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

Peter K. Burian  |  Apr 1, 2002

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

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.

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.)

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.

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.)

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.

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.)

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.

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

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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