The field of view of a camera lens remains unchanged underwater when the nodal point of the lens is positioned at the centre of internal curvature of the dome port. A dome's internal radius of curvature is consequently a key measurement. A dome port with the appropriate radial specification is therefore the best option but, where this is not available, an extension of suitable length can be added. Specifications for dome ports generally include a figure for the internal diameter of the sphere from which the hemispherical dome was notionally cut. This measurement is not the diameter of the port assembly, but the inside diameter of the complete sphere from which the dome is notionally cut. A port specified as having an 20cm dome consequently has a 10cm internal radius of curvature.
The position of the virtual image created by a dome port is a function of this same radius of curvature. Small radius domes produce a virtual image closer to the camera than those with larger radii. Optical calculations reveal that the virtual image created by a dome with a subject positioned at infinity will be located at a distance in front of the outer surface of the dome equal to approximately 2.25 times the relevant radius of curvature. Note that the precise figures are dependent upon the refractive indices of the dome material, typically 1.5, and those of air and water which are typically 1.0 and 1.33 respectively. Subjects positioned closer to the dome result in a virtual image even closer to the dome. A camera lens used with a small radius dome is therefore more likely to be fitted with a supplementary close-up lens - which is not ideal. However, large-radius domes tend to be fragile delicate and also contain a larger amount of air and hence buoyancy. An inherent problem with any virtual image is that it does not lie in one plane. It is a curved image formed on the surface of an imaginary sphere concentric with the dome and having a radius of about 3.25 (2.25 +1) times the dome's radius of curvature. Consequently, depth of field may be an issue when using a camera and lens designed to focus on a flat plane.
A lens used behind a dome port should ideally have a range of focus extending from the front of the dome to the virtual image of a subject located at infinity (maybe 3.25 times the radius of curvature). A compromise between a dome's size and its practicality may therefore have to be made. A large dome may be fragile, buoyant and cumbersome and imposes a limit on the closest focus distance, and a small dome may make it difficult to allow a lens to achieve focus without a supplementary lens.
The focus range of a camera's lens can effectively be recalibrated with a supplementary close-up lens of the correct dioptric power. An ideal situation changes the normal focus range of a lens, from minimum focus distance to infinity, to one extending from the radius of curvature of the dome port to the distance of the virtual image (about 3.25 times the radius of curvature of the domed port). The correct dioptric value for the supplementary lens is calculated by the equation:
Dioptres = 1,000 / (VI)
where VI is the distance to the virtual image - approximately 3.25 times the internal radius of curvature of the dome port in millimetres.