PHOTOMACROGRAPHY
A major problem with the photography of small subjects is the lack of depth-of-field. The use of small apertures helps, but has its limitations and introduces the problem of reduced sharpness of the image (see Chapter 12
for more on depthof-field). There is a way to overcome this problem-a technique called Scanning light photomacrography (SLP) or deep field photomacrography.
The technique has been around since the early 1960s and has been written about by several authors (MacLachlan,
1964; Root, 1985; and Sharp and Kazilek, 1990). 1 have not seen the tech-nique used on minerals or gems, but with some restrictions it could produce excel-lent results. It is an ideal technique for photographing small fossils.
The specimen is mounted on a stage that moves perpendicular to the film plane ( Fig. 7-11). During the exposure, the subject moves either toward or away from the camera, passing (scanning) through a thin plane of light. This proce-dure is done in a darkened room so that the camera only records what is illumi-nated by the thin plane of light. In order to work properly, the thickness of the plane of illumination must be less than the depth-of-field at the aperture and magnification used. The result will be that all parts of the subject will be in
focus (Fig. 7-12).
The components for an SLP system are available from the Irvine Optical Corporation, which calls its system Dynaphot. It is built for use with a 4 x 5 camera or 35 mm and with either two or three illuminators. The system is not cheap, but for those with a small budget or who like to tinker, a usable system can be put together for less than $200 if you are a good scrounger or can borrow or rent some of the components.
In Sharp and Kazilek's setup, they use three Kodak Ektagraphic AF-2 slide projectors as light sources. Each is outfit ted with 102-152 mm zoom lenses and
Figure 7-12 Seashell, Argobuccinum pulchellum, photographed with scanning light photography.
(Photo by Charles Kazilek, courtesy of Arizona State University)
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ELH lamps that are rated at 3360 K. To create the thin horizontal beam of light, they mount portions of double-edged razor blades in plastic side mounts. The slit is centered in the slide opening in the long dimension. Any gaps that can pass light are covered by two layers of metal-lic slide masking tape. The projectors are spaced equally around the subject and are mounted on equipment jacks. These jacks look like a car's scissors jack, and can be bought from scientific supply houses or borrowed from a university or appropriate business if you have the con-nections.
The first step is to level the projectors by placing a level on top of the slide trays both front to back and side to side. Sharp and Kazilek's test subject was a three-sided pyramid of clay oriented so that its edges were aligned with each projector lens. Final adjustment is done with the jack controls and focus and zoom
adjust-ments on the lenses. The three light beams should form an even band around the test subject. The subject should also be centered on each of the beams by placing a white card on the far side of the pyramid from the projector and centering the projected beam on the subject. This last step is critical so that illumination is even and to assure that no part of the subject will be missed during the scan.
Sharp and Kazilek use a modified microscope dissection stage for mounting the subject. l t moves up and down by a focusing motor attachment controlled by a foot switch. They also suggest using a
telescope eyepiece focusing assembly
con-nected to a motor. Any smooth rack-and-pinion movement that can be motorized will work. Be sure that the device you are using will travel a distance at least equal to the height of the subject to be scanned.
Determine the rate of travel for the unit you are using. This figure is needed to determine your exposure times.
The camera is mounted on a copy stand looking down on the subject. Be sure that it is parallel with the stage and perpendicular to the vertical movement of the stage. When making the exposure, the room lights are turned out and the shutter opened and locked on B. Color-compensating filters have to be used to balance the light to the film. At 3350 K, the lights are 150 K too cool (see Chapter 10, Filters). Exposure is best determined by doing a series of test exposures in half-stop increments. If proper color balance is critical, you may wish to determine prop-er color filtration for each batch of film you buy. Examine your test exposure roll on a balanced light box using color-compensating filters until proper color is achieved. To assure that all your film is of the same batch, buy it by the "brick." A brick contains 20 rolls of film.
There are several problems to over-come when using SLP Since the tech-nique records great detail, the subject must be very clean. The technique also produces very contrasty images. The use of three light sources instead of two helps reduce contrast. If possible, use a low contrast film. If slides are not necessary, print film is inherently less contrasty.
Another problem is "light piping," where
transparent-to-translucent subjects trans-mit the light through to other portions of the subject. This can create ghost images if these other portions are not within the light beam. Light piping is a major prob-lem with crystals and gemstones because many such subjects are transparent to translucent. It may be best to restrict yourself to opaque or just slightly translu-cent subjects.
Another problem arises with subjects that are very complex in form so that por-tions shade other porpor-tions, or if there are caviúes present that the light can not enter. Such shaded areas or caviúes will come out black.
Viewers of SLP photographs some-times have problems with perspective and depth because such photos lack the usual reference points. Portions out of focus indicate depth, and shape is often defined by shadows. The best way to deal with this situation, if it is seriously disagreeable, is to take a stereo pair of the subject with SLP Gerakaris (1986) recommends mounting the subject on a tilting stage.
The subject is mounted over the axis of the stage so that when it is tilted from one extreme to the other, the subject moves the required amount to create good threedimensional modeling. For more on stereophotography, see Chapter 14.
One final limitation is that the back-ground will be black. This condition is
generally not a problem, unless the subject is dark and tends to blend in with the background. You may wish to experiment with double exposures as a way to intro-duce a background other than black. The double exposure must be done at maxi-mum extension of the subject towards the camera or you will get ghost images around its edges. Be sure that the illumi-nation on the background does not strike the subject. To be sure of a shadowless background, the subject should be elevat-ed above it on a pelevat-edestal.