Flat Port vs Dome Port Theory
One of the most important and often misunderstood features of an underwater camera housing is the lens port. Flat ports were all that were available for underwater photography from its beginning in 1893 up until 1931 when A. Dratz designed hemispherical dome ports in order to correct for water refraction. Flat and dome ports both have their place in underwater photography and it is important to know the theory and practice of each. For straight underwater photography, the dome port is the best choice, but if the shot starts or ends above water or if you need to shoot an extreme close-up, the flat port should be used.
The Flat Port:
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The flat port is unable to correct for the distortion produced by the differences between the indexes of refraction of light in air and in water. Using a flat port introduces a number of aberrations when used underwater. They are:
This is the bending of light waves as they pass through different mediums of optical density (I.E.: The air inside the camera housing and the water outside the lens port). Light is refracted 25% causing the lens to undergo the same magnification you would see through a facemask. The focal length of your lens also increases by 25% and the refractive effect produces an apparent shift of the subject towards the camera to 3/4 its true distance. For flat port underwater photography, if you measured your camera to subject distance at 4 ft. you should set your lens at 3/4 the distance or 3 ft.
Because flat ports do not distort light rays equally they have a progressive radial distortion that becomes more obvious as wider lenses are used. The effect is a progressive blur which increases with large apertures on wide lenses. Light rays passing through the center of the port are not affected because their direction of travel is at right angles to the water-air interface of the port.
White light, when refracted, is broken up into the color spectrum. The component colors of white light do not travel at the same speed, and light rays passing from water, to glass, to air will be unequally bent. When light separates into its component colors the different colors slightly overlap, causing a loss of sharpness and color saturation which is more noticeable with wider lenses.
It should be noted that shooting through a window or port of a tank or pool is the same as using a flat port, but the camera lens axis must be at 90 degrees to the window surface. Camera moves will be very limited and panning and tilting should not be done unless an extremely distortive effect is desired. Camera moves in this situation can be accomplished by dollying and booming.
The Dome Port:
The dome port is a concentric lens which acts as an additional optical element to the camera lens. The dome port eliminates the problems of refraction, radial distortion and axial and chromatic aberrations when the curvature of the dome's inside radius is placed as close to the entrance pupil or nodal point of the lens. The dome port also allows the "in air" lens to retain its acceptance angle, but optically an "aerial or virtual image" is created inches in front of the lens. To photograph a subject underwater with a dome port you must focus the lens on the "aerial or virtual image", not the subject itself, as the dome makes the lenses footage marks totally inaccurate.
To focus a lens with a dome port, set your lens at 16" in the housing, to begin calibration for underwater photography. If a lens cannot focus close enough to take advantage of the dome port, use a plus 1,2, or 3 diopter to shift the lens focus back. Ultimately your lens should be able to focus down to at least 10 inches to be able to follow focus from 1 ft. to underwater infinity. When using anamorphic lenses with the dome port, you will have to add a +3 diopter to the lens to attain close focus. To guarantee a sharp image when using either the flat or dome ports, it is best to calibrate the lenses underwater using a focus chart. Set the housing on a tripod underwater and secure a focus chart on a C-Stand. It is best to do the focus test at night or in an indoor pool and cross light the focus chart with two underwater 1200W HMIs or 1Ks. This allows for the best viewing of the focus chart for critical focus. Using a measuring tape, place the chart 2' from the camera film plane and eye focus for the sharpest image. Mark a line and the number "2" on the data ring with a soft lead pencil. Repeat for 3', 4', 5', 6', 8', 10', 12', 14', etc. When focus calibration is complete, surface, dry the data ring and go over the pencil marks with a black "Sharpie". Repeat this process for each lens to be used.
Cleaning The Glass Dome Port:
This port should receive the same care as any professional motion picture lens. Blow off the dirt particles first and use standard lens cleaner and tissue to clean. When not shooting, leave the port cover on, to protect the glass port.
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