I started to do a How-to post that shows a technique to Double Process a Single Raw File inside a Photoshop document. Since the technique relies heavily on the use of Smart Objects I thought it would be helpful to explain a little about Smart Objects first.
In Photoshop a Smart Object is a layer that contains a copy of the original referenced Photoshop or Illustrator file. (And yes it does make you file size a little larger but IMHO that a small price for the huge benefits.) From a practical point of view this means that you can edit the version of the file in the Smart Object nondestructively by opening the file in it’s application. For example, if you bring a raw file into Photoshop as a Smart Object, you can re-open the file in ACR and make edits to it. When you do this the original version of the file is not modified. Only the version within your Photoshop document, the Smart Object, is edited. That means that the preview for the raw file in either Bridge or Lightroom will remain unchanged because the edits are not applied to this version of the file. The Smart Object version of the file is independent of the original file.
Likewise if you place one Photoshop document into another, you can then double click the original document’s layer thumbnail, to open it in Photoshop. When you save and close the document the edits will be updated in the second Photoshop document.
You can create Smart Objects by using the Open As Smart Object Command, the Place command, or converting a Photoshop layer into a Smart Object. A Smart Object Layer can be identified by the icon in the lower right hand corner of the layer’s thumbnail See the two red circles in the figure above). You can also open an image as a Smart Object from Lightroom (Photo > Edit in > Open As Smart Object in Photoshop).
Another useful feature that Smart Objects is the ability to create Smart Filters. Without the use of Smart Objects all filter edits are destructive. But filters used within a Smart Object, a Smart Filter, are editable (including the ability to completely remove the filter effect.) In the figure above the arrow point to Gaussian Blur being applied as a Smart Filter. To edit the filter simply double click on the Gaussian Blur effect and the filter’s dialog will reopen.