The Keystone effect is defined as, an optical effect by which parallel lines appear to converge because of their varying distance from the camera. Due to their sophisticated circuitry, our eyes automatically correct for this effect with normal viewing, but because our cameras record everything on a flat plane, we are forced to take additional measures to correct for this phenomenon of visual imperfection.
The most common place we try to correct for the keystone effect is in architecture and commercial photography, although there are additional places this can occur. Images of buildings and architecture are classified as either record or interpretive photographs. A record shot is one that attempts to represent all of a structure’s essential details from a relatively point of view with a minimum of distortion. An interpretive shot attempts to convey an impression of the structures character and meaning by adopting a unique point of view and emphasizing certain features of the structure.
Perspective should always be controlled at the time of exposure for optimum negative quality, but this is not always possible with the limitations of some equipment. We are going to look at the steps available to us to take better architectural photographs. They say the serious photographer uses a field camera to control perspective at time of exposure, dealing with the problem up front with a full compliment of swing and tilt adjustments on both the film and lens planes. Having this equipment in your inventory is the most expensive, but effective way to get the best results.
There are some other alternatives to achieve the same results with out the heavy investment of a field camera. To achieve the same results with a 35mm camera, a perspective control (P-C) lens may be used. This wide-angle lens is designed so that its optical elements may be shifted off center and the entire lens rotated on its axis. With these adjustments, some control is available, but not to the same degree as a field camera.
Two more alternatives will not cost you anything, except a lot of walking to compose your image just right. The first is to move far away from the subject and use a very long lens. This will help reduce the keystone effect, but will not eliminate it completely. The second is to locate yourself in a building adjacent from the subject your photographing, about half way up the height of the subject building. At this point, using a wide-angle lens will eliminate the keystone effect. The down side to this, is locating the second building, or tall structure to position yourself for the shot, and then fitting your subject into the compositional frame with your wide angle lens.
With the advent of modern technology, there is one other new alternative to consider. For those who have a computer, Photoshop software, and a scanner at their disposal, you can scan in the negative of your subject, and use a verity of tools in Photoshop, primarily the skew tool, to adjust the ends of each vertical plane, to get some impressive results.
For further detailed reading on this subject, I suggest the same book from which the information for this article was extracted, Introduction to Photography– Forth Edition, Marvin L. Rosen & David L. DeVries.