A common example of crooked lines that should be straight is in an architectural photograph. Even if you're not an architectural pro, sometimes the goofy convergence that shows up in shots doesn't do a scene justice. When this optical distortion strikes, try these few simple steps to correct for that vertical convergence-and then apply the same principles to fix all sorts of other distortions in your pictures.
STEP 1: If you are able to work with layers, create a duplicate of your shot on a new one. This will make it easier to monitor progress-or even revert to the prefixed version should the need arise. If you can't work in layers, simply save your shot as a new document to avoid accidentally overwriting the original file. Then get to work.STEP 2: Enlarge the canvas, making it bigger than the original shot. Without an expanded canvas, it's easy to lose valuable picture information outside the edges. For a 4x6 photo, consider adding a couple of inches on each side. You can always crop it back to normal in a later step.
STEP 3: View guides and grid lines if possible, because they act as ideal reference points for what's actually straight and vertical. Remember that you don't always have to make the lines perfectly vertical or parallel to get a pleasing effect. Sometimes just minimizing the distortion is enough to improve the shot significantly, without further compromising the integrity of the scene.
STEP 4: Stretch the top of the image from the center to the edges to correct a structure that bends toward the middle-almost as if you were widening a shot of train tracks as they converge toward the horizon. In Photoshop, choose Edit > Transform > Perspective. Grab the image in a box at the top corners, and as you pull one corner out, you'll see the opposite corner move as well. If you need to correct one side of the image without the other, simply use the Edit > Transform > Distort tool. This tweak is a great way to adjust off-center vertical subjects.
STEP 5: You've now pulled the top edges of the frame outside the original boundaries, effectively changing the proportions and making the shot look squished. To return the shot to an accurate proportion, use the Transform > Scale tool to lift the top of the image (from the center) until the building no longer seems squat.
STEP 6: Now choose the crop tool and drag the selection to cover as much of the image as possible, without including any empty canvas. The top corners should look almost like wings outside of the cropping box. If those areas contain essential image information-anything you don't want to lose-go back and readjust the correction to leave more original but make less perspective change.
STEP 7: Finally, merge the layers into one and save the file (with a new name if you didn't do that way back at the beginning). Before flattening, don't forget to take advantage of those layers and toggle between the old and new versions, marveling at your technical ability and remarkable talent for accurate architecture photography!