Flat port vs.
By David Knight
In the early days of digital compact cameras (prior to about
the majority of models on offer had zoom lenses with a maximum
wide-angle setting equivalent to about f=35mm on the 35 mm stills
format. This corresponds to a field-of-view (FOV) of
air, which is about the maximum acceptable for use behind a
port underwater if the optical degradations caused by the water-air
boundary are not to be noticeable at moderate image enlargements.
The coverage obtained underwater is then 46°,
hardly be called wide-angle; but various manufacturers also brought-out
wide-angle converters (so-called 'wet lenses') that could be mounted to
the outside of the underwater housing, and these supplementary optics
not only gave true wide-angle coverage, they could also correct for the
chromatic aberration pincushion distortion due to the port (actually,
they usually over-correct for pincushion distortion and display barrel
distortion). Thus, with the aid of cute little polycarbonate
housings, pioneered by Olympus and later followed by Canon, Sony, Fuji
and Panasonic, underwater photography came of age, and people with
compact cameras could match (or, frankly, improve upon) the results
obtained using expensive Nikonos equipment.
Camera manufacturers however, will vy
commercial advantage; and while most consumers don't understand the
specifications that matter (i.e., optical performance figures) they can
be swayed by megapixels and super-zooms (i.e., by things that result in
image files many times larger than is warranted by the actual
information content, and lenses that give wide f-range at the expense
of optical correction). The less extreme examples of cameras
subject to this trend still constitute good designs however, and while
the move away from the traditional 2.5× optical zoom to
4× or even 5× is cause for some
in-air results are generally not too bad.
So it was that in 2004, Olympus brought
C-5060 compact camera with a 35 mm equivalent focal length range of 27
to 110 mm, and a (non-underwater) wide-angle converter giving an
equivalent focal length of 19 mm. This was quickly followed
the upmarket C-8080 compact, with a 2/3" (3.9× crop factor)
a 28 - 140 mm equivalent zoom, and a wide converter
the equivalent of f=22.4 mm. In both cases, the companion
underwater housings had interchangeable ports, with a flat port for use
with the built-in lens, and a wide-angle port allowing the camera to be
used underwater with the wide converter fitted. Unfortunately, the
wide-angle ports were also flat, and so, while suitable for above water
photography in wet conditions, they gave spectacularly bad results
underwater. Fortunately however, ikelite brought out housings
with optional dome ports for both cameras, giving a sensible
alternative. The author also set out to promote an excellent
for flat-port users in the form of computer
, although few were keen to
take the trouble to use it.
The problem, of course, has not gone
Many modern fixed-lens cameras zoom-out to 28 or even 24 mm,
making it extremely difficult for wet lens designers to avoid
vignetting while still prividing good edge resolution. Hence,
those interested in taking high-quality wide angle photographs
(regardless of camera system) the following direct comparison of flat
port vs. dome port should be instructive (even though the C-8080 camera
used for the tests is no longer available).
: Angle of coverage (FOV) figures
given below are
nominal, i.e., on the picture diagonal when focused at infinity.
Some loss of FOV occurs with a dome port due to lens
i.e., the lens must focus on a virtual image at a distance always less
than 4 dome radii in front of the centre of curvature of the dome.
lens and flat port
C-8080 (2/3" CCD) camera in PT-023 housing. Standard flat port.
Full wide (f=7.1mm), 35mm equiv. f=28mm. Aperture as indicated on test
ikelite DS-125 with 4100.5 TTL slave sensor.
underwater FOV = 54° (camera FOV = 75°)
detail is full size, cropped from 3264 × 2448 pixel
from test card to entrance pupil (estimated) = 0.58m
(removal of pincushion distortion and chromatic
aberration). Same photographs as above, corrected in Adobe
Photoshop using the Panorama Tools 3rd party filter plugin.
picture taken at f/2.8 (mixed lighting) has also been corrected for
Chromatic aberration visible in the uncorrected pictures is almost
entirely due to the air-water boundary and is unavoidable when using a
lens with 75° coverage behind a flat port. It will be very
noticeable in large prints and displays with significant edge detail.
The apparent degree
distortion is somewhat less than will be introduced by a flat port when
the camera lens is rectilinear. This is because the C-8080
5× zoom shows some barrel (fisheye) distortion at the 28mm
its range (see below). Most compact cameras, and many
interchangeable zoom lenses, show barrel distortion at the wide
setting, and so it could be argued that the flat port has a correcting
effect. Chromatic aberration however, is a far worse defect
deviation from rectilinearity.
6" dome port
||C-8080 camera in ikelite 6130.80 housing with 9306.38
port. Full wide (35mm equiv f=28mm). Aperture as indicated on
DS-125 flash with diffuser & TTL sync cord.
material: Acrylic, nD=1.492.
Inner radius: 76.2mm. Thickness: 5.54mm.
UW FOV: ca. 75°
detail is cropped from 3264 × 2448 pixel image
from test card to entrance pupil (estimated) = 0.4m
photographs as above, corrected using PanoTools.
The original barrel (fisheye) distortion is due to the camera lens.
