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I've imported some raw images from camera and made changes. When I close and reopen LR I can still see the changes, but my original important RAW file is intact. So I'm curious, where/how LR keeps my changes on top of RAW file? Can I make my changes on imported RAW file? Say want to crop imported file and keep just cropped version.

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up vote 9 down vote accepted

Lightroom does not make changes to the original file. It stores the actions of each edit in the LR catalog. When you Export an image to, say a JPEG, LR performs each of those actions, creating a new file, in this case a JPEG. It never alters the original. The RAW file isnt really an image, but data that was recorded by the sensor. You see an image in preview because the RAW file actually stores a small JPEG preview image within the file. So 'cropping' the RAW isn't meaningful. You instead use the RAW data to create an image that you can crop as your final output. The output can be JPEG, TIFF, PSD, DNG, etc.

The catalog location can be found by looking under Catalog Settings.

You can also ask Lightroom to store these same edit actions in a 'sidecar' file, which will be stored alongside (in the same directory) of your RAW image, by selecting "Automatically write changes to XMP" in the Metadata tab of Catalog settings.

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Thus by having sidecar file I can migrate to another PC and have the same action history for each RAW file? – Pablo Jul 31 '11 at 19:33
Theoretically yes. However, there are not many programs that can consume this XMP data, even though it is standardized. You can do this within LR of course, but I find exporting the images and their catalog simpler (a function in LR) – cmason Jul 31 '11 at 19:35
I mean only LR of course and for the situations when I want to backup my image corrections. Say I dropped my Macbook accidentally from window but having sidecar and raw will give my efforts back. – Pablo Jul 31 '11 at 19:39
Backup in 3s my friend, that includes your RAWs and your LR catalog. [3 copies: 1 ea two different media, 1 stored offsite] – cmason Jul 31 '11 at 20:42
@Michael: "Thus by having sidecar file I can migrate to another PC and have the same action history for each RAW file?" - no, the XMP file only stores the current settings for the file, not the complete history (i.e. as seen in the History panel in Lightroom). The history is stored in the catalog, regardless of whether you use sidecar files or not. – Mark Whitaker Aug 1 '11 at 11:11

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