JPG File Format
SUMMARY: A beginners guide to JPG images, images compressed in a lossy-compression format.
The .JPG (or .JPEG) file extension normally represents an image file stored in a format developed by the Joint Photographic Experts Group. JPG files usually represent photos, and these files are compressed, thus saving space on a hard drive or other media. An image such as a BMP file that may take up 250k of hard drive space, for example, may be represented as a JPG image possibly only requiring 35k of storage.
Unlike some other compressed image formats such as PNG and lossless TIF, JPG files perform what is called "lossy" compression. This means that to help save space, its encoding algorithm (procedure that takes the image and reduces the amount of space it requires on a hard drive or other media) removes some detail from the image. The amount of loss is customizable, and JPG images saved with a low amount of compression can sometimes be indistinguishable from their non-compressed brethren.
Highly-compressed JPG images may show traces of artifacts, or noise due to the compression process. Also, JPG images may seem blurred or out-of-focus depending on the amount of compression.
Virtually every image and photo editing program supports JPG files. However, editing programs and image display software may support different versions of the JPEG standard such as Progressive JPEGS that allow for display of image files before they are completely transferred.
Most all JPG image editors allow users to adjust the amount of compression. Highly compressed files take up less space on a hard drive or other media but may lack noticeable detail and contain many artifacts. Less-compressed files will take up more space but contain more detail.
Once detail is lost from a JPG image it cannot be returned by resaving the file in a format that does not perform lossy compression. Also, if a JPG image is resaved as a JPG, the file will be recompressed, losing more detail in the process. Even if low compression is used, resaved JPGs can quickly lose noticeable detail. However, depending on the image size and editing software, certain modifications can be made to some JPEG files that require no recompression such as image rotation and cropping.
Note that digital photographers may choose to store photos as JPG files since these require less space than native or non-compressed formats such as RAW. This also allows cameras to store photos quicker, sometimes supporting a "burst" mode where multiple images can be stored in quick succession. This is useful in wildlife and sports situations, among others.
Digital cameras may also store information inside JPEG photos called EXIF metadata (EXIF stands for Exchangeable Image File Format). These tags may store information including what camera was used to take the photo, as well as exposure settings, aperture, metering mode, focal length, and other information when the photo was taken. Most modern operating systems can view the data, or one can use software such as IrfanView (see the screenshot to the right).
Windows users: NOTE that unless you have configured your computer with the file extension viewing tweak (Windows Vista instructions for the file extension viewing tweak) (Windows 7 instructions for the file extension viewing tweak), it is possible that a filename that looks like it ends with ".jpg", especially one attached to an e-mail message, actually has a hidden OTHER extension, meaning that if you double-click the file, it may actually run a computer virus!
For more information (listed for informational purposes only):
* Joint Photographic Experts Group - http://www.jpeg.org
* JPEG image compression FAQ - http://www.faqs.org/faqs/jpeg-faq/part1/
Return to the File Extensions page.
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