Comparison of M67 underwater wide-angle adapters
Introduction:
These tests were carried out in order to compare the performances
of the various M67 supplementary wide-angle lenses which were
commercially available in June 2004 (The Sea & Sea 0.56x
lens is now discontinued). The camera used was an Olympus C-5050
(5Mp, 2560 x 1920 RAW) in an Olympus PT-015 housing (both now
discontinued). |
 |
When examining the test results,
note that the interpretation of the images depends on the angle
of coverage achieved. Extremely wide-angle lenses, i.e., the Inon
UWL-100 with its dome-lens fitted, cannot be expected to produce
the same apparent corner sharpness as lesser lenses, the reason
being due to curvature of field (the zone of sharpest focus is
not flat). Also be aware that a low magnification ratio corresponds
to a large angle of coverage, i.e., a 0.56x lens is wider than
a 0.58x lens.
In each case, a photograph was taken
with the camera zoom-lens set at full-wide (35mm equivalent f
= 35mm) with the aperture set at f/8. Numerical values for geometric
distortion and chromatic aberration were then obtained by correcting
for these effects using the Panorama Tools radial correction function.
In this way, the manufacturer's claims regarding correction for
chromatic aberration can be verified, and the lenses can be ranked
in order of 'fish-eye-ness'. Note that the radial corrections
obtained are not meant to be definitive (more time spent adjusting
the coefficients might have produced slightly better results),
but they are good enough for the present purpose.
Test conditions:
Zoom setting = 35mm (35mm equiv.)
AF mode: Normal.
Camera mode: Aperture priority with external flash.
Aperture (all photographs): f/8
Light source (underwater): Sea & Sea YS50TTL/N with
Matthias Heinrichs DA-IR-N TTL converter.
C-5050 camera, in air (no housing)
Note: The purple tinge of the ruler is due to its high reflectivity
in the ultraviolet part of the spectrum (the picture was taken
outdoors with no UV filter). A UV filter is not necessary underwater.
Radial Correction
Before
 |
After
 |
|
PanoTools
radial correction
coefficients: |
 |
Camera and Housing (flat port, no supplementary lenses
or filters)
Note: The illumination hot-spot in the full picture is due to
the use of a torch as a focusing-assistance light.
Radial Correction
Before
 |
After
 |
|
PanoTools
radial correction
coefficients: |
 |
Epoque DCL-20 (x 0.56)
Radial Correction
Before
 |
After
 |
|
PanoTools
radial correction
coefficients: |
 |
INON UWL-100 (x 0.57)
Radial Correction
Before
 |
After
 |
|
PanoTools
radial correction
coefficients: |
 |
INON UWL-100 with Dome Port (128°)
Radial Correction
Before
 |
After
 |
|
PanoTools
radial correction
coefficients: |
 |
Sea & Sea Wide Lens 0.56X/L
Radial Correction
Before
 |
After
 |
|
PanoTools
radial correction
coefficients: |
 |
UN PWC-01 (x 0.58)
Radial Correction
Before
 |
After
 |
|
PanoTools
radial correction
coefficients: |
 |
Collected Results
C-5050 Camera in air
PT-015 housing (flat glass port)
Supplementary lenses ranked in order of increasing coverage
UN PWC-01 (x 0.58)
Inon UWL-100 (x 0.57)
Epoque DCL-20 (x 0.56)
Sea & Sea WL 0.56x/L
Inon UWL-100 with Dome Port
Optical parameters:
(supplementary lenses ranked in order of increasing
coverage)
|
Lens system |
magnif
-ication |
angle
of
coverage |
Geometry |
Aberration |
|
a |
b |
c |
a+b+c |
Δd (red) |
Δd (blue) |
|
Camera |
1 |
63.4° |
0 |
-0.02 |
0 |
-0.020 |
+0.0005 |
+0.0005 |
|
Camera+Port |
- |
46.2° |
0 |
0.005 |
0 |
+0.005 |
0 |
+0.003 |
|
PWC-01 |
0.58 |
|
0.01 |
-0.06 |
-0.005 |
-0.055 |
+0.0011 |
0 |
|
UWL-100 |
0.57 |
91.4° |
0 |
-0.06 |
-0.035 |
-0.095 |
0 |
+0.002 |
|
DCL-20 |
0.56 |
|
0 |
-0.055 |
-0.04 |
-0.095 |
0 |
+0.002 |
|
S&S WL0.56 |
0.56 |
|
0.015 |
-0.05 |
0.005 |
-0.030 |
0 |
+0.0015 |
|
UWL-100D |
|
127.9° |
0.02 |
-0.08 |
-0.16 |
-0.220 |
+0.001 |
0 |
Discussion
The C-5060 zoom lens set at maximum-wide gives a small amount
of barrel distortion in air, but this distortion is almost completely
corrected underwater by the pincushion distortion introduced by
the flat port. This results in a very small third-order radial
correction coefficient ( b=0.005) to remove the residual pincushion
distortion. This happy coincidence, incidentally, is almost certainly
accidental. Compact camera zoom lenses are not designed with underwater
ports in mind, and other cameras will not necessarily turn out
to be rectilinear underwater. The flat port moreover, does introduce
some chromatic aberration (as it must), as indicated by the shift
in the first-order coefficient (Δd) for blue.
