back-scatter

  • Lighting Underwater Pictures

     

    Flashguns mounted on long mounting arms. Underwater photography requires special considerations and it's best to read a book on the subject before you jump in!
    How often do we hear divers, ecstatic after coming back from a dive and describing the visibility as being ‘gin-clear’? The horizontal visibility might have been exceptional and offered clear vision for thirty-metres. Great! How often do we hear of fog warnings on major roads with visibility reduced to thirty-metres? That sums up the difference between being in air and being under water. Water is full of detritus, tiny planktonic life-forms, and other contents for the ‘soup’ in which we normally dive. Not only that but natural daylight from above tends to be flat and dull. Yet when you look at pictures in diving magazines that are crisp and there is usually no hint of this poorer visibility. The photographs look sharp and clear. Why is that?
    The author's lighting rig. (Picture by Ivan Petrov) The author's lighting rig. (Picture by Ivan Petrov)
    Well, your pictures can be equally crisp once you have an understanding of the limitations imposed by making them water. The same rules apply whether you are using flash to shoot still photographs or a continuous light source to shoot moving images. The first rule is to get as close to your subject as possible, eliminating as much of that water as you can. This is where a wide-angle lens, a fish-eye lens or a macro lens can help. The second rule is to light your subject with a full spectrum of colour and that normally needs to be supplied by a light or strobe (electronic flash) that you take with you. You will need to be sure that the light coverage of that light source is adequate for the lens you are using. Plastic diffusers fitted over the front of the light ensures an even wide-angle of light output. Although many underwater photographers opt for two flashguns to give them even lighting, a thoughtfully positioned single flashgun with such a diffuser will usually give just as pleasing results. Some subjects such as sharks actually benefit from the added contrast of a solitary light source.
    A hawksbill turtle feeding on soft coral and photographed by a solitary flash. A hawksbill turtle feeding on soft coral and photographed by a solitary flash.
    How you position this ancillary light is crucial and at Ocean Leisure we can supply you with all manner of problem-solving devices in order to mount your lighting to your camera rig. The major difficulty that people encounter is inadvertently lighting up this detritus in the water. We call it ‘back-scatter’. There are two basic philosophies to how you might position your lights. I subscribe to a solution that positions my light or light as far away from the lens axis of the camera as possible. Imagine if you will, your light or lights illuminating a cone of detritus directly in front of them for a distance of, say, thirty centimetres. You need to be sure that these cones of back-scatter do not intrude into your picture area. This means that with a wide-angle or fish-eye lens you need to get them as far from the lens as possible. I use long flash-mounting arms, angled in such a way that the light sources are not ahead of the lens. They are angled slightly backwards but I can point the flash heads back directly at the subject and get the full effect of the light emitted. An alternative philosophy is to angle your flash heads in such a way that the cone of back-scatter is angled out of the picture. Lighting cameramen in the movie industry call this ‘edge-lighting.’ You use the edge of the light and that reflected off the back-scatter in the ‘cone of light’ adds to the amount of light arriving at the subject. This allows you to use shorter flash-mounting arms with the added convenience that brings with it, but reduces the amount of light illuminating your subject. The standard one-inch ball and clamp systems effect many different solutions.
    My flashguns are mounted on the long arms that ensure they are far from the lens axis of the camera. My flashguns are mounted on the long arms that ensure they are far from the lens axis of the camera. (picture by Saeed Rashid)
    Be coincidence, Saeed Rashid and I both photographed a turtle at the same time and each shot reveals the two alternative ways to light a subject. His flashguns are angled outwards for ‘edge lighting’ whilst mine are angled for a direct effect. The choice is yours but Ocean Leisure has a large stock of mounting arms and trays by a range of different manufacturers.
    Saeed Rashid angles his lights outwards a little in a semblance of 'edge lighting'. Saeed Rashid angles his lights outwards in a semblance of 'edge lighting'. (picture by John Bantin)

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