What is perspective correction as it applies to architectural photography? In a nutshell, perspective correction works to make buildings in photos appear more as they truly are. This is why you may not notice the use of perspective correction; the photo will look "right."
Here's the problem perspective correction addresses: Buildings can be pretty tall and are usually not found in the middle of vast, open landscapes. Odds are that you're not going to be able to back up far enough to frame the whole building with a standard lens. A wide angle lens is often the only way to go.
Wide angle lenses are wonderful. They artificially increase the distance between you and your subject. Depending on what you're trying to photograph, this can be a blessing and a curse. In the case of taller buildings, a wide angle lens allows you to get the entire building in your frame. Since the building is so much taller than you, though, you'll have to tilt the camera up to do it. But, since the lens exaggerates distance and the film plane is now much closer to the bottom of the building than it is to the top, perspective distortion is the result. The top of the building appears significantly farther away and the photo looks as if the building is falling over. In the case of wide, square buildings, it may even appear that one side of the building is shorter than the other. For a tourist snapshot, this is no big deal. For an architect or preservationist, however, it's a terrible way to depict a building.
To better represent a building in a photograph requires the use of special skills and equipment. A key element in eliminating perspective distortion is keeping the film plane parallel to the subject, thus greatly reducing the differences in distance that wide angle lenses exaggerate. Doing this with a standard camera and wide angle lens, however, will give you a perfectly corrected photo of the foreground and front door, and nothing above it.
Here is where the special equipment comes in. You need the ability to move the lens to a point where it can "see" the top of the building without tilting the camera. Notice all the foreground in the lower half of the illustration above. If we could just pull that image down, we'd get the top part of the building into the frame again. With a PC (perspective correction) lens for 35mm cameras, or a large format view camera, this is exactly what you do. By shifting the lens up while keeping the film plane parallel (plumb) to the building, you can pull in the top of the building and keep the parallel lines of the building parallel.
In addition to the feature of a movable lens, PC lenses and view camera lenses rely on a very large coverage area to allow these movements. A normal lens will project an area just large enough to cover the size of the negative. A PC lens or good view camera lens can project an image sometimes twice the size of the negative it needs to cover. This means that in almost any position you might shift the lens to, you won't go beyond the coverage area of the lens.
pc lens for 35mm slr
4x5 view camera
|Is this all there is to it? No! But I hope this explains the basic principles involved. The bottom line is, your eye is an incredible lens and your brain can run circles around the newest version of PhotoShop. To get a photograph that even comes close to what your eye sees and your brain knows to be true often takes special techniques and equipment.