Before we start, it's worth repeating that Perspective Warp is as much a creative tool as it is a photo correction tool. If you have unwanted distortions in your photos that stem from the use of a particular lens or focal length, you may want to try the Lens Profile Corrections in ACR, the Upright feature in ACR, the Adaptive Wide Angle filter in Photoshop, or some combination of these first. The Upright and Lens Profile Correction options are also available in Lightroom. Manual corrections are also possible in both ACR and Lightroom, but tend to require more trial and error than the options noted above.
Perspective Warp can be used in many photographic workflows, but I think the most interesting from a photographic perspective, is the ability to subtlety "shift the camera position" after the fact. However, there are a few caveats to this workflow that you should consider before starting.
First, this feature is best adapted to modifying the perspective of architectural imagery, although it can certainly be used on other kinds of pictures. The reason it works better with architecture is that it requires you to define planar shapes in your image, before warping anything. Absent the straight-edged corners, the rooflines, and other geometric features common to man-made structures, it may be a little tricky to properly "map" the photo. It’s still possible, but only with significant trial and error.
The other thing to keep in mind is that this command is not magic. It can only modify the angle of the shot by a few degrees, and ideally the original photo should show two complete sides of a structure (along with some details in the distance). Last but not least, having some room above and below the structure can be useful when you want to warp things in a realistic way. Let's take a look!
Define the Perspective Planes Using Quads
The photo shown in Figure 1 is a good example of a shot that lends itself to the Perspective Warp workflow. This creative commons image of Lytham Hall (courtesy of Michael Beckwith) is a good example of a shot that can benefit from Perspective Warp.
Figure 1 A good example of a shot that is well-suited to Photoshop's new Perspective Warp command.
Shot on an angle, we can see two sides of the hall, and it's not cut off at the top or bottom. Perspective Warp allows us to shift the perspective so that it appears the camera was originally more "to the right" when the picture was taken. This will have the effect of stretching the front of the house and shortening the left side, while maintaining relatively accurate proportions and details. With your chosen shot open, choose Edit > Perspective Warp. This will display the Perspective Warp controls in the Options bar, shown in Figure 2.
Figure 2 The Perspective Warp options are quite simple and easy to use once you've experimented a bit.
You'll notice that by default, the Layout mode is selected in the Options Bar. First you need to place two or more "quads" (as Adobe calls them) onto the canvas, in order to define the surface planes of the object you will be warping. Think of each wall in a photographed structure as a plane sitting in 3D space, whose general dimensions and perspective lines must be defined for Photoshop. This is how Photoshop calculates the actual "3D space" that is represented by the 2D image, and thus understands how it should warp the pixels in two dimensions (later on).
To create a quad, move the cursor over the surface that you'd like to define, then click, drag, and release. It doesn't matter how large your quad is initially—just get it reasonably close to where you want to work. You'll resize and reshape things as you go. For this shot, I created a quad over the left side of the Hall (Figure 3).
Figure 3 To start, you must place quads onto the canvas, to define the surface planes of the object you want to warp.
Each quad can be moved around the canvas by clicking and dragging inside its border, and you can change the shape of the quad by clicking and dragging any of the corners individually. This shot contains several clearly defined straight edges on both sides of the house. I started by moving the quad down and placing the top corners at either end of the white trim beneath the roofline on the left side of the hall (Figure 4).
Figure 4 Line the edges of each quad up with the edges of the perspective plane you're trying to match.
From there I used the edges of the white bricks on the corners of the house, as well as the tops of the windows, to rough out all the edges of my first quad. Note that you may have to go back, after setting three or four corners, to straighten out your original corners a little bit more. It is an iterative process to match the perspective of your quads to the sides of the structure.
Once the perspective lines of your quad are a good approximation of the surface it covers, you can hold down the Shift key and then drag any of its outer edges, in order to extend that edge without losing the shape of the quad. That is, it maintains the proportion of all edges. Ultimately, the idea is to cover most, if not the entire structure, unless you're trying to warp one area and not the others). When your quad extends to the top or bottom of the structure, you may notice that the perspective no longer matches precisely on one or both edges.
This is OK; it happens because the base of the structure is closer to the lens than the top (sometimes much closer depend on your subject), so the perspective lines will skew slightly as you increase the size of your quads. Our eyes don't notice this skewing until we try to match things up on a larger scale. Figure 5 shows a simple correction at the bottom of the wall; in most cases the additional adjustments required are small ones, especially if the initial quad covers a large portion of the plane you're matching.
Figure 5 As you extend the quad to the top or bottom of the structure, you may need to tweak your perspective lines a bit.
From this point, you can create as many additional quads as you need, in order to cover the parts of the image you'd like to warp. Here I added one more: to the front of the hall (Figure 6).
Figure 6 Two sides of the structure, two quads ready to go!