Flatport, domeport, fisheyeport or macroport, all underwater photographers or filmers are asking for the right port for the right lens for usage underwater. This question depends on the size of the scene, the financial possibilities and the costs for transporting the equipment.
Strong zoom lenses are inappropiate for underwater photography
A common solution for landphotographers is the usage of zoom lenses with a focal length of 28-500 mm. These lenses offer a wide range of shootings but they are not usable for underwater photography. Because of their change of length while zooming, the ports must have an according size. The port pane should cover the angle of view in the wide angle area and becomes disproportionally big in diameter. If the pane is too small there will be vignetting. The corners of the picture are black because the port is getting photographed at the inside. It´s only feasible for the small Kit lenses like the 18-55 mm for APS-C.
Classics in the underwater photography – macro- and wide angle lenses
The classic underwater lenses are macro or wide angle lenses. For these lenses there are two port Systems, the flatports and the domeports. For focal lengths over 28 mm (APS-C) or an angle of view smaller than 75° you use flatports. The refraction of the water extends the focal length with factor 1,33. If you use a wider angle of view with a flatport this results in blurred edges and vignetting. Macro lenses with their tight viewing angle has no such results. The edges are very sharp. The focal length of the macro lenses can be extended by using close-up lenses. The lenses can be mount with a swivel adapter on a flatport or screwed directly on a macroport. Both systems are screwed together via a M67 mount.
Domeports deliver sharp-edged pictures while using wide angle lenses.
If the angle of view is smaller than 28 mm or higher than 75° you´ll use domeports. They deliver sharp-edged pictures and increase the fun of underwater photography. The size of the domeport depends on the angle of view of the lens.
Physically two basics should be noted, they are decisive for the image quality :
- The domeports have got a lens-effect. The camera shoots a virtual picture through that lens. The distance from the scene to the lens depends on the radius of curvature of the pane. The wider the radius the further away the virtual picture is. With increasing distance of the picture the depth of field increases too. Wider radii of curvature deliver more depth of field and that increases the sharpness of the image, especially in the corners.
- The other physical basic is the position of the lens regarding the dome. If the center of the dome-pane coincides with the nodal point (the front entry point of lenses) of the lens, this will result in good image quality.
With these two basics you can describe all possible compromises of the image-sharpness, the size of the domeport and the costs. In case of zoom-lenses the nodal point often changes position. For delivering sharp-edged images you´ll need wide dome-radii because the problem of basic rule 2 can be compensated by the wider field of depth. In case of fixed focal lengths smaller radii can be used because only basic-rule 2 has to be respected. Considering Half&Half shootings basic-rule 1 outweighs. Two scenes with different distance to the lens have to be photographed, mostly endless above water, about 2-8 m under water. This problem can only be solved with a maximum of depth of field. The compromising of the photographer depends on his claim on image quality and his financial possibilities. Big domeports are much heavier combined with the size some of the flights can become much more expensive..
In reference to underwater ports our approach is flexibility
For approaching the needs of the huge amount of possibilities, we, BS Kinetics, revised our port system completely in 2014. The ports are basically a two-piece construction, a spacer ring on the one hand on the other a pane. The pane can be a flatport for macro- and standard zoom-lenses or a domeport. These domeports can be offered with different radii of curvature and different materials. Beside the glass-domeports there are also plexiglass-domeports. They are much lighter and cheaper. Small scratches can be polished out. But these domes will never reach the quality of a glass-dome. By using this system everybody will find the fitting port for his underwater housing and a lot of scenes can be photographed.