Appreciating Exif

Wrote a parser to extract image metadata once, and got massively frustrated with the amount of undocumented, semi-documented, wrongly documented, or partially documented attributes. You’ll find references online, but most of them lack half of what you encounter in images. Every image processing app under the sun adds its own range. Some use metric values, some imperial; finding out which can be guesswork. Aperture is given in f-stops, decimals, or literal fraction strings. Some attributes hold sentinel values. Some vendors have custom conventions for undefined data.

It’s a jungle out there.

I worked at a company that displayed user-uploaded photos online and in email newsletters. Some small portion of photos displayed very incorrectly only in the email newsletters as displayed by outlook. They were fine everywhere else.

After a lot of investigation, we discovered that only photos uploaded by one specific (prolific) person had this issue. And it was caused by their software putting some nonsense exif DPI data in the image that was ignored as nonsense by every renderer except outlook. The format is a minefield of features with inconsistent support.

But I suppose that's part and parcel with actually being used. And that's somewhat better than the alternative.

As with most standards, there are three almost overlapping standards for image metadata. EXIF, IPTC and XMP. IPTC incidentally is hot again because certain generative AI providers add IPTC metadata[1] to the images they generate. Incidentally, OpenAI uses another standard for the same purpose, C2PA[2].

All of these, the author mentions. Now for SynthID[3], which both OpenAI and Google embed in the pixels of the images they generate. Needless to say, in mutually incompatible ways.

[1]: https://iptc.org/news/draft-for-public-comment-new-photo-met...

[2]: https://c2pa.org/

[3]: https://arxiv.org/abs/2510.09263

FYI: I just recently added simple Exif viewing/editing/clearing to my "xv"-inspired image editor pxv: https://github.com/linsomniac/pxv

My primary goal was to have my core "xv" muscle-memory usable through a simple tool that didn't require me building the original xv (since you can't just apt install it), because these days I'm not using xv much.

But I've since added a few features that xv doesn't have like the Exif and also image annotation, plus beefed up the image enhancement to be very much like XVs.

Exif is technical debt in the most flattering sense. Messy, old, and still quietly useful decades later.

Exif is great but here is your obligatory reminder that if you are publishing images you should strip out some of the identifying information that cameras and image editing software likes to embed.

In particular, you probably don’t want the GPS coordinates of your house publicly available on your blog for everyone to see.