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Understanding XMP in Lightroom

Discussion in 'General Technical Discussion' started by joelincoln, Sep 5, 2008.

  1. I'm using LR 2 now and I love it. But I want to understand EXACTLY what's going on when I edit a NEF.

    Apparently (please correct me if I'm wrong), LR creates an auxiliary xmp file to store all edit info and database info for a NEF. This includes everything from cropping, exposure changes, clone tool changes, keywords, meta data changes, etc... it's all in the xmp file.

    Question 1 - The NEF file itself is unaffected by anything I do in LR?

    Question 2 - As long as the xmp file is in the same folder as the NEF, any application that I use to open the NEF will automatically display it modified based on what's in the xmp (not just Adobe apps)?

    Question 3 - In order to "see" the original NEF, I only need to rename or move the xmp file?

    Question 4 - Are there any long-term down-sides to relying on the xmp file to modify the NEFs?
  2. billg71


    May 4, 2007
    Atlanta, GA

    First of all, Lightroom stores metadata and image processing instructions in the catalog, which is just a database. By default, LR does not create .xmp sidecars, you can turn this on in Catalog Settings under the Metadata tab. Most leave this off since the program will re-write the .xmp file every time you change any setting, slowing performance more or less depending on the computer/drive system, etc.. You can write an .xmp file for an individual file by right-clicking and selecting Metadata > Save Metadata to file from the context menu.

    Regardless of whether or not you use sidecar files, all the metadata and develop information LR creates for a file is stored in the catalog.

    As to your questions:

    1) Yes. LR will not modify your original file in any way.

    2) No. Bridge will read the .xmp file info and Camera Raw will apply develop changes made by LR, but there's no guarantee any third-party application will will pick up on ratings, flags, etc. I'm not sure, but I think most image browsers/DAM apps will pick up metadata and keyword changes but I don't use any so don't take this as the gospel. Develop changes will be ignored in apps that don't use the ACR engine, which pretty much limits it to Adobe.

    3) To view your original .NEF in LR all you have to do is go into the Develop module, scroll down on the left panel to History and select "Import". Other apps won't read Develop changes so there's no need to move or delete the .xmp file.

    4) The performance loss and file clutter could be called "downsides" IMO.

    I don't use third-party apps, I do all my keywording, rating, metadata changes and organization in LR so I don't use .xmp. Some folks use a browser/organization tool like Expression Media to sort,rate and keyword their image collection and I'm pretty sure the other tools use sidecars.

    IIRC, the metadata fields in an .xmp file relating to IPTC info and keywords are strictly defined so that info is pretty much shareable between different programs. But different manufacturers have different systems and locations for ratings, flags and labels so that info may or most likely may not be read by other programs.

    To sum up, if you work strictly within LR there's no advantage to using .xmp's and you will slow down the program. If you use Bridge, then using sidecars will let you view and change your LR metadata and ratings. But, when you go back to LR, you'll have to select "Read Metadata from file" to update the LR catalog with any changes you made in Bridge.

    That's pretty much the extent of my knowledge of sidecar files and the way they work with Lightroom. I'm not really well-versed in the ins and outs of .xmp and their use, but there are some members here who are and I'm hoping they'll chime in and correct me if I''ve made any mistakes in my answer.

  3. Lowolf


    Jan 26, 2006
    you do in fact want the side cars .xmp files if you loose that catalog at least with the sidecars you will not loose your edits just reimport and you are off and running once more.
    Always create your sidecars should not impact performance that extremely, does not on my systems with over 100k images in catalogs on 3 computers
  4. Wow, This is a lot of good info... thanks.

    I need time to digest this and decide what to ask next.
  5. Tom a very thorough answer, but does it answer the questions??
    By the way DNG is used by more camera manufacturers than any other raw format.
  6. I guess the bottom line question is would you rely on xmp to "hold" post process image information long-term? That is, should I save the changed file as a psd or tiff or can I be confident that I'll be able to access those changes in years to come?
  7. billg71


    May 4, 2007
    Atlanta, GA
    A third option is to back up your catalog regularly along with the image files. No .xmp, .tif, .whatever required, just your original .nef's and the LR catalog.

  8. Lowolf


    Jan 26, 2006
    Actually Hasselblad uses there software to convert to dng and were one of the first to offer dng for there raw files.
  9. Thanks, Tom, for the insight.

    I don't have a way to save NEF. So TIFF is an option.

    So it sounds like I can eliminate the xmp files and rely on the catalog...I already back up the catalog using the auto backup feature and store it off-line.

    But I'd be at the mercy of Adobe's ability/willingness to maintain long-term compatability of their current catalog with future products.

    I think I'll start another thread on long-term data retention. It's a topic that most people ignore or underappreciate I think.
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