the wide apertures required with slow
film and a polarizing filter, the 20-40mm
Sigma zoom produced exceptional image
quality. Under a 10x loupe, even the letters
in the corner of the frame appear razor
sharp and crisply defined in this image.
(At 24mm; f/5.6; Fujichrome Velvia at
incorporate an inner focusing system that offers several
advantages. It helps to maintain top quality at all
focused distances, prevents rotation of the front element,
and assures minimal barrel extension so optimum weight
balance is maintained. The minimum focus distance (a
short 11.8") allows photographers to take maximum advantage
of the unusual near/far relationship possible with an
ultra-wide or super wide angle lens.
are also equipped with a Dual Focus (DF) mechanism.
Push the focusing ring forward and it provides autofocus
operation. The ring does not rotate and operation is
fairly quiet and very fast. To switch to manual focus,
simply pull the focusing ring back toward the camera.
The benefit of this DF mechanism? The ring has a lot
of torque, for a familiar "feel" in manual operation.
The 15-30mm zoom has a new mechanical design, said to
minimize astigmatism at all focal lengths. Its iris
diaphragm is made with eight blades to render out of
focus highlights as circular and not hexagonal as with
some wide angle lenses.
Do note that this is a true rectilinear lens, so you
won't get the barrel distortion that fisheye lenses
produce. As long as I held the camera perfectly level--with
the film plane parallel to the subject--all lines were
rendered as very straight and true. As with any super
wide lens, there is some limited barrel distortion at
the edges of the frame at the shortest focal lengths.
This is not obvious and this zoom is not intended for
formal architectural photography, in any event. Sigma's
14mm lens is even more effectively corrected for linear
distortion, but this zoom is a far more versatile lens.
was minimal even at the widest apertures and completely
gone when I stopped down by a single stop. Flare is
more of a problem, as with many super wide angle lenses,
for two reasons: a lens hood is not very effective and
the front element is very large. I had to take extra
care to avoid images with lower contrast and flare patterns
of various colors. On sunny days, I looked for a shooting
position where overhead branches or architecture would
shade the lens. When working with a tripod, I used my
hat to cast a shadow, preventing any stray light from
striking the front element.
aperture from f/8-f/16, this lens produced images suitable
for excellent 16x24 prints with high edge to edge sharpness.
The slides also exhibit great clarity and impressive
definition of intricate detail. At the widest apertures
at the shortest and longest focal lengths, edge sharpness
does not quite match central sharpness. By one stop
down, image quality is adequate for a technically excellent
8x12" print or full-page spread in a magazine. For the
ultimate in sharpness, contrast, and resolution, I would
shoot in the mid range of focal lengths at f/8, taking
care to avoid flare.
the incredibly wide field of view that you get with
a 35mm camera, you may not consider 15mm as an all-purpose
focal length. Owners of most digital cameras may, as
discussed earlier. I didn't shoot at the shortest focal
lengths often, but I did appreciate the super wide angle
for some subjects. It was great for certain cityscapes,
for large groups of people, and in cramped interiors
where I wanted to include as much of the subject as
its problem-solving abilities, a 15mm focal length can
produce special effects, with expanded spatial perspective
and apparent distortion of lines if you tilt the camera.
Some of your pictures will seem "weird" but others will
be "dynamic" or "dramatic." If you want to expand your
horizons and are willing to experiment, this Sigma lens
can open new doors to creativity. Although the AF 15-30mm
f/3.5-4.5 EX DG Aspherical zoom is not inexpensive,
its "street price" is moderate (under $600). In my opinion,
that's a bargain for a solidly constructed zoom of high
quality, especially considering its unique range of
This model includes the cosmetic, optical, and mechanical
features mentioned in the previous sections, but its
corner-cut lens hood is removable. Because of the large
diameter optical elements, it's a bit more expensive
and 1.6 oz heavier than the 15-30mm zoom, but a lot
shorter. It also accepts front-mounted filters such
as my polarizer, so I found myself using it more frequently.
Digital camera owners will prefer the 15-30mm model,
but the 20-40mm range will meet many 35mm shooters'
of overall performance, it's similar to the 15-30mm
zoom when that lens is used in the 20-30mm range. At
any focal length, stopping down to f/4 produced excellent
image quality, superb by f/5.6 and stunning by f/8 with
extremely high sharpness across the entire frame. Because
this 20-40mm zoom does not include the super wide focal
lengths, flare was rarely a problem and any barrel distortion
AF 20-40mm f/2.8 EX DG Aspherical is clearly a pro caliber
lens. Considering the rugged construction, problem-solving
wide aperture plus impressive optical performance, it's
also a very good value. ("Street" price is under $650.)
These factors should combine to make this zoom a best
seller among serious photo enthusiasts.
The adapter tube is threaded and it can accept 82mm
filters, useful for those who own certain digital SLR
cameras. Especially with cameras that increase the effective
focal length by 1.5x or 1.6x, you should not get any
vignetting at most focal lengths. Before making any
important images using a filter, take some test shots
of a white wall. For tests with a polarizer, use a "thin
ring" or "slim line" model to reduce the risk of vignetting.
Check the slides closely with a loupe for any darkening
at the corners.
information, contact Sigma Corporation at (631) 585-1144
or visit their web site at www.sigmaphoto.net.