Amazing Underwater Pictures DefinitionSource(Google.com.pk)
The techniques involved in using photographic equipment underwater. By far the greatest percentage of underwater photography is done within sport-diving limits in the tropical oceans.
Underwater photographers are faced with specific technical challenges. Water is 600 times denser than air and is predominantly blue in color. Depth affects light and creates physiological considerations for the photographer. As a result, underwater photography requires an understanding of certain principles of light beneath the sea.
As in all photography, consideration of the variables of light transmission is crucial to underwater photography. When sunlight strikes the surface of the sea, its quality and quantity change in several ways. As light travels from air to a denser medium, such as water, the light rays are bent (refracted); one result is magnification of underwater objects by one-third as compared to viewing them in air. The magnification effect must be considered when estimating distances underwater, which is critical for both focus and exposure. Light is absorbed when it propagates through water. Variables affecting the level of light penetration include the time of day (affects the angle at which the sunlight strikes the surface of the water); cloud cover; clarity of the water; depth (light is increasingly absorbed with increasing depth); and surface conditions (if the sea is choppy, more light will be reflected off the surface and less light transmitted to the underwater scene).
Depth affects not only the quantity of light but also the quality of light. Once light passes from air to water, different wavelengths of its spectrum are absorbed as a function of the color of the water and depth. Even in the clearest tropical sea, water serves as a powerful cyan (blue-green) filter. Natural full-spectrum photographs can be taken only with available light in very shallow depths. In ideal daylight conditions and clear ocean water, photographic film fails to record red at about 15 ft (4.5 m) in depth. Orange disappears at 30 ft (9 m), yellow at 60 ft (18 m), green at 80 ft (24 m), and at greater depth only blue and black are recorded on film. To restore color, underwater photographers must use artificial light. Seawater
The water column between photographer and subject degrades both the resolution of the image and the transmission of artificial light (necessary to restore color). Therefore, the most effective underwater photos are taken as close as possible to the subject, thereby creating the need for a variety of optical tools to capture subjects of various sizes within this narrow distance limitation.
There are two types of underwater cameras—amphibious and housed. Amphibious cameras may be used either underwater or topside, although some lenses are for underwater use only (known as water contact lenses). A housed camera is a conventional above-water camera that has been protected from the damaging effects of seawater by a waterproof enclosure. The amphibious camera is protected by a series of O-rings, primarily located at the lens mount, film loading door, shutter release, and other places where controls are necessary. The O-rings make the system not only resistant to leaks but also impervious to dust or inclement weather when used above water.
Deep-sea underwater photography—approximately 150 ft (35 m)—requires the design and use of special camera and lighting equipment. Watertight cases are required for both camera and light source, and they must be able to withstand the pressure generated by the sea. For each 33 ft (10 m) of depth, approximately one additional atmosphere (∼102 kilopascals) of pressure is exerted. At the greatest ocean depths, about 40,000 ft (12,000 m), a case must be able to withstand 17,600 lb/in.2 (1200 kg/cm2). The windows for the lens and electrical seals must also be designed for such pressure to prevent water intrusion.
Auxiliary lighting is required, since daylight is absorbed in both intensity and hue. The camera must be positioned and triggered to render the desired photograph, and the great depths preclude a free-swimming human operator. Operation is often from a cable via sonar sensing equipment or from deep-diving underwater vehicles. Bottom-sensing switches can operate deep-sea cameras for photographing the sea floor, and remotely operated vehicles (ROVs) can incorporate both video and still cameras. Underwater vehicle
When an observer descends to great depths in a diving vehicle, the camera can assist in documentation by recording what is seen. Furthermore, the visual data will assist in accurate description of the observed phenomena. Elapsed-time photography with a motion picture camera in the sea is important in studying sedimentation deposits caused by tides, currents, and storms. Similarly, the observation of biological activity taken with the elapsed-time camera and then speeded up for viewing may reveal processes that cannot ordinarily be observed.