Can I export a 16-bit image or an XMP file from iPhoto?
At the present time, iPhoto cannot export 16-bit color or XMP (Extensible Metadata Platform) files. If you export an image as a TIFF file, the resulting photo will be an 8-bit TIFF derived from your edited JPEG image. The iPhoto RAW workflow is designed for simplicity—not expert level control. If you export a photo as an original RAW image file (by choosing Original as the image format in the Export Photos dialog), you will get an unmodified RAW file without an XMP sidecar (no metadata will be embedded into the file).