Dome and Flat Ports
What’s the Difference?
Dome ports give the photographer and researcher an increased angle of coverage over flat ports and better conserve the image geometry of the main camera lens. Dome ports reduce the incidence of chromatic aberration (color fringing) for correctly positioned lenses and require less post-processing for wide-angle underwater photographs and video. The geometry of a dome port (round) also makes them well suited for imaging at greater depths versus similarly price flat ports.
Flat ports, through diffraction of the water/port/air interface, introduce severe chromatic aberration and radial distortion to images (both of which can be corrected to an extent in software). For imaging flat objects, flat ports may be preferred for corner image sharpness because the field of focus is flat (compared with the curved field of focus produced by dome ports). Focusing through a flat port underwater will be similar to focusing a lens in air though the effective field of view underwater will be reduced approximately 20%, making flat ports more useful narrow focal range for macro photography.
Dome Ports and Virtual Images
When imaging through a dome port underwater a camera must focus much closer than the object being imaged. This characteristic of dome ports is caused by diffraction through the water/dome/air interface and creates a virtual image which is smaller than the original object, closer to the camera, and projected on a curved (spherical) surface. A side benefit of the virtual image phenomenon is a compressed depth of field that allows shooting at wider apertures for increased light gathering capability.
The size of the dome and the distance to the object will both impact the ultimate location of an object’s virtual image and objects closer to the camera will have a corresponding closer virtual image.
Below is a rough rule-of-thumb formula for calculating the location of a virtual image for an object at infinity:
Virtual Image Distance from Dome Equator = (4 * Dome Radius) – (4 * Dome Thickness)
Dome Ports and Camera Alignment
Maintaining the correct camera and lens alignment within a dome is essential for optimizing underwater images. In an optical setup, a lens’ entrance pupil will be lined up with the nodal point of the dome. For a full hemispherical dome, the nodal point will be located at the center of the equator. This point is equidistant from all points on the curved surface of the dome. When the lens and dome are aligned correctly, all light rights entering the lens will be traveling at an angle perpendicular to the surface of the dome, thereby limiting chromatic aberration due to refraction. Improperly aligned lenses will result in blurry images, image artifacts, and difficult to correct distortion.
Designers at Sexton can help you choose which domes, lenses, and cameras are appropriate for your application. Please contact us with any questions you have about integrating dome ports into your housings.