When correcting a supplementary wide-angle lens for chromatic
aberration, the starting point is something of a moving target
because it depends on the degree of correction of the camera lens.
When the supplementary lens is designed with no particular camera
in mind, the only logical approach is to try to reverse the chromatic
aberration introduced by the flat air-water boundary; and so we
should expect to see less chromatic aberration than for the camera
and port alone (if the lens maker has used a correction scheme),
provided that the camera is reasonably well corrected. In this
case, the camera lens is very well corrected, and the d-coefficient
shifts (Δd values) indicate that all of the supplementary
lenses produce an improvement, even though only the Inon lens
is claimed to be an achromat. In fact, we might have guessed that
all of the lenses tested would be achromats, since all have more
elements than they need for the degree of barrel correction achieved.
We should say however, that the Inon UWL-100 with its dome lens
fitted, achieves a remarkably high degree of correction in view
of its 128° nominal coverage.
If we leave the Inon lens with the dome aside for a moment;
there is little difference in performance between the four remaining
lenses. The Sea & Sea lens shows the best rectilinear correction,
as indicated by the quantity a+b+c, but this benefit is offset
somewhat by the fact that this lens is no longer manufactured
(unavailability is a serious practical disadvantage). The UN PWC-01
has slightly less chromatic aberration than the others in numerical
terms, but the overall aberration when used with the C-5050 is
of the more noticeable red-cyan variety (see corner detail images
above), and the other lenses offer a greater angle of coverage.
The Epoque DCL-20 and Inon UWL-100 are practically identical in
behaviour, except that the Epoque lens gives slightly greater
coverage (and is the cheapest of the bunch). The indications are
therefore that there is no great reason to favour one lens over
any other (especially if the user intends to carry out radial
correction); except on the basis of price or (in the case
of the Inon lens) the ability to accept an add-on dome lens in
order to achieve super-wide-angle coverage. In this latter respect,
the UWL-100 with dome is a thoroughly remarkable device, and it
is important to understand how different it is from the other
lenses in order to appreciate its merits. The apparent low resolution
of the corner detail is due to curvature of field, which is actually
exhibited by all of the lenses*, but exaggerated in the ultra-wide
photograph because, although the test-card looks distant, the
camera was only a few inches away from it. With the lens pulled
back to photograph a very large object, this curvature of field
effect will be compensated by the depth of field. Hence, while
the Epoque DCL-20 is the best choice (amongst M67 lenses) for
a general-purpose wide-angle adapter, the Inon UWL-100 and dome
port is the lens of choice for photographing such subjects as
giant marine animals and whole shipwrecks.
* The effect of using a wide lens aperture is
shown in a set of supplementary lens tests which were carried
out using an Olympus μ410 camera in an Ikelite housing. These
pictures were taken using an aperture of f/3.1 and show a pronounced
softening of focus at the edges in close-up wide-angle pictures.
In closing, it should be pointed out that none of the lens setups
tested actually requires radial correction for normal purposes.
All are good enough to produce large prints, and the fish-eye
effect is an accepted feature of underwater photographs. Radial
correction however, can be used to remove fisheye distortion if
so desired; and it may also be used selectively to remove only
the chromatic aberration.
To learn more about radial correction, and to find out how to
obtain, install, and use Panorama Tools, see the radial correction
article.
D W Knight,
© Cameras Underwater 2004, 2006
Magnification: 0.56x
Max. depth: 60m
Achromatic afocal wide-angle converter with anti-reflection coatings
front and back.
Max.
coverage without vignetting: 131° underwater (when camera
lens 35mm equiv: focal length is 31.5mm). Actual coverage depends
on camera and housing.
M67 filter thread. Useable with 40.7, 46 and 48mm filter threads
by means of supplied stepping rings.
