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A few days back, I was extolling the virtues of the Sidecar (.XMP) file format that Adobe uses for storing updated metadata and edits to digital images in Adobe Camera Raw (ACR):

“It turns out that Bridge (together with ACR) is exactly what I needed to organise my images, open them in ACR (and optionally Photoshop) to perform non-destructive edits, with the changes (and associated metadata) stored in Sidecar (.XMP) files alongside the original image (avoiding the need to maintain multiple copies of images.”

Well, soon afterwards I found out that, for raw image files, ACR does indeed create XMP files (which are also used by Adobe Photoshop Lightroom and are visible in Bridge) but, if ACR is used for JPEGs (or TIFFs), then the original files are modified.

In a blog post from February 2007, Adobe’s John Nack explains why non-destructive edits to JPEGs may be considered an oxymoron – basically Adobe appends the metadata that would normally be stored in the .XMP file to the JPEG. That means that, if I view an edited file using an Adobe product, it can see the changes but other viewers are unaware of the additional data and ignore it.

The accompanying images show a JPEG file that I opened in ACR (via Bridge) to adjust the exposure and to straighten the image. Bridge shows the updated file as being 2848×1894 pixels in size and the image edits are visible in the preview:

Meanwhile, the Mac OS X Finder (or any other image viewer) sees the original 3008×2000 pixel image, still underexposed and leaning to one side:

If, like me, you like your digital asset management software to maintain the original images untouched and only perform non-destructive edits, then this may come as a bit of a shock.