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:

  • A sheet of 3/16" thick black (both sides and interior) foamcore, such as Hunt's 20x30" Sturdy Board (800-879-4868; www.hunt corp.com) from an art or office supply store. This or similar foamcore remains virtually warp-free during construction and use, is very easy to cut with an art knife and goes together easily with only white glue. Save the leftover pieces for future project ideas.
  • A T-mount for your camera, available from your photo dealer or by mail from many Shutterbug advertisers.
  • A 70-105mm enlarging lens. They can often be found used at camera shops and photo swap meets for very little money.
  • Less than a yard of lightproof cloth, such as the black vinyl-clad material used for upholstering, from a fabric shop.
  • Several sheets of black construction paper.
  • Basic hand tools, black photo tape, white glue, and flat black paint.

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.


The finished DIY PC lens at its zeroed position. Glue the T-mount in its hole so that the panel is squarely aligned with the camera when installed. When the glue is thoroughly dry, give this joint at least two lightproofing coats of flat black paint inside and out. After pre-focusing the stack of shims to the proper hyperfocal distance, create a flexible bag bellows by taping lightproof fabric to the front and rear panels. Use large office binder clips or the method described to hold the front panel in place.
Photos © Tom Fuller, 2000

 


The camera must be level for a pleasing rendering of architectural subjects. However, while this eye-level composition keeps the church door straight and upright, it cuts off the top of the building and includes too much foreground. The 75mm focal length of the enlarging lens I used placed the tripod-mounted camera across the street.

 



The PC lens in its raised position. Although I originally used binder clips to hold the front panel, I found it is also possible to "hand hold" it during use. With the camera securely mounted on a tripod, shift the lens as needed for the desired composition and hold it firmly in place with one hand. Start the self-timer with the other hand, then bring it up to gently and evenly squeeze the panels together before exposure. However, go back to the clips if you notice any decrease in image quality. Use manual focus and manual or aperture-priority automatic exposure modes. Most meters give accurate stop-down exposure readings, but refer to your camera instruction manual if this metering method is new to you. Exposure is controlled by shutter speed, film speed choice and, if needed, a neutral-density filter placed in front of or behind the enlarging lens.

 


The trick is to simply leave the camera body alone and raise the lens to include the desired subject area. This action eliminates the "falling over backward" effect that would result from tilting the camera (with conventional lens) upward. Our PC lens can be slid downward to include more foreground if desired, or moved left or right to shoot past a car parked in the spot where you would normally place the tripod. I'm sure sharp-eyed readers will wonder, as did I when taking the picture, if this is really St. George Church.

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