An arm is used to firmly attach a strobe or light head to the camera housing. They usually provide a degree of movement to enable the angle and position of the light heads to be changed underwater, whilst retaining a degree of rigidity to avoid accidental repositioning.
Longer arms are used to ensure adequate lighting whilst minimising backscatter.
Backscatter is mainly caused by light from a camera rig lighting up particles in the water in between the lens and the subject. It can also be caused by lighting up open water directly behind a subject and is usually easier to see against a dark background.
The most backscatter can be seen when a strobe is positioned right next to the lens, for example like the internal flash or a compact camera. The backscatter will be worse when the subject is far away, or if the water is filled with plankton or sand.
To reduce backscatter when using a compact camera, use a diffuser, or better still use an external strobe. Make sure you completely block your internal flash when using an external strobe, otherwise there will still be backscatter from the internal flash.
To minimise backscatter further, two strobes should be used far from the lens, that light up the subject with only the soft edge of the light beam.
A flat port on a camera housing acts as a prism. When it is hit by white light, the different colours that comprise the light are refracted unequally, creating chromatic aberrations from some of the frequencies overlapping with one another.
To minimize the effects of chromatic aberration, a Dome Port should be used.
When a Dome Port is used, the focus is on a "virtual image" of the subject in front of the Dome Port itself. This is due to the concave curvature and optical properties of the Dome Port. The distance of the virtual image from the dome port can range from less than a foot to about 18 inches depending on the port.
Some wide angle zoom lenses do not have the ability to focus at these short distances and therefore a close-up diopter (screwed onto the front of the lens) must be used in order to achieve sharp edge to edge focus on the virtual image.
Also known as point-and-shoot camera. They are designed primarily for simple operation and most use focus free lenses or autofocusing, and automatic exposure settings. They often have flash units build in and apart from zoom control they require minimal operator interaction.
Unlike SLR cameras, they usually have no viewfinder, and rely on a rear of camera digital screen to display what is currently in lens view.
Although they provide less "manual" control when compared to SLR cameras they do have some major advantages underwater, primarily due to their compact nature and simplicity of use which simplifies the requirements of compatible underwater housings which makes them cheaper to manufacture.
Dome / Dome Port
A Dome Port is added to an underwater SLR camera housing to allow the use of wide angle "dry" lenses. They are usually "dome" shaped to minimises optical distortion caused by refraction, radial distortion and chromatic aberration.
The curvature of the dome should be aligned to the optical focal point in the lens. Therefore, it is important to make sure that the right dome port and accessories are deployed for the lens in use in order to produce sharp images from edge to edge.
Underwater filters tend to be used for colour compensation and colour conversion. Due to the nature of water filtering out colours at varying depths, filters are primarily used to compensate. The filter to be used is based on the colour of the water being dived, the depth being dived and the light source. In natural light the filter will subsequently be a variant colour of magenta, with green sea filter having more bias towards red, and blue sea towards.
Most cameras used underwater are standard land based cameras that have been placed into third party waterproof housings. The more popular makes and brands of camera usually have housings available for them.
The housing usually provides external buttons and levers that manipulate the standard controls found on the camera.
The cost of the housing is usually determined by the material used to manufacture the housing and the complexity of the housing.
High end housings are often machined from a single piece of aluminium, which provides a light but robust casing, and employ buttons and levers that enable complete control and use of all the features of the camera within.
Cheaper housings tend to be made from moulded plastic and may only provide access to the basic main functions of the camera.
A lanyard physically connects the camera to you. In the event of you having to let go of the camera for whatever reason, a lanyard will prevent your camera dissappearing into the depths.
Two types of lanyard are usually available:
A wrist lanyard, which attaches to the camera housing via a larks head knot at one end and provides a loop which the hand is placed through and then the loop locked in place around the wrist with a toggle. These are ideal for lighter smaller compact camera setups. For larger, heavier setups where the weight hanging off of a wrist would restrict movement of that arm, a clip lanyard should be used.
