Glossary
IFD
An Image File Directory is the table structure EXIF uses to store tags, inherited from the TIFF specification.
Updated
Also written: Image File Directory.
An IFD is a count of entries, that many fixed-size entries each pairing a tag number with a value or a pointer to one, and finally an offset to the next IFD. Chaining them is how one EXIF block holds several directories.
A typical photo has a main IFD for the image, a second for the thumbnail, a sub-IFD for the detailed EXIF fields, and a GPS IFD for location.
Values larger than four bytes are stored elsewhere in the block with the entry holding an offset to them. This is why EXIF cannot simply be edited in place — moving anything invalidates offsets — and why rebuilding a block is safer than patching one.
Related terms
- EXIF — EXIF is the metadata standard cameras use to record how, when and where a photo was taken, stored inside the image file itself.
- GPS IFD — The GPS IFD is the sub-directory of an EXIF block that holds location data, kept separate from the camera and exposure fields.
- Maker note — The maker note is a proprietary, largely undocumented section of an EXIF block in which a camera manufacturer records whatever it likes.