November 2002

Physics under Water

Tauchen am Great Barrier Riff The students of a new course at James Cook University in Cairns in Australia have a practical study “classroom”, for which they are to be particularly envied: The Great Barrier Reef. Here, in this wonderwork of nature, the practical component of the course "Introduction to Underwater Photography" takes place. Apart from diving and taking photos, some physics is also included.

Under water, everything is different: there are fewer noises, movements will become gentle and calm, the colors will disappear and everything appears blue. On their diving trips, the students meet swarms of fish, fascinating corals, sea anemones and giant shells. There is also the chance to dive with rays, sharks and - during particular seasons - with whales.

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The task for the students is clear: to take professional quality photos. "That is not as easy as one might think at first", says Peter Kennedy, the director of the new course. He himself has dived for two years and has only just begun to discover underwater photography. Therefore he secured the services of two professionals as lecturers in the course: Bob and Dinah Halstead, who have dived and photographed for thirty years and for some years actually lived more on their dive boat Telita, than ashore. Kennedy knows the difficulties of the beginners from his own experience: "You have to be able to dive completely without using hands because you need them for the camera and strobe lights."

For many types of cameras, including digital and single lens reflex types, housings are available which withstand water pressure and thus it is possible to make completely normal land cameras suitable for shooting under water. For the photographer, of almost as much importance as the camera is the strobe lighting and an understanding of the physical laws that govern the behavior of light under water.

For example, water refracts light differently from air. Due to the higher refractive index, all objects under water appear to be about one quarter closer and thus one third larger than they are in reality. A typical beginner error is to adjust the focus on the lens to the wrong distance, but with time, the photographer learns to estimate distances correctly.

The real traps for the underwater fotografer, however, lie elsewhere: first of all, it may be too dark, secondly, the colors disappear with depth, and thirdly, the light under water can be scattered by particles so that the pictures are degraded. Physicists use the terms "reflection", "extinction" and ”scattering” for these photographic annoyances.

The reasons for them are simple physical phenomena. Firstly, by reflection, light bounces off surfaces, including the surface of the water, and is deflected. Thus not all light from above penetrates the water. It is best to photograph in the middle of the day, because then the sun is high in the sky and the rays of light meet the water surface more or less perpendicularly, and under water it is brighter. However, in addition to this factor, wave action has an important influence on the lighting conditions: the more wave action, the less perpendicular the light can enter the water, and the darker it becomes.

Already one hundred years ago, the pioneers of underwater photography tried to overcome the poor lighting conditions by bringing their own source of light under water. This light source, which at that time was still a separate monstrous lighting apparatus, belongs today with each good camera: strobe lighting, otherwise known as electronic flash. It is not only important for bringing light into the darkness, but it also counteracts the extinction of the colors under water.

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For many holiday-makers the problem of color absorption expresses itself in practical terms as discoloration in their underwater pictures. Novice divers eagerly buy a water-resistant disposable camera in holiday places, but after developing the film, they are disappointed: all the pictures are blue toned. That is because the colors of the spectrum are rapidly and differentially absorbed. Most red is absorbed already at a depth of three meters. In approximately five meters, orange begins to fade, and by ten meters, the yellow is almost gone. At greater depths, green and blue also gradually disappear. This has the effect that a bright yellow snorkel appears an indistinguishable greenish color at thirty meters depth. However, if the diver directs their torch toward the snorkel, it shines yellow again.

Interestingly, scientists now take advantage of this factor in order to investigate the sea and its productivity. Absorption of light is dependent upon dispersion and therefore upon the amount of matter suspended in the water. If, for example, many algae are in the water, the rays of light are dispersed through more particles (the algae) and so the colors of the spectrum disappear faster. Thus, with much suspended matter, red may have disappeared completely after only one meter. In Oldenburg, marine physicists use this effect and work with photo-sensitive bio-optical sensors, which measure how much light of which color still arrives at different depths. From these data then, conclusions over algae, bacteria or minerals of the sea can be derived.

For the average photographer, absorption is, however, rather annoying. With the use of electronic flash, the problem is only partly solved. Red, in particular, cheats many beginners again and again. For example, if the photographer wants to photograph an object which is 1.5 meters away, the light from the electronic flash must travel to the object and then back to the camera, a distance of three meters under water, and the red is progressively absorbed in the water. Many underwater photos show the result of this problem. For example, red coral appears bright red in the foreground, but the redness fades in the background.

When much suspended matter is in the water, the necessity for strobe lighting can become a genuine problem, such particles often mutate into ugly large squares in the picture. Here again, reflection is at work, because the particles in the water are like small mirrors, which reflect the light of the strobe onto the film. In this case, photographers can hold individually mobile strobes far away from the camera, so that the light does not reflect directly onto the film. The precise positioning of the strobes, the type of lens used, and the positioning of the camera are also of critical importance.

However, some researchers are using even more sophisticated methods: they use recently developed video cameras, which are equipped with lasers. To put it in a nutshell, these cameras function because they are extremely fast. Both the pulse duration of the laser, and the shutter speed of the camera are of only five nanoseconds duration, which amounts to one five-millionths of a second. When a picture is taken, the scattered light (known as “backscatter”), which was reflected by particles on the way to the object, returns to the lens earlier than the light reflected from the object. Since the shutter speed is so unbelievably short, it can be adjusted precisely to the window in time when the backscatter has already passed and the light reflected from the more distant desired object strikes the camera. Thus the marine physicists of the University of Oldenburg take completely accurate pictures without backscatter.

The students in Cairns are not occupied with such highly technical equipment. For them, the conventional camera with strobe lighting is sufficient. However, just the like for the scientists, avoidance of backscatter and the appropriate use of lenses and strobe lightening are of majour concern for the students. Next year, the course is to be expanded to include underwater videography.

Author:
Copyright of images [in order of appearance]: Maya Holme | Praveen Wignarajah | Rebecca Hayllar

German version of the text

Hyperlinks

Weitere schöne Fotos von den Studenten finden Sie direkt auf der Seite der JCU
Mehr Infos zu dem Kurs der JCU
Viele praktische Tipps zu Fotografie allgemein und UW-Fotografie liefert diese kanadische Website (Englisch).
Zwei begeisterte Unterwasserfotografen haben ausfürliche Informationen zu Kameras, Gehäusen, Blitz, Reisen, Wettbewerben, Reisen und vielem mehr zusammengestellt.
Wie sieht ein Studiengang Meeresbiologie in Deutschland aus? Infos von der Uni Rostock.

Literature

Halstead, B. (2000). Coral Sea Reef Guide. Sea Challengers. Danville.
www.seachallengers.com
Der Autor dieses Buches, Bob Halsted, unterrichtet bei den praktischen Stunden des Faches Underwater Photography der JCU.
Edgar, G. (2001). Australian Marine Life. New Holland/Struik. Sydney.
www.newholland.com.au
Odewald, L. (1999). Fotografie. Der Weg zum guten Foto. Jahr Verlag. Hamburg.
Fiedler, W. (2001). Unterwasserfotografie. Ausrüstung, Technik, Motive. Könemann Verlagsgesellschaft. Köln.
Köhler, A. & Köhler, D. (1999). Handbuch der Unterwasserfotografie. Inklusive Unterwasser- Videokameras. Delius Klasing Verlag. Bielefeld.