After talking about housings last post , part 2 of the under water cam system , is the PORT.
A lot is going to be discussed here and i thought just before ur heads start aching id give u a real brief summary to help u read more easily. :)
There are basically 2 types of ports :
1) Flat Ports : work for macro lenses and normal lenses .
2) Dome ports : work for wideangle lens.
It is to be noted that the are variable lengths for ports even of the same measurement this is because :
1) Some lenses change considerably in length as they focus .
2) U may need to add a macro lens on top of ur existing one when ure using a dome port , because dome ports make objects seem closer , so in case ur lens doesnt focus under a certain distance u may need an additional macro lens (this will be discussed in more detail further in the post ) .
6"Dome port of different lengths
Flat (Macro) Ports:
A flat port used underwater does not simply act as a window. Instead, the air-water boundary causes it to act as a lens. The effect of this compulsory extra lens element is to reduce the angle of coverage and increase the magnification provided by the main camera lens. The extra magnification is useful with macro lenses, which is why flat ports are sometimes called macro ports. It also means however, that if (for example) you want the same angle of coverage as a 50mm lens, you must fit a 35mm lens. A table showing the reduction in coverage for a lens of a given focal length is given in the angle of coverage article in the optics section.Flat ports are not ideal for use with wide-angle lenses for several reasons: Firstly the port is likely to cause vignetting (cutting off the picture at the corners); secondly the port may introduce noticeable pincushion distortion (although this may be offset by barrel distortion of the lens); and thirdly the edge-sharpness of the picture will begin to deteriorate as the angle of coverage increases. The loss of sharpness is caused by chromatic aberration (colour fringing in off-centre detail). Assuming a 36 x 24mm picture format, or 35mm equivalent focal lengths, a general rule-of thumb is as follows: With 50mm or longer lenses, chromatic aberration is negligible. A 35mm lens will give acceptable results with a flat port (same configuration as the Nikonos W-Nikkor 35mm), but is noticeably better with a dome port. A 28mm lens with a flat port will give poor results. For those who process their images digitally however, chromatic aberration is reversible (see the lens correction article for details) allowing sub-optimal lens-port combinations to be used without penalty except for the loss of coverage (provided that there is no vignetting).
Dome Ports:
Dome ports were introduced in the 1960s, by the photographer Flip Shulke and others, and provided a solution to the problem of vignetting with fish-eye lenses. It soon became obvious however, that the dome port has another desirable property, which is that it corrects 
for the reduction in angle of coverage caused by the air-water boundary. A dome port is set-up for wide-angle photography by placing the entrance pupil of the lens at the centre of curvature of the dome (ie., the point at the middle if the dome were a complete sphere). In this case, light rays heading for the entrance pupil always strike the port surface at right-angles, no refraction occurs, and the angle of coverage of the main lens is conserved. This convenient situation comes at a price however, which is that the port acts as a strong de-magnifying lens.
for the reduction in angle of coverage caused by the air-water boundary. A dome port is set-up for wide-angle photography by placing the entrance pupil of the lens at the centre of curvature of the dome (ie., the point at the middle if the dome were a complete sphere). In this case, light rays heading for the entrance pupil always strike the port surface at right-angles, no refraction occurs, and the angle of coverage of the main lens is conserved. This convenient situation comes at a price however, which is that the port acts as a strong de-magnifying lens.Based on an original illustration by Gale Livers of Ikelite Inc.
