Interchangeable
Lens Ports
Introduction:
Underwater SLR housings from all manufacturers (those known to
the author at least) have interchangeable lens ports. Some compact
camera housings also have interchangeable ports. Potential purchasers
of such housings are therefore faced with the problem of how
to choose from a confusing array of components in order to arrive
at an optical system which fulfils requirements. In general,
manufacturers offer a variety of flat ports, dome ports, extension
rings, ports with additional controls, and mechanical linkages
(gears etc.); and a brief summary of their properties and applications
is given below. |
Sea & Sea SLR ports (NX-bayonet fitting) |
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). |
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Aquatica housing with macro port. |
Hugyfot housing with macro port |
Chromatic aberration of a 50mm lens and flat port.
Shown below are 3 images scanned from a 35mm (36 x 24mm) transparency.
The photograph was taken underwater using a Sigma 50mm macro
lens and a flat port, and was recorded on Kodachrome 25 film.
The camera to subject distance was about 0.6m. The first image
gives an overview of the photograph, the second image is a detail
from the exact centre of the picture, and the third image is
a detail from the top left-hand corner. |
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2) Centre detail scanned at 126 pixels / mm (3200ppi). |
3) Top left corner detail scanned at 126 pixels / mm (3200ppi).
There is no colour fringing in the centre of the picture, but
the corner image shows that detail in the blue colour channel
is displaced outwards, and the red colour channel is displaced
inwards (relative to the green channel), i.e., the magnification
due to the lens-port combination is greater at the blue (short
wavelength) end of the visible spectrum than it is at the red
(long wavelength) end. This is classic chromatic aberration due
to an underwater port. For pictures recorded using a 50mm lens
(35mm format), the effect is noticeable to the trained eye in
large prints, but is negligible for most purposes. For perfectionists,
such aberration can be corrected in software (see the radial
correction article). |
Macro Photography:
Serious macro photography is done with a proper close-focusing
macro lens rather than with the 'macro' setting of some general-purpose
zoom lens. Macro lenses tend to change length somewhat radically
as the focus setting is adjusted, and so it is important that
the length of the port stem is chosen so that the lens cannot
crash into the optical window. Manufacturers produce ports of
varying stem length, or extension rings, to allow for this requirement.
One potential problem with macro lenses and long ports is that
the lens will be a long way back in the port when set to focus
at infinity. Hence it is not always possible to prevent vignetting
when the lens is to be used for normal-perspective photography.
This is a particular problem of 1:1 macro lenses of focal length
in the 50-60mm range.
Nowadays, many photographers are content to use auto-focus for
macro photography and are perfectly happy with the results. AF
is not necessarily optimal in this application however, firstly
because focusing may be unreliable due to lack of light (an auxiliary
focusing light is recommended) and secondly because the depth
of field in macro photographs is often very limited. When photographing
a three-dimensional object, it is best to focus on a point one
third into into its depth, whereas an AF system will tend to
focus on the nearest detail. Hence, you may prefer to use manual
focusing, in which case it will be necessary to fit a gear or
clamp around the focusing ring of the lens and connect it to
a focus control. Since macro lenses are often physically long,
the focus ring may be out of reach of the control actuators provided
in the camera housing. For this eventuality, manufacturers produce
ports with an auxiliary focus control. |
Ikelite Macro Ports
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#5501 Flat Port |
#5502 Flat Port |
#5506 Flat Port With Control |
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. |
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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. |
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Aquatica housing with 8" dome
port |
Ikelite housing with 6" dome
port #5503 |
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Ikelite 6" dome port #5503 |
Ikelite extended 6" dome port
#5503.80 |
Ikelite housing with 8" dome
port assembly #5510.82
Half-In Half-Out:
If a camera housing with a dome port is used above water, the
apparent position of the object will be very little affected
by the presence of the dome. This presents a problem when taking
half-in half-out photographs, because the camera must be able
to focus both on the normal object above water and on a nearby
virtual image of the object below water. The large dome has a
clear advantage in this situation, because it ensures that the
virtual image is as far away as possible. A large depth of field
is still required however, so half-in half-out photographs are
best attempted using a wide angle lens operated at a small aperture. |
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Note that, because a flat port does not preserve the angle of
coverage of the camera lens, a half-in half-out photograph taken
using a macro port will show the underwater part of the image
magnified relative to the above-water part. Shown right is a
half-in half-out picture taken using a 50mm lens behind a flat
port. The effect obtained is generally considered to be undesirable,
which is why a dome port is normally used. |
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Use of Zoom Lenses:
The author must concede that, with the advent of computer-aided
design, some zoom lenses are capable of underwater optical performance
which is actually superior to the smashed-off bottom of an old
beer-bottle. This concession can be made in the case of wide-angle
zoom lenses of limited zoom range, which can be used behind a
dome port in much the same way as a fixed focal-length wide angle
lens. In such cases the zoom ring is fitted with a gear and connected
to a control provided on the camera housing, and the increase
in versatility in comparison to the use of a fixed focal-length
lens is definitely worthwhile.
For longer focal length "standard" zoom lenses (e.g.,
28-70mm), the optical situation is usually not so good. Some
people like to use these lenses because they have a built-in
'macro' facility and seem to offer everything in one package.
The problem however is that most of these lenses change length
as the zoom control is rotated, and are physically shortest at
the wide-angle setting. The port length has to be chosen to allow
for the lens at its maximum extension, but then the front of
the lens will be a long way from the port at the wide-angle setting
and vignetting will result. The solution to the vignetting problem
is to use a dome port, but then the optimum magnification will
not be obtained in macro mode. Wide-range zooms also tend to
have poor minimum focusing distances, which means that macro
photographs have to be taken from some distance (resulting in
foggy pictures with a blue colour cast), and the lens may not
be able to focus on the virtual image produced by the dome underwater.
It then becomes necessary to add a close-up lens to the front
of the zoom lens, which reduces the lens resolution slightly
and prevents the system from focusing above water.
Some zoom lens designs are physically longest when set for widest-angle.
These lenses can often be used to good effect with a flat port
(bearing in mind chromatic aberration at the wide-angle end),
but care is needed in selection of the correct port length.
Most housing manufacturers provide only a rotary coupling to
the lens zoom barrel. Hence push-pull type zoom lenses cannot
be used (except perhaps by pre-setting the focal length before
closing the housing).
DWK July 2006. |
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