Do It Yourself
Build A Perspective Control Lens
Our project this month is a Perspective Control (PC) lens for 35mm cameras. This optic is often used by architectural photographers to eliminate the distortion caused by tilting a camera upward to include the top of a building. This is accomplished by moving the lens elements laterally with respect to the film, while keeping the optical axis of the lens at right angles to the film plane, in order to include more of the upper scene area (and less of the bottom) with a level camera body. Upward movement is called rise, and downward movement (to include more of the subject bottom) is called fall. This lens also provides sideways movement, known as shift, to take in more of the left or right without having to rotate the camera horizontally on its tripod.
This homegrown lens does all of the above and may even allow more movement than an off-the-shelf model. In addition, while most PC lenses come in only 28mm or 35mm focal lengths, our 70mm (or longer) model requires a greater distance from the subject and keeps you safely on the sidewalk. Although this is a relatively inexpensive Level 3 project made from very simple materials (see the April 2000 issue for an explanation of my DIY complexity scale), it performs remarkably well if built with care.
You will need the following:
The T-mount, glued into a hole in the center of a square piece of foamcore, forms the base of the PC lens when attached to the camera body. The enlarging lens is mounted in the center of another foamcore square that can be slid in all directions across the T-mount board, creating the moving optic. Because the medium format enlarging lens produces an image circle considerably larger than needed for 35mm, it will cover our 24x36mm frame even when shifted considerably from its zero position. An oversized fabric bag bellows, taped over the edges of the boards, keeps out light as the panel is moved.
Rather than incorporate a complicated mechanical focusing system, our lens relies upon hyper-focal focusing--a preset distance at which all subject elements from infinity to the nearest possible distance will be in focus at a given aperture. Mount the camera on a tripod and face it toward an object the proper distance away for the enlarging lens focal length, see the accompanying chart. Center the lens panel over the rear panel and press them together. Most likely the image seen through the SLR viewfinder will be well out of focus. Add focusing shims, made from foamcore and construction paper as shown in the diagram, until the pressed-together stack brings the target object into sharp focus at maximum aperture. Double-check this focus, then glue the shims together and to the rear panel, making sure that the top shim is made of foamcore.
To keep this article within space limitations, I'll let the photos and diagram guide you through the construction and use of the DIY PC lens. As it has no mechanical screw movement adjustment, any combination of rise/shift or fall/shift can be had by simply sliding the lens panel by hand as required. Shoot at f/16 or smaller apertures for maximum depth of field and uniform illumination over the entire 35mm frame.
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