1. Open an affected image and choose Filter: Distort: Lens Correction. Image below shows the filter's dialogue box. Notice the grid that assists us in lining things up.

2. The first job is to straighten the image. It's impossible to fix all the other angles if the image is not straight first.

Choose the straighten tool for the left side of the dialogue box as shown in image.

Click-&-drag across the image to define the new horizon. Try to follow a horizontal line in the image if there is one.

3. The image suffers from keystoning it's narrower at the bottom than the top so we'll adjusting the Vertical Perspective to make the top and bottom the same width.

Adjusting the Horizontal perspective appears to rotate the image through 3D space. What's really happening is that we're making one side narrower than the other to combat perspective problems. You can also change the Angle to counter a diagonal distortion. Image shows the perspective repaired.

4. Notice how the image appears to bulge. Move the Remove Distortion slider to the left to 'bulge out' and move to the right to 'pinch in'. This also works by choosing the Remove Distortion tool, which is the topmost tool icon on the left of the Lens Correction window. Once the tool is selected you can drag in the image, although I find that the sliders give you more control.

After adjusting the distortion, you may need to go back and again tweak the perspective. The Vertical Perspective has been readjusted to compensate.

5. Finally, choose an edge option:

Transparency: Produces transparent pixels outside of the distorted rectangle

Background Color: Fill the empty pixels with the current background colour

Edge Extension: Stretches edge pixels to fill the background (see Figure 5).

Click OK to apply

6. The before-&-after shows the power of the Lens Correction filter. To finish off the effect, crop the image to suit your needs. As you can see this really is a great tool for fixing distorted images.