The bisected frame of the "over/under" photo, capturing topside and underwater elements in a single shot, is one of the most striking formats available to underwater photographers. While the results can be dramatic, the execution is tricky. To create your own over/under, also known as the "split" or "50/50," you'll want a housed camera with a wide-angle lens and dome port as well as a good understanding of what happens when you use that dome port simultaneously in air and water.

Over/under photos are plagued by complications in both exposure and focus. Focus is complicated by the fact that the above-water portion of the photo is essentially unaffected by the dome port, while the underwater portion is significantly affected. In terms of exposure, the topside portion of the shot is invariably brighter than the underwater half. Here's a quick lesson in over/under physics.

Focus Factors

The dome port offers no special optical properties above water, functioning essentially as a clear window. This means that a subject above water, 10 feet away, will be in sharp focus when the lens's distance scale is set to 10 feet. Lenses are calibrated to work in air, unless they are amphibious water-contact lenses like the Nikonos 15mm or 13mm (for the RS), in which case they don't work well above water at all. In a housing, the topside portion will have air in front of the lens and air in front of the port, so there is no differential medium.

For the portion of the dome port that's in the water, however, the rules change. Because the underwater portion has air in front of the lens, but water in front of the port, the dome port acts essentially as another lens element. A "virtual image" of that portion of the photo is created inches in front of the lens.

The point of focus for the virtual image is dependent on the size of the dome. In ballpark terms, the virtual image exists at twice the diameter of the dome away from the lens. For a six-inch dome, the virtual image is about 12 inches away, and for a nine-inch dome, the focus point can be assumed to be 18 inches away.

Of course this means that a lens that can't focus as near as the virtual image won't be able to focus at all. Some wide-to-telephoto zoom lenses may have a minimum focus of nearly two feet, so to use them behind a six-inch dome requires the use of a supplementary close-up lens known as a diopter. This will change the range of focus, accentuating the near focus capability and therefore allowing focus on the virtual image.

Exposure Factors

Because water is substantially more dense than air, light is absorbed as a function of depth, time of day, water clarity and surface conditions. There may be as little as a half-stop difference between the topside and underwater portions of the photo if you're working in very shallow water with a white sand bottom, or the difference can be two or more stops if you're working in open water without the benefit of light reflected off the bottom. Particularly early in the morning or late in the afternoon, when the light rays strike the water surface at an oblique angle, there can be a significant exposure difference between the topside and underwater portions of the image.

The Optical Solution

Once upon a time a photographer could buy a special filter designed for over/unders called a "split diopter," but I'm not aware of a commercial source for these today. You may have to employ the services of an optician to create a split diopter to your specifications. This filter uses neutral-density glass on the top (to hold back the sunlight and make it more closely approximate the light striking the underwater scene) and a diopter (usually a +3 or +4) to balance the focus. We know that with the wide-angle lens focused on the virtual image for the underwater portion of the frame (say 16 inches for an eight-inch dome), anything more than about two feet distant in the topside half will be out of focus.

The answer is to focus the lens nearly on wherever the topside subject is likely to be (usually infinity) and use the strength of the magnifying lens on the bottom half to pull focus back to the distance required by the virtual image. The choice of diopter strength for the bottom will depend on the size of the dome (as that controls where the virtual image will occur) and the likely distance of the primary topside subject from the lens.

UPSIDE: The split diopter is the best solution to match both light and refraction. Under perfect conditions such as slick calm seas, this is the ideal option.

DOWNSIDE: The filter cannot be rotated under water, so the photographer must choose whether to shoot all verticals or all horizontals in each session. Also, because of the way most split diopters are ground and glued into the filter ring, the split between topside and under water must be exactly symmetrical. The intersection of the two pieces of glass will show unless the composition places the surface of the water at that precise point.

The Practical Solution

With a fisheye lens, the over/under doesn't need to be exactly symmetrical.

A more forgiving means to shoot over/unders is with a full-frame fisheye lens (16mm Nikkor or 15mm Canon). Recently, I have had good luck using the Nikon 12-24mm zoom on my digital SLR cameras as well. The depth of field with these wide lenses is fantastic, enough so that with an aperture of f-8 or smaller it is possible to hold focus on both the topside and underwater scene. I usually focus on the foreground part of the scene, which tends to be the underwater section, and let the background go a little soft if necessary.

UPSIDE: The photographer has the choice of vertical or horizontal compositions because the water/air interface can be anywhere in the frame. Also, this optic is more forgiving of surface chop as there is no fixed intersection of diopter and neutral density to contend with.

DOWNSIDE: The downsides include the extreme distortion typical of fisheyes, and the lack of exposure correction. If you're shooting a shallow reef with lots of reflecting sand, the f-stop difference may be very slight. In this case, you should expose for the highlights (topside) and let the underwater portion record slightly dark. Local controls in Photoshop can bring up the levels in the underwater scene.

Other Variables:

> STROBE FILL: Another way to match the exposure to the topside section is to use strobe fill to augment the underwater portion. This also gives the benefit of adding color and illuminating the mask when photographing divers.

> DOME SIZE: Large-diameter domes tend to spread the water over the surface area more evenly, thereby diminishing the effects of surface chop. In bumpy seas a wider dome is much easier to use.

> DOME MATERIAL: Water sheets off glass more quickly and completely than off Plexiglas. This makes glass ports better for shooting over/unders.

> WETTING AGENTS: Whether you're using a glass or Plexiglas dome, a wetting agent like Rain-X will facilitate the sheeting action of water and minimize unwanted water drops.