The use of a flash or strobe is often regarded as the most difficult aspect of underwater photography. Some misconceptions exist about the proper use of flash underwater, especially as it relates to wide-angle photography. Generally, the flash should be used to supplement the overall exposure and to restore lost color, not as the primary light source. In situations such as the interior of caves or shipwrecks, wide-angle images can be 100% strobe light, but such situations are fairly rare. Usually, the photographer tries to create an aesthetic balance between the available sunlight and the strobe. Deep, dark or low visibility environments can make this balance more difficult, but the concept remains the same. Many modern cameras have simplified this process through various automatic exposure modes and the use of through-the-lens (TTL) metering. The increasing use of digital cameras has reduced the learning curve of underwater flash significantly, since the user can instantly review photos and make adjustments.
Color is absorbed as it travels through water, so that the deeper you are, the less reds, oranges and yellow colors remain. The strobe replaces that color. It also helps to provide shadow and texture, and is a valuable tool for creativity.
An added complication is the phenomenon of backscatter, where the flash reflects off particles or plankton in the water. Even seemingly clear water contains enormous amounts of this particulate, even if it is not readily seen by the naked eye. The best technique for avoiding backscatter is positioning the strobe away from the axis of the camera lens. Ideally, this means the flash will not light up the water directly in front of the lens, but will still strike the subject. Various systems of jointed arms and attachments are used to make off-camera strobes easier to manipulate.
When using macro lenses, photographers are much more likely to use 100% strobe light for the exposure. The subject is normally very close to the lens, and the available sunlight is usually not sufficient.
There have been some attempts to avoid the use of flash entirely, but these have mostly failed. In shallow water, the use of custom white-balance provides excellent color without the use of strobe. In theory one could use color filters to overcome the blue-green shift, but this can be problematic. The amount of shift would vary with depth and turbidity, and there would still be a significant loss of contrast. Many digital cameras have settings that will provide color balance, but this can cause other problems. For example, an image shifted toward the "warm" part of the spectrum can create background water which appears gray, purple or pink, and looks unnatural. There have been some successful experiments using filters combined with the raw image format function on some high-end digital cameras, allowing more detailed manipulation in the digital darkroom. This approach will probably always be restricted to shallower depths, where the loss of color is less extreme. In spite of that, it can be effective for large subjects such as shipwrecks which could not be lit effectively with strobes.
Natural light photography underwater[6] can be beautiful when done properly with subjects such as upward silhouettes, light beams, and large subjects such as whales and dolphins.
Although digital cameras have revolutionized many aspects of underwater imaging, it is unlikely that flash will ever be eliminated completely. From an aesthetic standpoint, the flash emphasizes the subject and helps separate it from the blue background, especially in deeper water. Ultimately the loss of color and contrast is a pervasive optical problem that cannot always be adjusted in software such as Photoshop.[citation needed]
Split images[edit]
Another format considered part of underwater photography is the over/under or split image, a composition that includes roughly half above the surface and half underwater, with both in focus. One of the pioneers of the traditional technique was National Geographic photographer David Doubilet, who used it to capture scenes above and below the surface simultaneously. Split images are popular in recreational scuba magazines, often showing divers swimming beneath a boat, or shallow coral reefs with the shoreline seen in the background.
Over/under shots present some technical challenges beyond the scope of most underwater camera systems. Normally an ultra wide angle lens is used, similar to the way it would be used in everyday underwater photography. However, the exposure value in the above water part of the image is often higher (brighter) than in the one underwater. There is also the problem of refraction in the underwater segment, and how it affects the overall focus in relation to the air segment. There are specialized split filters designed to compensate for both of these problems, as well as techniques for creating even exposure across the entire image.
However, pro photographers often use extremely wide or fisheye lens that provides extensive depth of field - and a very small aperture for even more extensive depth of field; this is intended for acceptably sharp focus both on the nearby underwater subject and the more distant elements above water. An external flash can also be very useful underwater, on a low setting, to balance the light: to overcome the difference in brightness of the elements above and below the water.
Over/under photos necessitate the lens or port to be partly below and partly above the surface. When bringing the outer optical surface out of the water, droplets can be left on the surface which can distort the image. This can be avoided to some extent by wiping off the droplets with a chamois leather cloth above the water and lowering the camera to working position. Keeping the port fully wet is an alternative option, which requires the shot to be taken before the water on the top part of the lens surface separates into droplets. Which approach works better will depend on the surface tension of water on the lens surface.
David Doubilet explained his technique for split field images in an interview for Nikon Corporation. "You need to use a D-SLR and a super wide-angle or fisheye lens and a sophisticated housing that has a dome, not a flat port. Underwater images are magnified by 25 percent, and the dome will correct for that. The technique requires a small f/stop—f/16 or smaller—for great depth of field, plus a lens capable of close-focus; you always focus on the subject below the water line. You also have to balance the light. I look for a light bottom—white sand is best—or a light underwater subject. I'll put the strobes down below and light the bottom and then expose for the top. If you shoot at, say, ISO 400, you'll have plenty of exposure for the top, and the strobes will take care of the bottom. Of course, you need subjects that suit the technique."[7]