The correction needed for chromatic aberration is negligible.
lens and flat port
C-8080 with WCON-08D 0.8× wide converter, in
housing with PPO-05 wide (flat) port. Zoom at full wide,
DS-125 with 4100.5 TTL slave sensor
UW FOV = 62.5° (Camera + WCON FOV in air = 88°)
detail is cropped from 3264 × 2448 pixel image
Distance from test
card to entrance pupil (estimated) = 0.51m
Radial Correction. Same photographs as above, corrected using Panorama
The addition of the wide conversion lens gives the C-8080 a nominal
angle of coverage of 88° in air (equiv. f=22.4mm) and
underwater . The use of such a wide angle lens behind a flat port
underwater results in a degree of pincushion distortion and chromatic
aberration which will be noticeable even at small magnification. Hence,
for underwater photography, software optical correction is almost
lens and 6" dome port
Olympus C-8080 with WCON-08D in ikelite 6130.80 housing with 9306.38
DS-125, diffuser, TTL sync cord.
UW FOV: ca. 88°
detail is cropped from
3264 × 2448 pixel image.
Distance from test card to entrance pupil (estimated) = 0.32m
Softness of corner detail at f/2.8 is due to curvature of
This effect will be less apparent at greater lens to subject distances.
Radial Correction . Same photographs as above, corrected using Panorama
DP with WCON-08D,
It could of course, be argued that test cards are intended to show-up
optical defects, and that the inadequacies of the flat port will not be
noticeable in practice. That this is not the case is
by the picture below, which was taken using a 28mm lens (35mm format)
behind a flat port.
The picture is of a moray eel hunting
It is somewhat cropped from the original because the animal
moving fast and careful composition was not possible. The
contains no straight lines, and so in the absence of geometric cues,
there is no need to correct for pincushion distortion.
for chromatic aberration has however been applied, for reasons which
become apparent from the corner detail images.
Corner detail cropped from 4335 × 2914 pixel image
Corrected for chromatic aberration
|Pano Tools radial
(chromatic aberration only
- no geometric correction)
In the uncorrected picture, it can be seen that the red, green and blue
images of fine detail on the animal's body have failed to superimpose.
Such degradation is easily visible at moderate magnification.
The main advantage of using a dome port in wide angle underwater
photograpy is that the angle of coverage and overall image geometry of
the main lens is conserved. Also we might add that, although
chromatic aberration is introduced at the air-water boundary, is is
slight and barely noticeable in practice.
The flat port, on the other hand,
chromatic aberration and pincushion distortion; and while barrel
distortion is sometimes considered desirable (i.e., when it is referred
to as the 'fisheye effect'), pincushion distortion is only acceptable
if it cancels existing barrel distortion.
After radial correction however, the
appears to produce better corner sharpness at large aperture (i.e.,
f/2.8) than does the dome. The reason is that the test card
flat, and the flat port focuses in a flat field; whereas the field of
focus for the dome port is curved. Hence, to make the test
sharp at the corners when using a dome port, it is necessary to
maximise the depth of field by stopping-down the lens.
The question is whether the curvature
of field of
the dome port optical system matters in practice. For the widest angle
photographs given above (f=22.4mm with dome port), the distance from
the entrance pupil to the test card is about 0.32m, i.e., the card is
24cm from the front of the port, which is very close. For
wide-angle photography applications, the distance will be greater and
the field curvature problem will be much less severe.
Overall, we can commend the dome port
as a device that allows the production of wide-angle underwater photographs
without the need for further processing. The flatness of
the flat port after radial correction may however be of interest for
some types of scientific or technical photography.
One curious irony uncovered by this study is that the Olympus housing
designers achieved nothing useful (from the end-user's point of view at
least) by producing a flat port for the WCON-08D lens. Not only was the
uncorrected image quality terrible, we can also note that the addition
of the WCON increased the underwater FOV from 54 to 62.5°,
changing to a dome port (without fitting the WCON) increased the
coverage to 75° with negligible optical degradation.
dome port is a type of wide conveter in its own right; and a simple
moulded acrylic hemisphere (albeit highly polished) radically exceeded
the performance obtained using an exotic piece of ground multi-coated
optical crown glass.
||The fact that, in a world where flat ports are the
dome on its own is a wide converter, has led some manufacturers to
produce devices called 'dome converters' (see illustration left). These
are more accurately described as plano-convex air lenses, being
constructed as a section cut from a spherical dome, closed by a flat
optical window, filled with air and sealed by one or more O-rings.
If the flat window is placed close to and parallel to the flat port of
an underwater housing, then a light ray heading for the camera first
encounters a dome port, then it meets an air-water boundary followed by
a water-air boundary. If the path through the intermediate
layer is reasonably short (a few mm), then the optical defects
introduced by the air-water boundary are almost exactly cancelled by
water-air boundary. The effect is almost the same as that of
fitting a dome port to the housing, except that the wide converter can
be fitted and detached underwater.
|Wet mounting dome
(plano-convex air lenses)
If the camera zooms-out to 28 or even 24 mm, one advantage that the dome
converter has over the external multi-element wet lens is its very
large rear optical aperture. This means that the system is
unlikely to vignette.
© David W Knight. 2004, 2012. Updated Feb. 2018.
David Knight asserts the right to be recognised as the author of this