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Sigma Wide Angle Zoom
AF 15 to 30mm f3.5 to 4.5 EX DG Aspherical And AF 20 to 40mm f2.8 EX DG Aspherical
By Peter K. Burian • April, 2002
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On a conventional camera, a 20mm focal length provides an
ultra-wide angle of view (below) but a 15mm focal length
produces a super wide angle effect (above). Any obvious
"distortion" is caused by the camera angle and not by an
optical flaw. (At f/8; Fujichrome 100F; Flagler College,
St. Augustine, Florida.)
Photos ©
2001, Peter K. Burian, All Rights Reserved
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The Sigma engineers never seem
to take a break, coming up with new lenses with surprising frequency.
Late last year, they released two short zooms that certainly attracted
a lot of attention. Their AF 15-30mm f/3.5-4.5 EX DG Aspherical boasts
the shortest focal length of any zoom on the market, at the time of this
writing. The new AF 20-40mm f/2.8 EX DG Aspherical is less unusual but
includes the longest focal length of any f/2.8 ultra-wide zoom.
The 20-35mm zooms are common,
and two camera manufacturers offer a 17-35mm f/2.8 model, but some photographers
want a zoom with even wider angles of view. Because many digital SLRs
multiply focal length by 1.5x or 1.6x, few zoom lenses provide a very
wide angle effect with such cameras. The shorter focal lengths of the
Sigma 15-30mm will do so, making it ideal for digital photographers. Naturally,
a 15-30mm zoom is also great with a conventional SLR camera for creating
super wide angle effects.
I tested the Sigma zooms with
an EOS-3 and a 550EX Speedlite while shooting stock images in central
Florida. They proved to be fully compatible with all of the high tech
capabilities of both Canon models. The flash unit could not cover the
super wide field of view of the 15mm and 16mm focal length, so in dark
locations I used only available light at these settings. When using flash
outdoors, this was not a problem. Because the sun provided most of the
light, the 550EX with its wide angle adapter produced excellent fill flash
without any obvious darkening at the edges of the image.
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When used on a digital
camera that increases effective focal length by 1.5x
or 1.6x, the 15mm end of the Sigma zoom still produces
a very wide angle of view. For this image, I set 23mm
to illustrate the approximate effect that you'll get
with such a camera. (At f/8; Fujichrome Provia 100F;
Spanish Quarter Museum, St. Augustine, Florida.)
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15-30mm Zoom Characteristics
Mechanically and cosmetically, this is clearly a high grade lens,
as suggested by the EX designation, used only for Sigma's premium
grade products. The barrel is made of aluminum alloy, the mount
is stainless steel, and this zoom appears to be built for long-term
reliability as a professional tool. Its handsome matte-black finish
offers a professional look plus scratch-resistance. In spite of
the small maximum apertures, this zoom is certainly large and heavy.
You'll find a distance scale and a depth of field scale for apertures
from f/5.6-f/16. Models intended for some brands of cameras include
an AF/MF switch and an aperture ring for setting f/stops.
The maximum aperture
is not overly wide and gets smaller as you zoom toward the long
end, a compromise that keeps weight, size, and price acceptable.
Frankly, the longer exposure times at the smaller apertures do not
pose a huge problem in many kinds of wide angle photography. Even
in handheld shooting, we can get sharp pictures at shutter speeds
such as 1/15 sec.
In order to reduce the
risk of flare--and as a bit of protection for the bulging front
element--there's a built-in metal lens hood. It's notched: cutout
at the corners to prevent vignetting of the image area. In order
to mount a lens cap you do need to add an adapter tube as with most
super wide lenses. Remove this accessory before taking any pictures
when using a 35mm camera. Otherwise, your photos will have severe
darkening of all corners.
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The 20-40mm
Sigma zoom has one major advantage over the
15-30mm zoom. When used on a 35mm camera, it
accepts front-mounted filters, including a polarizer,
a real plus in outdoor photography. However,
owners of most digital cameras can also use
such filters, as discussed in the text. (The
20-40mm zoom; Tiffen polarizer; f/8; RC Basilica,
St. Augustine, Florida.)
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Lenses such as
this--with an extremely wide angle of view--do not accept
front-mounted filters because of the protruding element and
a vignetting problem. They would darken the corners of the
image. Hence, this zoom has a slot for rear-mounted gelatin
filters. Naturally, you won't find some filters as gels: polarizers,
graduated neutral density or special effects filters, for
example.
Optical Formula
And Construction
Both of these EX series zooms include sophisticated optical
formulas and advanced mechanical components. They incorporate
elements with a non-spherical surface in both front and rear
lens groups for high image quality at all focal lengths, noticeable
especially at wide apertures.
Aspherical elements
are used for several reasons: to correct distortion of lines
and to control optical flaws such as astigmatism, halo, comatic
flare, and spherical aberration (curvature of field). The
result should be greater clarity and higher--and more consistent--sharpness,
noticeable especially at the edges of the frame. This technology
also reduces the total number of elements required for optimum
results, so it minimizes size/weight as well.
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Even at
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
EI 40.) |
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Both lenses
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.
These zooms
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.
15-30mm
Zoom Evaluation
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.
Light falloff
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.
At every
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.
Considering
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
possible.
Aside from
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
focal lengths.
20-40mm
Zoom Evaluation
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'
needs.
In terms
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
was negligible.
The Sigma
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.
A Useful
Tip
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.
For more
information, contact Sigma Corporation at (631) 585-1144
or visit their web site at www.sigmaphoto.net.
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Sigma
AF 20-40mm f/2.8 EX DG Aspherical
Construction: 17 elements, two aspherical,
in 13 groups
Angle Of View: 94 to 57°
Maximum/Minimum Aperture: f/2.8; f/22
Diaphragm Blades: Not stated
Minimum Focus Distance: 11.8"
Maximum Magnification: 1:4.6
Filter Size: 82mm
Lens Hood: Removable, corner cut
Dimensions/Weight: 3.5x4.2"; 1.3 lbs
Mounts: Sigma SA, Minolta (D), Nikon (D), Canon
EF
List Price: $820
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Sigma
AF 15-30mm f/3.5-4.5 EX DG Aspherical
Construction: 17 elements, two aspherical, in
13 groups
Angle Of View: 110.5 to 71.6°
Maximum/Minimum Aperture: f/3.5-f/4.5; f/22-f/32
Diaphragm Blades: Eight
Minimum Focus Distance: 11.8"
Maximum Magnification: 1:6
Filter Size: Rear, gelatin, insert
Lens Hood: Fixed, corner cut
Dimensions/Weight: 3.42x5.12"; 1.4 lbs
Mounts: Sigma SA, Minolta (D), Nikon (D), Canon
EF
List Price: $720
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