What type of metadata should I add to my photos? IPTC, XMP, or EXIF? I want to add copyright info to my photos.
Date/Time are the only fields that are present in all three metadata formats (Exif, IPTC-IIM & XMP).
Generally, when to use which
Metadata Working Group's recommendation is to use
- Exif for device properties
- IPTC-IIM for IIM properties
- XMP for IPTC Core & IPTC Extended properties, cached Exif properties and all other custom properties.
General answer on metadata reconciliation
I.e. if two or more are present, which wins?
If both (native) Exif and XMP are present, the aforementioned four fields are the only fields that get mapped between Exif and XMP:
Exif ∧ ¬XMP ⇒ read Exif ¬Exif ∧ XMP ⇒ read XMP Exif ∧ XMP ⇒ read both, but prefer Exif
IPTC-IIM and XMP can have a lot of more data in common. Good things to note are:
- IPTC encourages the use of IPTC Core, which can be written to both IPTC-IIM blocks and XMP blocks; and use the (older) IIM data just in the IPTC-IIM block.
- Moreover: it is recommended to write the IPTC Core data to XMP, unless backward compatibility is needed.
- IPTC-IIM has length limitations, while XMP hasn't.
- to help solve possible conflicts, a checksum of the IPTC-IIM data should be written.
So, if both IPTC-IIM and XMP data are present:
¬IPTC-IIM ∧ XMP ⇒ read XMP IPTC-IIM ∧ ¬XMP ⇒ read IPTC-IIM IPTC-IIM ∧ XMP ∧ (Checksum match) ⇒ read both, prefer XMP IPTC-IIM ∧ XMP ∧ ¬(Checksum match) ⇒ read IPTC-IIM IPTC-IIM ∧ XMP ∧ ¬Checksum ⇒ read both, prefer XMP
Above suggests that Exif and XMP are the most preferred. In my opinion, Exif is great for technical details of the photo (time, aperture, focal length, white balance, GPS etc.) and XMP is great for the rest (keywording, post-processing, rating, description etc.). While the Exif data could be written to XMP, I wouldn't trash the Exif as it is so much more widely used and supported than XMP.
Date/Time exist in all three metadata formats and the reconciliation guidance is specific for each property —
Date/Time being the least straightforward.
Copyright (33432, 0x8298), IPTC
CopyrightNotice (IIM 2:116, 0x0274) and XMP
dc:rights are (should be) mapped together. The mapping happens in the program that reads the data, so it cannot be 100% guaranteed. However,
CopyrightURL should be stored in XMP
So the ultimate answer is: whatever fits best to your workflow. How do you plan to use the info? The copyright info is, well, just info and as @Roland said you might need to mention the copyright explicitly, which metadata generally isn't. If in doubt, write the info to all three.
I usually write the copyright info in to the IPTC Core, which Lightroom most probably (being an Adobe product and Adobe being a member of the Metadata Working Group) ultimately writes into XMP.
IPTC core defines a few pieces of meta data information that, I think, is appropriate to your purpose:
- Copyright notice
- Creator's contact info (including email)
And a whole lot more. Read the spec, I think you'll see that it covers your needs more than adequately and there are plenty of tools (Adobe's various options, like Lightroom or Bridge, or exiftool) that can do this in batch.