I want to tag some images. I'm wondering which is the right place in the EXIF to store specific information about the scene. For example: Cat, Nature, etc.
There are a number of metadata standards used in photography. EXIF is one of them, but there are others in common use as well. Today's image files often include multiple metadata sections in different formats.
EXIF is most concerned with metadata from the camera about the shooting situation: exposure parameters, image data structure, and etc. It has a few fields that are descriptive, but these have some serious limitations (for example, this is supposed to be pure ASCII — Unicode is not allowed, so no characters outside the basic English alphabet). It's not really very rich for description and categorization.
Instead, I encourage you to use XMP for this. XMP is a modern, flexible standard understood by a wide variety of modern software.
In XMP, the tag "Subject" is meant to be a list of keywords, just like you've provided. This is very commonly used in the real world — for example, if you upload images with XMP Subject tags to photo-hosting service Smugmug¹, those tags will automatically get turned to Keywords. ("Title" and "Caption" are also widely understood.)
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The question is a bit too broad but for me the place to store this king of information is tag:
Of course you can select own tag like
More tags, appropriate for such info can be:
0x927C MakerNote 0x9286 UserComment
You can check also in Exif specification here
Alternatively, you could leverage something called IPTC. It's like EXIF, but it's used a little bit differently.
Designed for working professionals, IPTC is primarily used to tag a type of photo and to embed copyright information into it. IPTC allows you to change all sorts of information, including
- Overwriting or attaching a caption or description
- Adding a headline
- Attaching copyright and contact information
- Licensing information,
- Manual location entry (location, sublocation, city, state, country, ISO country code, continent)
- Models or artwork information
- and more!
IPTC, EXIF, and other metadata forms can be stored in something called sidecar files, which take the form of XMP files, or eXtensible Metadata Platform. When you change the metadata, even through editing in Photoshop (yes, the metadata changes behind the scenes), you may have noticed that the software creates a second file that can be opened in Notepad or the macOS equivalent. That's the sidecar file, and it's carrying all of the juicy metadata information as a "backup" of sorts, helping the main image file. Most of the time, you may not even see this sidecar file, but it's there.
You can use programs like Photoshop or Photoshop Lightroom to edit this metadata, though some softwares handle metadata editing and saving better than others. My weapon of choice is Photo Mechanic, as it handles sidecar files very well.
If you got curious about what XMP metadata looks like for photos:
<x:xmpmeta xmlns:x="adobe:ns:meta/" x:xmptk="Adobe XMP Core 5.6-c140 79.160451, 2017/05/06-01:08:21 ">
xmlns:exif="http://ns.adobe.com/exif/1.0/" and on and on and on. It's several hundred lines of information!
Edit: Tl;dr: Information about what the photo contains should be packaged in the IPTC Description/Caption field. EXIF is a type of metadata, but its purpose is NOT to describe the contents of the photo, but rather the camera and its settings.
In passing on adding keywords:
If you have the keyword
cat you will get your pussycat, but also caterpillar (insect); caterpillar (earth moving equipment); caterpillar (brand) catastrophe, every girl named Catherine...
There are two approaches to this. Not mutually exclusive.
You use a hierarchical keyword list.
Birds > Falconis different from
Cars > Ford > Falconis different from
Ships > British war ships > Falcon > ...
You facet you keywords by using prefixes. E.g
P: what follows is a person
L: what follows is a place name
WE: what follows is weather
SK: what follows is a sky attribute.
LAND: what follows is a landscape attribute
T: what follows is a time of day lighting attribute.
That last one: Time of day is standard Exif data for any digital image. But pics during the golden hour or blue hour or civil twilight have a certain quality to them.