UW Wide Converters - Discussion
The digital compact camera is remarkable for its ability to facilitate,
and to some extent automate, the process of producing good-quality
underwater photographs in a way that no film camera ever could. Thus,
when technically mature digital camera models began to appear (in about
2004), they immediately swept away the dreary focus bracketing,
exposure bracketing, 3 good shots per film if you were lucky, tradition
of Nikonos photography. Such cameras nevertheless go against a well
established principle, which is that results will suffer if you try to
do everything with a single (zoom) camera lens.
The first good compact cameras had a minimum focal
length setting equivalent to 35 mm. In other words, they couldn't
do wide-angle photography at all. Put one into an underwater
housing with a flat port, and the built-in lens will give a reasonable
performance at any zoom setting. If you want wide angle however
(and since water is cloudy, you do) you have to fit an adapter. The
apparent logic of this situation is that the camera and housing
combination is optically nearly-perfect, but lacking in FOV. Thus
it is possible to fall into the trap of thinking that the adapter might
possibly be eliminated, if only the camera could be given a wider
minimum zoom setting.
The reality is radically different. It has
been established over about 60 years of collective experience with 35
mm film cameras (and is confirmed by
theory),
that there is a break point at about f = 35 mm. At longer focal length,
the use of a flat port is acceptable, because the optical defects
introduced are not serious, and the reduction in FOV corresponds to an
increase in magnification, which is useful for macro photography. At f
< 35 mm however, the pincushion distortion and aberration caused by
a flat water-air boundary start to become offensive, and it is
necessary to use either a dome port, or something akin to a Rebikoff
corrector.
The unavoidable conclusion is that camera designers
have rather spoilt things for the fixed-port underwater housing
designers (even sometimes within the same company) by making zooms go
to ever shorter focal lengths. Optical quality suffers at f < 35 mm,
it's not the housing maker's fault, and there is an issue of how to
solve this problem in a way that users will understand and be prepared
to accept.
Ease of operation is a primary consideration. Users
want to use the maximum-wide zoom setting as a point of reference, and
then fit an adapter to obtain even more coverage. Supplying an adapter
that vignettes at the wide setting and telling people that they have to
zoom-in to use it then appears to defeat the object. It does no such
thing of course, because, as explained in the introduction article, the
function of the adapter is primarily to correct for the water-air
boundary, and secondarily to increase the coverage. This point however,
sounds like an unscrupulous marketing ploy, and customers are likely to
remain unconvinced. Adapter lens designers have therefore been forced
to respond by trying to solve the 28 or even 24 mm lens vignetting
problem.
The results given in the preceding articles show that, unless the
converter lens has an extremely large exit aperture, the best optical
quality is always associated with the cameras (or camera lenses) that
have a small zoom range and a moderate FOV at the wide setting. The
reasons are that a large zoom range prevents the housing designer from
placing the lens port close to the entrance pupil at wide-angle
settings of the zoom, and a large FOV makes the adapter difficult to
correct for aberration.
The results obtained are however, a little more
equivocal than it might seem at first pass. It is perfectly possible
that, with a well matched camera and adapter combination, the problem
of having a large FOV at the camera might be surmounted. The
drawback however, is that the camera manufacturers change their designs
every few months, and so the details of the optical path are
effectively undefinable. Thus, if we were to test every possible
combination, we might find some good ones involving cameras having
minimum equivalent focal lengths of 28 mm or even 24 mm. So far
however, when using a compact camera in a housing with a fixed flat
port, it appears that, of the options discussed above, the most
reliable way to produce wide-angle underwater pictures that do not
require computer correction is to use a converter designed for 35 mm
lenses and zoom in to eliminate the vignette. One thing we have not
produced however, is any evidence that the adapters designed for 28 mm
are at fault. It is just, perhaps, that they pursue a goal (no
vignetting) that is not necessarily compatible with best optical
quality.
The problem of aberration, of course, evaporates if
radial correction is carried out as a matter of routine. The use of
Panorama Tools in Photoshop is probably a bit too technical for most
people, but lens correction with Adobe Lightroom is easy (and the
program provides RAW converters for a large range of cameras).
An important consideration that seems to get lost in the general
discussion of wide adapters is that a camera that can zoom out to f =
28 or even 24 mm can perfectly well be used at f = 35 mm. The Canon
S100, for example, can be set up so that specific focal lengths
(24, 28, 35, etc.) can be selected using the front control dial. This
means that, provided that the camera can reach (or go to less than) the
adapter's target focal length, then the adapter can always reach its
target angle of coverage. Thus, apart from the convenience of using the
extreme wide zoom setting as a point of reference, nothing is lost by
having to zoom-in until vignetting disappears.
DWK
See also:
Radial image correction.
© David W Knight. 2012, 2018
