Sigma’s 18-200mm f/3.5-6.3 DC OS Lens; The First Multipurpose Sigma Zoom With A Built-In Image Stabilizer
Peter K. Burian, December, 2007
Although Sigma released their first lens with a built-in Optical Stabilizer
(OS) system in the spring of 2004, the company employed this technology in only
one pro-grade lens, the 80-400mm f/4.5-5.6. That has changed with the introduction
of a more compact/affordable (digital-only) 18-200mm OS zoom. Sigma will not
comment about future plans, but a reliable source indicates that additional
OS lenses will be developed to compete with the growing number of Canon IS and
Nikon VR lenses. Coincidentally, Tamron has also announced their first zoom
with a “Vibration Compensation” optical system, a 28-300mm f/3.5-6.3
multi-platform model that I’ll be testing soon.
Sigma’s newly-developed OS system employs two sensors inside the lens
to detect vertical and horizontal movement and then shifts a group of elements
to compensate. It automatically detects panning (lateral-only) movement—common
in action photography—and then compensates for up/down camera movement
to provide smooth/sharp pan effects.
It’s just as important to note that this Sigma lens incorporates advanced
optical technology, including a piece of Special Low Dispersion (SLD) glass
plus three aspherical elements. That combination was intended to provide effective
correction for all types of aberrations and distortion in order to produce a
high level of optical performance at all focal lengths. As you would expect
from a digitally-optimized lens, numerous elements are “super multilayer”
coated to reduce flare and ghosting, which can be caused by a highly reflective
CCD or CMOS sensor. Naturally, these coatings are also useful for minimizing
flare from external light sources.
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Particularly when shooting indoors, or from the deck of a ship
or ferry, I really appreciated Sigma’s effective Optical
Stabilizer technology. While this system adds to the size/weight/price
of the lens, it provides extra value in terms of peace of mind,
maximizing the number of photos that will be “keepers.”
(Image made in Program mode at ISO 800.)
All Photos © 2007, Peter K. Burian, All Rights Reserved
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Design And Construction
Like all of the recent Sigma DC (digital-only) models, this one is beautifully
finished in matte black, with a very wide rubberized zoom and focus rings. It
also offers an on/off switch for OS and AF plus a zoom lock that prevents the
internal barrel from protruding while the lens is being carried. Sigma also
includes a lens hood as part of the kit; it’s corner-cut to avoid intruding
into the image area at short focal lengths. The barrel is marked with highly
visible numerals for focused distance, focal length, and maximum reproduction
ratio numerals; the latter are relevant only in close focusing to 1.48 ft and
differ depending on the focal length in use.
In spite of the broad range of focal lengths, obviously rugged construction
(including a metal lens mount), and the built-in OS system, this very solid
zoom is acceptably lightweight and compact. Compared to the non-stabilized 18-200mm
f/3.5-6.3 DC model, it’s 6.8 oz heavier, 0.3” wider in diameter,
and 0.8” longer. The internal barrel does protrude when zooming to longer
focal lengths, by 2.5” at the 200mm setting. Weight distribution does
not shift very much, however, because the internal barrel is made of a lightweight
polycarbonate material. The zoom mechanism is very well damped, with plenty
of friction but a smooth feel.
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While this was not my favorite Sigma lens for action photography,
the 18-200mm zoom’s autofocus performance—with an
EOS Digital Rebel XTi—was adequately fast and reliable
for the vast majority of other subjects. In travel and family
event photography, I very rarely missed capturing exactly the
desired instant. (JPEG capture at 42mm at f/8, using ISO 400
and a Hoya S-HMC polarizer.)
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Manual focus operation is quite smooth, with just enough friction for a familiar
feel. Because this zoom does not include the ultrasonic HSM focus motor available
in some other Sigma lenses, the focus ring rotates during AF operation, producing
a low-pitched hum that is not problematic. Focusing is internal so the front
element does not rotate; hence the effect of a filter never changes, a real
benefit when using a polarizer.
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Edge sharpness and brightness are quite acceptable at wide apertures,
making f/5.6 a “useable” f/stop for images to be
printed at the most common sizes: 4x6” or 5x7”.
By f/8 however, image quality is even better, suitable for making
fine prints in much larger sizes. (Image made at f/5.6 at ISO
400, at an 85mm focal length, using a Hoya S-HMC polarizer.)
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Autofocus Technology
During my tests with a Canon EOS Digital Rebel XTi, autofocus acquisition was
usually quick though not lightning fast, as it is with the Sigma HSM zooms.
In travel photography, I was rarely frustrated except when shooting at long
focal lengths inside dark cathedrals. Focus tracking of cycle race competitors—at
the 200mm focal length—was not very reliable; especially when the group
was close to the camera, the AF system was rarely able to keep up with their
motion. That’s typical performance for a “slow” f/3.5-6.3
all-purpose zoom with a conventional AF motor. (By comparison, Sigma’s
“fast” f/2.8 HSM 50-150mm and
70-200mm zooms are far more effective in action photography.) Of course, those
wide aperture lenses can transmit a lot more light to the camera’s AF
sensor and their ultrasonic motors (USM) are substantially more efficient.
Most D-SLR specs indicate that autofocus is available only with lenses featuring
a maximum aperture of f/5.6 or wider. At the longest focal lengths, this lens—like
many of today’s all-purpose zooms—has a smaller maximum aperture
of only f/6.3. In spite of that factor, autofocus does function thanks to some
electronic wizardry provided by the computer chip in the lens barrel. Of course,
it’s not very fast at long focal lengths, especially in low light or when
using a polarizer that reduces light transmission. For the best AF performance
with this 18-200mm OS zoom, use focal lengths shorter than 150mm where the maximum
aperture is wider.
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