A clip lanyard attaches the camera housing to your BCD or Harness via metal clips hooked onto the D rings of your BCD or Harness and thus freeing both arms.
Underwater lighting falls into two distinct categories, flash/strobe and always on.
Flash/Strobes are only used with photo cameras, providing a split second of otimal lighting to ensure correct exposure of the picture taken. Due to backscatter and to avoid shadowing, an ideal underwater camera rig consists of multiple strobes.
Always on lighting being required for underwater videography and night photography.
Unlike land based photography, switching lenses underwater is impossible or very limited (specialist underwater ports and wet lenses required). Subsequently as part of the dive plan a decision should be made to what type of photography is planned, be that Macro, Wide Angle or Zoom and the relevant lens setup for the dive.
Manual Slave Strobe
A normal slave strobe has to be "set" to ignore the small preflash and trigger only on the camera's main flash.
An o-ring is used to provide a water tight seal. They are usually made from rubber/silicon and rounded. They are used, along with grease, to provide a pressure seal between two solid edges. They are an inexpensive consumable item and should be replaced on a regular basis or at the first sign of wear/mis-shape to avoid leaks and housing floods.
If you tried to use a wide angle lens in a flat port, the effects of radial distortion would create a sweet spot of focus in the middle of the frame where the light hits both the port and lends at a perfect right angle, and the image would blur as you radiate outward from the centre.
Subsequently a Dome port should be used along with the wide angle lens to compensate the radial distortion.
As light passes from one medium of a particular density (water) to another (the air inside your camera housing) it is bent (refracted). When light enters your camera underwater it has been refracted by approximately 25%. Thus the focal length of your lens also increases by the same.
A Dome Port is used to compensate for this when taking wide angle shots.
Nickname given to a Slave Strobe. See Manual Slave Strobe, TTL Slave Strobe.
Single-lens reflex camera is a camera that uses a semi-automatic moving mirror system that permits the photographer to see through the lens and thus see exactly what is to be captured by the optical sensor.
A strobe, is a device that can emit a powerful "flash" of light on demand. The quality of the strobe will be determined by the levels of control over the strobe, power of the flash, length of the flash, and the maximum speed between flashes that the strobe is capable of. Recent strobes are usually powered by high powered rechargable batteries.
Strobes are only used to light the foreground subjects in wide angle photography, but this ensures the inclusion of color and detail. Wide angle photography requires stronger strobes with a wider angle of coverage than macro photography.
You can shoot wide angle with one strobe, but ideally two strobes gives more even coverage and increases the possibilities whilst reducing backscatter.
Unlike cameras, strobes donít get outdated every year or two, so an investment in the most powerful strobes within your budget range is always advisable.
This is the cable that connects the underwater housing to the strobe head.They are usually used where the camera has a degree of control over the strobe head via the cable to ensure correct exposure by setting timing and power of the flash.
Through-the-lens metering, also known as TTL, is a method of measuring light for photographic exposure purposes. In TTL metering, the light is measured using an in-camera light meter, eliminating the need to use a separate, hand-held light meter. The light enters the photographic lens and is read by the meter, which then outputs a display through a gauge or dial viewable through the lens.
TTL flash is a type of flash that is metered using through-the-lens metering. In TTL flash, the flash head works with the camera's internal metering system to automatically determine flash output for proper exposure of the scene.
TTL Slave Strobe
A TTL Slave Strobe has a sensor that can detect the preflash coming from the camera's small internal flash and "tell" the strobe to mimic it. The camera reads the light coming back to it (from the slave strobe) and sets the flash duration for correct exposure. It then fires the camera's main flash and the slave strobe mimics that too. To avoid backscatter the small flash on the camera is blocked or reflected up towards the TTL Slave sensor.
A Wet Lens is an optical, manual lens, thats attached to the camera housing via a port. As the name suggests, the lens is fully immersed and water exists between the camera/port and lens so care has to be taken when switching wet lens to minimise silt and other particles being trapped between port and lens.
Many compact camera housings can utilise wet lens increasing the range of use of the compact beyond that of its internal lens.