD Raw mode to record images using no compression. This format is transferred to the computer right off the sensor and interpolated there. Other cameras use a more universal format such as TIFF. JPEG (.JPG) The JPEG (Joint Photographic Experts Group) format, pronounced "jay-peg," is by far the most popular format for the storage of photographic images and displaying them on the Web. The JPEG format is optimized for the display of photographs and doesn't work as well as GIF for text or line drawings since GIF is optimized for those kinds of images. JPEG images have two distinctive features: Because JPEG compression affects the quality of the image, most cameras allow you to choose between different levels of compression. This allows you to choose between lower compression and higher image quality or greater compression and poorer quality. The only reason to choose higher compression is to create smaller image files so you can store more images, send them by e-mail, or post them on the Web; if you want to make prints, larger file sizes generally make better quality prints. Most cameras give you two or three choices equivalent to Good, Better, Best although the names change.
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