Lightroom Virtual Copies and Snapshots
Adobe Lightroom provides a great deal of flexibility for maintaining more than one version of the same image. Indeed, so much flexibility that it can be confusing which method to use in a given situation. Here's a quick rundown of Virtual Copies versus Snapshots.
The concept of a snapshot is straightforward enough, especially if you're a longtime user of Photoshop. Both programs use the same term for the same concept. A snapshot in each is a record of what an image looked like at some point in time. You can go back to any saved snapshot easily just by clicking on its name in a list. Both programs maintain a History list that records everything you do to an image that can serve somewhat the same purpose, but it's easy to lose track of which History states represent significant milestones in the editing life of an image. Snapshots let you keep track of this.
You can find Lightroom's snapshot feature in the Develop module. To create a new snapshot, click on the Create Snapshot button in the Snapshot panel. It's the plus sign icon on the right hand end of the Snapshot panel header. When you click on it, a dialog box will open given you the opportunity to name your new snapshot. Type whatever seems appropriate and click on the "Create" button. The entire Snapshot panel can be collapsed down such that only the header shows so if you don't see your new snapshot in the list, you probably need to expand the panel by clicking on the rightward pointing arrow to the left of the panel title. The expand button turns into a downward arrow when the panel is expanded. To delete a snapshot, select it in the list with your mouse, and then click on the minus sign ("Delete Selected Snapshot") button in the panel header.
Snapshots actually exist simply as metadata saved as part of the image to which they pertain. Even though you can toggle between any your saved snapshots, there's really only a single image underlying them all. As such, snapshots require very little additional space on your hard drive. No matter how many snapshots you create for an image, the actual pixel data is only there once.
A virtual copy in Lightroom implements a similar concept but in a different way. Rather than appearing as part of a list while editing an image, virtual copies show up as if they were separate images in the Library module. You can open a virtual copy in the Develop module exactly the same way you would an actual image. Edits made to a virtual copy are maintained completely separately from those made to the original image from which that copy was made. In almost every way, you will think the virtual copy is a separate image, but the truth is, it isn't. As with snapshots, virtual copies exist simply as additional metadata for the image on which they exist. You still have just a single image, but the Lightroom catalog makes it seem as if you had more than one.
To create a virtual copy, simply right click on it in either the Library or Develop module and select "Create Virtual Copy" from the pop-up menu. You can also access the same functionality under "Photo" in the Lighroom menu. Keyboard shortcut fans can also use Control-apostrophe (Command-apostrophe on Mac OS X). Virtual copies are automatically named based on their underlying source image with a suffix designating "Copy 1" and so on. The filmstrip icon for virtual copies have a page curl overlay in the lower left to help you keep track of which is the original and which the copy. To delete a virtual copy, select it and hit the Delete key, just as you would to delete an actual image. Lightroom will prompt you to make sure really want to delete the virtual copy. If instead you are greeted with a more lengthy message about whether you want to delete a master photo you clicked on the real image rather than a virtual copy.
So, when should you use a snapshot and when should you use a virtual copy? Sometimes it really doesn't matter. In both cases you only have a single underlying image so neither wins in terms of space usage. Both allow you to try out various editing options for an image to decide which you like better. You can create as many snapshots or virtual copies as you want with each representing a unique look for the image you are working on.
Since virtual copies behave as if they were images in their own right, you can create snapshots on a virtual copy. On the other hand, if you create one or more snapshots on an image and then create a virtual copy of that image, the virtual copy will inherit the same snapshots as the original. If you then create a new snapshot on the copy, it will show up as a snapshot on the original as well.
Because virtual copies function like independent images, you can add them to separate Collections if you want. Suppose you make a virtual copy of an image and convert the copy to black and white. You could create a Collection of all your black and white images with this one included, while at the same time your original image lives elsewhere. For the same reason, virtual copies also make it easier to do side by side comparisons between image versions. Simply select both in the Library module just as if both were regular images.
Snapshots can make life with Photoshop easier for Lightroom users. It takes a couple of steps, but it's possible to open a Lightroom image in Photoshop with its snapshots intact. Normally, the snapshot metadata lives in the Lightroom catalog. To get it into the image file itself so you can make use of it in another program such as Photoshop, select Photo >> Save Metadata to File. Now when you open that image from Lightroom for editing in Photoshop as a Smart Object, the snapshots will be part of that smart image. If you click on the smart object in the Photoshop Layers panel it will open in Adobe Camera Raw. You can then go to the Snapshots tab in ACR and select from the desired entry from the list. Unfortunately, if you create a new snapshot in Camera Raw, it won't show up back in Lightroom. Creating another snapshot on the image back in Lightroom won't make it available to the Photoshop smart object in Camera Raw either since the available list contains only those snapshots that existed at the time you first opened it in Photoshop. Note that you can't save snapshot metadata to the file for virtual copies since they don't even have files per se. This only works for snapshots on actual Lightroom images, not virtual copies.
Snapshots and virtual copies have a lot in common, but each has its own peculiar behaviors that might cause you to prefer one over the other for different situations. It's good to knowing how both work so you can take advantage of those behaviors when the need arises.