 underwater Photo Course :: (2) Equipment Guide :: Underwater Camera Housings :: Digital Underwater Cameras :: D-SLR Ports
Ports for Underwater Camera Housings
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U/W Photo Course
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The removable port is an integral feature of the underwater camera housing for
DSLRDigital Single Lens Reflex Camera. A camera that uses a mirror to redirect the image projected onto the film into the viewfinder. The mirror flips out of the way when the button is pressed to take the picture. cameras. There are
two basic types, the dome port (for a
wide angle lensA wide angle lens is one that has an angle of view significantly wider than a normal human perspective. Typically less than 28mm (on the 35mm system).) and the flat port (for
macro lensesAn extreme closeup lens.).
- Flat ports are necessary for focal lengths longer
than 28mm. They are especially important to flat-plane lenses for macro
photography.
-
Dome ports are used to maintain the angle of
view with wide-angle lenses. See lenses for further details.
 On land a dome port acts like a window but underwater it behaves like a negative lens.
It forms an apparent image of the subject for your camera to see (which appears closer)
but which corrects for
refractionThe bending of rays of light as they pass from one medium into another e.g. from air to water or from air to glass.. The apparent object distance varies according to the
curvature of the dome. The laws of physics therefore dictate that bigger domes give
sharper pictures!
Ports comprise of an optical element held in an aluminum tube which is physically
attached to the housingA casing or box with waterproof seals designed to contain a camera or other equipment in such a way that it can be used underwater. usually by bayonet mount (and sealed by a user serviceable
o-ringThe rubber ring that seals underwater camera equipment.).
For dome ports there is usually a Shade to avoid
flareFlare is unwanted streaks of light on your photos caused by lens imperfections and/or pointing the camera at the light source. and maintain contrastthe difference in desity between parts of an image. High contrast for example denotes larger differences with blacker blacks and whiter whites.,
which may be removable to suit different lenses.
Port elements are made from
acrylicPoly methyl methacrylate. A clear plastic with good optical properties. Used for many dome ports. Trade names Perspex Plexiglass. or glass but, contrary to what you may think,
acrylic is the best material to construct a port from because it is easier to repair
scratches, does not tend to mist up, and is cheaper. The best acrylic ports are injection
molded rather that vacuum formed.
 The optimum size for a dome is 8". This corrects for refractionThe bending of rays of light as they pass from one medium into another e.g. from air to water or from air to glass. above and below
water. This is useful for over/undershots that cross the surface boundary i.e the camera lens is half-in and half-out of the water showing subjects above and below shots. Domes smaller than 8" may require
you to put a
diopterA unit used to express the power of magnifying glasses. Also used as a name for screwin magnifying lenses which can be fitted to the front of a camera lens. lens on in order to focus properly (and this is a piece of optical
trash you should avoid). A 6" dome is a good compromise.
Zoom lenses are a special case. You can use zooms no
wider than a 28mm behind a flat portA flat glass or plastic port that fits onto the front of the video housing (with no diopter). Zooms wider than 28mm
must be behind a dome portLens port constructed as part of a sphere of uniform thickness for wide-angle underwater photography. (with diopter).
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