Migrating Keywords Between Photoshop/Bridge and Lightroom
Both Photoshop/Bridge and Lightroom are made by Adobe, and both allow users to keyword their photos. But not in the same way.
Adobe Photoshop has a long history, starting out as a basic bitmap editing program and only later adding features for keeping track of its user's ever-growing collection of image files. In its earliest releases, every change you made to an image was committed to that file. Only later did Adobe give Photoshop users the Bridge add-on and the XMP Extensible Metadata Platform the provided the ability to organize images and track information about those images.
Photoshop and Bridge have no common place to store information, instead relying on the computer's underlying operating system to track the files and folders that make up a user's collection of images. XMP metadata including keywords can be embedded directly into image files of a number of formats, but not all support it, and fewer still did when Adobe first introduced XMP. As such, they chose to utilize "sidecar" files as a means of avoiding the issue completely. Every image file that has metadata to be tracked therefore has an accompanying file of the same name with the "xmp" extension file type to keep the keywords and other metadata.
This worked well enough at first for Photoshop users as it allowed them a great deal of added features without the need to risk changes to their actual images. Keywords and metadata lives alongside those images in separate sidecar files. But the limitations of this approach quickly became evident. A user who copied images to a new location had to copy the pair of files to avoid losing their keywords. If they copied only the image file itself, all the metadata in the matching sidecar file would be left behind.
By contrast, Adobe Lightroom was built from the ground up centered on a workflow that includes not only optimizing images but also cataloging and using them. At its heart is the Lightroom Catalog that keeps track of everything you do including the addition of keywords. This catalog exists as a true database in the modern sense of the word and can scale to support the storage of keywords and metadata on thousands and thousands of images in one central place. Among numerous benefits, this allows for quick image searches, the creation of collections to help organize images without regard to where they are physically stored. Rather than having to open and read through countless XMP sidecar files, images could be found by keyword easily by querying the catalog database.
For various reasons, more and more Photoshop users I talk with lately are migrating to Lightroom. Those who make a clean break from their Photoshop/Bridge past will encounter few problems with keywords, but those who attempt to make use of both will likely have some challenges. The method of tracking keywords differs sufficiently between Bridge and Lightroom that you can lose work unless you understand what is going on. When you import images to which you have previously added keywords in Bridge or Photoshop, any metadata saved in accompanying XMP sidecar files will correctly be added to your Lightroom catalog. But if you then add more keywords to those images in either Lightroom or Photoshop/Bridge, those keyword changes will not automatically synch to the other metadata system.
To keep your XMP sidecar file up to date with changes made to your Lightroom catalog is easy, but it is not the default. If you want to keep your sidecar files in synch, change the settings for your Lightroom catalog to turn on the option to do so. Select Catalog Settings from the Edit menu and then click on the Metadata tab. There you will find an option to "Automatically write changes to into XMP." When unchecked, you will find a warning below the option stating "Warning: Changes made in Lightroom will not automatically be visible in other applications." Turning on this option not only makes the warning go away, it tells Lightroom to record changes you make to both the catalog database and XMP sidecar files for each image. Those sidecar files will be read when you later work with any of those images in Bridge.
Keeping things in synch the opposite direction takes a bit more work, but not much. An image whose metadata has been changed in another application such as Bridge will be flagged in Lightroom with an icon in the upper right of the Library grid view as shown here. Additionally, the Metadata panel will show the image's status as "Changed on disk." Clicking on either will present you with a dialog allowing you to "Import Settings from Disk" or "Overwrite Settings" to resolve the conflict. Note that the choices probably aren't worded as best they could be since both the disk XMP sidecar file and the Lightroom catalog contain "settings" so "Overwrite Settings" is ambiguous at best. Thankfully the other choice of "Import Settings from Disk" is clear enough as is the accompanying explanation in the dialog. You will need to do this for each image you need to re-sync.
If you are unlucky enough to have changed metadata both in Lightroom and another application before noticing things are out of sync, the upper right overlay icon in the Library grid view will become emphatic with an exclamation point and the Metadata panel status will show as "Conflict detected." As above, clicking on either will present you with the same dialog to allow you to fix the problem. Unfortunately, only the same two choices are provided. There is no option provided to attempt to merge the changes. You must either "Import Settings from Disk" or "Overwrite Settings."
Of course, the easiest approach is to switch to using Lightroom alone for your metadata tasks, but not everyone is ready to make a clean break from their prior workflow habits.