The effect of the dome port is to make the subject appear much smaller and much closer than it really is, and the camera has to be able to focus on this 'virtual' image. This is not a problem with true wide-angle lenses, which have a very short minimum focusing distance, but with moderate wide-angle (28 & 35mm) and standard lenses, it may be necessary to add a close-up lens to the front of the main lens, especially when using a dome of small-radius. Be prepared to add a close-up lens if your main lens cannot focus closer than about 25cm from the image plane. Needless to say, when using any underwater port, the distance markings on the lens are not to be believed.Dome Radius or Diameter:Some manufacturers use a nomenclature which attributes a number of "inches" to a dome port. This measurement is not the diameter of the port assembly, but is the inside diameter of the complete sphere from which the dome is notionally cut. Hence an 8" dome port has a 4" radius of curvature, measured from the centre of the notional sphere to the inside surface of the dome. Similarly, a 6" dome port has a 3" radius of curvature. Since the angle of coverage of the camera lens is preserved when the entrance pupil (the place where the iris appears to be when looking into the lens) is placed at the centre of curvature, the radius of curvature is the preferred distance from the entrance pupil to the inside surface of the dome. When purchasing a dome port, it is necessary to obtain something close to the required distance by selecting a port with the nearest appropriate stem-length, or by choosing an extension ring of the appropriate length.The proximity of the virtual image created by the dome is a function of the radius of curvature. Small radius domes place the virtual image closer to the camera than large radius domes. Hence a lens used behind a small radius dome is much more likely to require a supplementary close-up lens than a lens used with a large radius dome. The use of close-up lenses is best avoided if possible, but large radius domes have the disadvantage that they are delicate and vulnerable and contain a lot of air (i.e., additional ballast weight may be needed). Hence, instead of apportioning funds to the purchase of a large dome, it may be better to use a small dome and divert surplus funds to the purchase of a lens with a small minimum focusing distance.Assuming that the main camera lens is perfect (and therefore also mythical), a small radius dome port introduces more chromatic aberration (colour fringing in off-centre detail) than a large radius dome. An improvement in optical quality is not guaranteed by changing to a larger dome however, because the aberration introduced by the small dome may just as easily cancel some of the aberration of the main lens as increase it. Assuming that a close-up lens is not required, the lens data needed to determine which radius of port will give the sharpest pictures with a given lens is difficult to obtain, so it is probably best to choose the port radius on practicalities (size, buoyancy, vulnerability) rather than optical subtleties such as this.
The effect of the dome port is to make the subject appear much smaller and much closer than it really is, and the camera has to be able to focus on this 'virtual' image. This is not a problem with true wide-angle lenses, which have a very short minimum focusing distance, but with moderate wide-angle (28 & 35mm) and standard lenses, it may be necessary to add a close-up lens to the front of the main lens, especially when using a dome of small-radius. Be prepared to add a close-up lens if your main lens cannot focus closer than about 25cm from the image plane. Needless to say, when using any underwater port, the distance markings on the lens are not to be believed.Dome Radius or Diameter:Some manufacturers use a nomenclature which attributes a number of "inches" to a dome port. This measurement is not the diameter of the port assembly, but is the inside diameter of the complete sphere from which the dome is notionally cut. Hence an 8" dome port has a 4" radius of curvature, measured from the centre of the notional sphere to the inside surface of the dome. Similarly, a 6" dome port has a 3" radius of curvature. Since the angle of coverage of the camera lens is preserved when the entrance pupil (the place where the iris appears to be when looking into the lens) is placed at the centre of curvature, the radius of curvature is the preferred distance from the entrance pupil to the inside surface of the dome. When purchasing a dome port, it is necessary to obtain something close to the required distance by selecting a port with the nearest appropriate stem-length, or by choosing an extension ring of the appropriate length.The proximity of the virtual image created by the dome is a function of the radius of curvature. Small radius domes place the virtual image closer to the camera than large radius domes. Hence a lens used behind a small radius dome is much more likely to require a supplementary close-up lens than a lens used with a large radius dome. The use of close-up lenses is best avoided if possible, but large radius domes have the disadvantage that they are delicate and vulnerable and contain a lot of air (i.e., additional ballast weight may be needed). Hence, instead of apportioning funds to the purchase of a large dome, it may be better to use a small dome and divert surplus funds to the purchase of a lens with a small minimum focusing distance.Assuming that the main camera lens is perfect (and therefore also mythical), a small radius dome port introduces more chromatic aberration (colour fringing in off-centre detail) than a large radius dome. An improvement in optical quality is not guaranteed by changing to a larger dome however, because the aberration introduced by the small dome may just as easily cancel some of the aberration of the main lens as increase it. Assuming that a close-up lens is not required, the lens data needed to determine which radius of port will give the sharpest pictures with a given lens is difficult to obtain, so it is probably best to choose the port radius on practicalities (size, buoyancy, vulnerability) rather than optical subtleties such as this.
Article and photos from http://www.camerasunderwater.info/
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