When a digital camera stores a photographic image onto removable media it stores along with the image data some meta data such as exposure, ISO rating, whether or not the flash was used, the image width and length, and the date on which the image was created. It is useful to be able to read this meta data. For example, it allows the easy creation of a program to rename digital camera files to file names containing the date the photo was originally taken.

The files produced by digital cameras are usually EXIF format files.

The utility presented here allows a Common Lisp program to extract the meta data about a photograph from an EXIF format file.

http://www.exif.org contains information about the EXIF file format.

ISO/IEC 10918-1:1994 document.

jhead, which the author of this utility used to understand EXIF.