Sophisticated 4x5 view cameras have backs that tilt and swing to compensate for keystoning, and you can buy very expensive 35-mm camera lenses that correct this type of distortion as well. You can also avoid keystoning by positioning your camera in such a way that it is level with the plane of the building.

However, this isn't always possible and the image to the right illustrates my point. The shot was taken with a digital camera with a slightly wide-angle lens. In order to fit the entire building into the shot, I had to step back a good distance. The ground sloped behind me and I ended up shooting upward toward the building, creating the keystoning effect.

Luckily, it's not hard to fix shots like this with a perspective transform command. Here's what I did:

1. I copied the background layer containing the building (click on the Layer menu and select Duplicate Layer). I turned the visibility of my original background layer off so it wouldn't confuse me later when I applied the perspective transform. (Turn off the visibility of a layer by deselecting the eye icon in the leftmost side of the Layer palette.) I created a copy because I want to keep the original image intact, and transform commands aren't an option when working on a background layer.

2. I applied Auto Levels to the duplicate layer and increased the saturation.

3. I clicked on the View menu and selected Show Grid to get a series of 90-degree vertical references. The grid makes it easier to determine when the sides of the building are straight. Here's an example of how it looks.

4. I clicked on the Image menu, selected Transform, and then selected Perspective. If you hold down the Alt/Option key when you do this, a new layer containing the transformed image will be automatically created when you apply the perspective command. If you are not concerned about the original image appearing in the background as you adjust the perspective controls, this can save you time. However, with the Alt/Option method, it's not possible to turn off the visibility of the original layer before you apply the transform. I find it distracting when the original shows through, and that's why I chose to manually create a duplicate layer and turn off the visibility of the original layer as described in Step 1.

5. I didn't change any of the default choices in the Transform options bar. They are more applicable to other transform commands, which can also be accessed via the Options bar. You can also switch to other transforms by holding down the Control key and clicking anywhere on the image window. A pop-up window appears with your options.

6. To illustrate how the Perspective command works, I'll give you an example by first using it in the wrong way. This image shows what happens when I place the pointer on the bounding box handle in the upper right and click and drag it inward. This image shows what happens when I change the perspective by going the opposite way.

7. Here's the final image. It has the correct adjustment, confirmed by the vertical grid lines. Click the OK button in the Options bar or press Enter/Return to implement the changes.