Split-shot underwater images, also known as 'over-unders' or 'splits,' are underwater photos where the top half of the photo is above water, lending a dramatic and unusual contrast. This advanced technique requires great attention to detail and setting, as the problem in balancing contrast and sharpness is often difficult to correct in graphic editing software such as Photoshop.

This staple of underwater photography was pioneered by National Geographic photographer David Doubilet, who used it to image scenes above and below the surface of water simultaneously. Split images have become popular in marine biology textbooks and recreational diving magazines and books. Popular subjects often show divers swimming beneath a boat, or shallow coral reefs with bright tropical fish in the foreground.

The ideal conditions for a superb split image are good overhead natural light, relatively calm water, and little to no wind. The best time of day for clear photographs is close to noon when the sun is directly overhead as it lights up the top layer of water, greatly assisting in image quality. Sunny weather also provides a blue sky as well as good light penetration. Positioning the sun behind the camera will minimize the exposure difference between the over and under sections.

Even with good precautions, the brightness in the top half of the photo will usually be greater than the underwater half. Choosing lighter undersea backgrounds such as sand shallows will greatly assist in image clarity. The best image will result when the subject underwater is positioned as close as possible to the camera. Strobes may be used underwater to compensate for the difference in brightness. If light is lacking, one half or both halves of the split image may be out of focus or dull, rendering the image unsatisfactory.

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density filters may also be used to reduce the difference in contrast between the sea and the sky. Besides using a good quality wide angle lens and a small aperture setting (which admits less light to even the bright balance) a split focus diopter may be used to manipulate the shot when all else fails. A split diopter is a half convex glass that attaches in front of the camera's main lens and makes it possible to have one plane in focus in one part of the image, and a different focus in the other half.

Though the preparation may seem intimidating, it is necessary, as it is very difficult to artificially reproduce the appearance of a split image in post-production, without sacrificing quality and realism. Digital editing techniques on a program such as Adobe Photoshop may be used to mesh two underwater photos into one whole. Yet for those striving for a truly exceptional underwater photo, nothing can top a genuine split image brought to life with proper equipment, skills and preparation.