Adobe Photoshop CS5 for Photographers - Martin Evening [85]
Camera Raw cache
Whenever you open an image in Camera Raw, it builds full, high quality previews direct from the master image data. In the case of raw files, the early stage processing includes the decoding and decompression of the raw data as well as the linearization and demosaic processing. All this has to take place first before getting to the stage that allows the user to adjust things like the Basic panel adjustments. The Camera Raw cache is therefore used to store the unchanging, early stage raw processing data that is used to generate the Camera Raw previews so that the early stage raw processing can be skipped the next time you view that image. If you increase the Camera Raw cache size, more image data can be held in the cache. This in turn results in swifter Camera Raw preview generation when you reopen these photos. Also, because the Camera Raw cache can be utilized by Lightroom, the cache data is shared between both programs.
DNG file handling
Camera Raw and Lightroom both embed the XMP metadata in the XMP header space of a DNG file. There should therefore be no need to use sidecar files to read and write XMP metadata when sharing files between these two programs. However, some third-party programs may create sidecar files for DNG files. If ‘Ignore sidecar “xmp” files’ is checked, Camera Raw will not be sidetracked by sidecar files that might accompany a DNG file and cause a metadata conflict. Having said that, there are times where it may be useful to read the XMP metadata from sidecar files. For example, I sometimes have raw files on computer A and DNG versions of the same files on computer B. I can save the metadata to the xmp files on computer A, copy these across to computer B and read from the xmp metadata to update the DNG files there.
DNG files have embedded previews that represent how the image looks with the current applied Camera Raw settings. When the ‘Update embedded JPEG preview’ is checked, this forces the previews in all DNG files to be continually updated based on the current Camera Raw settings, overriding previously embedded previews. However, it is important to point out that DNG previews created by Camera Raw can only be considered 100% accurate when viewed by other Adobe programs such as Lightroom or Bridge and even then, they must be using the same version of Camera Raw. While DNG is a safe format for the archiving of raw data, other DNG compatible programs (that are not made by Adobe), or that use an earlier version of Camera Raw, will not always be able to read the Camera Raw settings that have been applied using the latest version of Camera Raw or Lightroom.
Sharing XMP metadata
While I may prefer to archive files in DNG, I am still wary of converting to DNG too soon. This is especially true when working with Bridge. I find if I keep my raw capture files unconverted I can easily swap the XMP sidecar files from one computer to another as a quick way to transfer and update the image edit information such as the Camera Raw settings and ratings. For example, when I work from a rental studio I'll have a computer in the studio with all the captured files on it and a backup/shuttle disk to take back to the office at my house, from which I can copy everything to the main computer there. If I make any further ratings edits on this main machine, the XMP metadata is automatically updated as I do so. It then only takes a few seconds to copy only the updated XMP files across to the backup disk, replacing the old ones. Back at the studio I can again copy the most recently modified XMP files back to the main computer (overwriting the old XMP files).