Making Photographs with a Zone Plate
Recently, Iíve started making images using zone plates. Zone plate photography is similar to pinhole photography in that itís a form of lensless image making and the photos produced by this technique have unique characteristics compared with conventional lens images.
* Since the area of each zone must be equal for even illumination, the concentric rings radii are proportional to the square roots of the integers. This is why the zones increase in width the closer they are to the center of the plate. Formulas and instructions for creating your own zone plates are available online. Briefly, a magnified set of alternating rings are drawn on paper (or printed from a computer drawing program) then photographed at the desired reduced size on a high contrast film like Kodalith. This small zone plate image is then mounted onto the camera. Of course the easiest way to obtain a high quality zone plate is to buy one. Several companies, including Zero Image, sell zone plates of various focal lengths ready to be installed on a camera of your choosing.
Whereas pinhole photography is a somewhat common form of alternative image making (I often require students to construct and use pinhole cameras in introductory photography courses and have conducted pinhole workshops) zone plates are less familiar to photographers and more often encountered in scientific applications where theyíre used to focus X-rays.
Simply stated, a zone plate, also known as a Fresnel zone plate after French physicist Augustin Fresnel who wrote about wave theory and diffraction during the early 19th century, is a set of alternating opaque and transparent concentric rings which diffract the light passing through them. This diffraction occurs when light skims the edges of the opaque rings. The size and number of rings required to produce a zone plate of a specific focal length is determined by a mathematical formula.*
Both pinhole and zone plate photography work on the principal of diffraction to focus light. Unlike the pinhole, which tends to produce sharp images, the nature of the zone plate allows a great deal of undiffracted light to reach the image plane resulting in photos that are almost soft-focus with distinct, glowing halos in the highlights.
Zone plate images are somewhat reduced in contrast and itís often necessary to increase film development times to produce a negative that prints well. Of course film selection, type of developer and the specifics of processing are all dependant upon the final effect the photographer desires.
Zone plates have smaller effective f-numbers than pinholes of similar focal length. The wide-angle zone plates I use are between f/45 and f/64 and the pinhole equivalent is around f/160 or f/180. This three to four stop difference makes for much shorter exposure times than with a pinhole; on a sunny day using 400 ASA film, an exposure for a zone plate is around 1/8 or a 1/15 of a second whereas a pinhole exposure may be in excess of two seconds.
While these shorter exposures can be a benefit - reciprocity departure is less of an issue than with pinholes and a tripod can often be avoided - it also means exposure times must be carefully controlled and the common strip of black tape or lens cap used for shutters on pinholes may not necessarily work with a zone plate as greater accuracy is required.
Of course itís possible (and rewarding!) to construct your own zone plate camera, but Iíve been using several designed by Zernike Au of the Zero Image Company in Hong Kong. These beautiful cameras are lovingly constructed of teakwood and brass and are as much a work of art as the photos one can create with them. Zero Image builds cameras ranging from 35mm to 4x5 inch format and I own both the 6x6 and 4x5 versions. I canít recommend these enough; there are few pieces of equipment I enjoy as much.
Iím finding the zone plate creates photos resembling those made by toy cameras with plastic lenses. Like the Diana and its clones, zone plate images are imbued with a dreamlike quality one is unable to obtain with a modern glass lens. The soft rendering of detail and the "weight" of the image is almost magical.
'A chair in my yard.' First image from a zone plate of my own design.