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In very tight close-ups, the 28-70mm zoom produced excellent
image quality at all focal lengths, particularly in the
mid range of apertures. The 15.7” minimum focus distance
is as short as most anyone would need with a zoom of this
range. (At 50mm at f/11; B+W Polarizer; Provia 100F.) |
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For a long, fast zoom that's
not terribly expensive, the optical formula is impressive. This lens contains
four Super Low Distortion (SLD) glass elements "for full correction of
chromatic aberration," according to Sigma. This is important with a telephoto
lens of wide maximum aperture, because that optical flaw can degrade image
quality in lenses with a less sophisticated optical formula. Other features
include a sturdy, removable tripod mounting collar, internal focusing
so the barrel length remains constant, a non-rotating front element for
convenience with a polarizer, plus very rugged construction.
Hypersonic Focus Motor
Like many recent Sigma lenses, this zoom is available with a built-in
Hypersonic Focus Motor in Canon EF, Nikon AF-D, and Sigma SA mounts, for
silent (40 dB or lower), superfast autofocus performance. This ultrasonic
motor employs vibrational or oscillation energy--instead of electromagnetic
energy --for rotational force. Note that models in Pentax AF and Minolta
AF-D mount do not include the HSM motor; they use focus drive provided
by the camera instead.
The non-HSM lenses feature
a focus clutch that's worth considering, because it's also incorporated
in the 28-70mm model. If you plan to shoot with autofocus, pull the focus
ring back for AF operation. Now, it will not rotate allowing for maximum
speed; as well, your hand cannot impede its progress. Push it forward
and you can focus manually. Naturally, the AF/MF switch on the lens (or
the camera body) must be set to the correct position first. This mechanism
allows for excellent manual focus "feel": it's well damped, with plenty
of torque or friction, as you'd find in a high quality manual focus lens.
As a bonus, the front element does not rotate during AF operation, thanks
to a helical focus system.
The HSM lens does not require
this mechanism because the focus ring does not rotate and manual focus
can be accessed at any time--even in AF mode--so you can touch up focus
slightly without the need to flip a switch. Other advantages claimed for
the HSM motor include better starting and stopping response, quicker and
quieter AF, more reliable follow-focus on a rapidly moving subject, and
longer motor life. Naturally, autofocus speed and reliability depend on
the camera to some extent: the latest high tech AF systems offer superior
performance over those with older AF technology.
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The 28-70mm zoom range does not offer the greatest versatility,
but is surprisingly useful for a broad range of subject
matter. At any focal length, move in close to the subject
and you can get satisfying images. (At f/5.6; Sigma EF-500SUPER
flash; B+W polarizer; Fujichrome Provia 100F.) |
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Tele-Zoom Evaluation
With the HSM model that I used, autofocus was indeed virtually silent.
I found the ability to quickly fine-tune focus manually at any time to
be a real plus--to get focus on the eye instead of the snout of a captive
lion, for example. In general, autofocus performance was highly reliable,
accurate and smooth when used with a Sigma SA-9. With vehicles traveling
at 60+ miles per hour, all frames in series of six were razor sharp in
Continuous AF. With more erratic motion--like mountain bikers racing and
jumping small hills--tracking focus might be more reliable with an SLR
camera with a multi-point AF system.
As expected from a premium
grade zoom, image quality was excellent overall with high contrast, sharpness,
and resolution of fine detail; I found little or no flare even under backlighting
conditions. Color rendition was neutral. I frequently worked without a
tripod, so I often shot at f/4 and was impressed with the resulting slides,
confirming the value of the four SLD elements. Even under an 8x loupe,
there was absolutely no color fringing around the edges of a subject.
Central sharpness was extremely high, while edge sharpness was quite high
at maximum aperture. Since I tend to center subjects in tight close-ups,
individual hairs in a giraffe's hide and the smallest feathers of a colorful
macaw are clearly defined. I did not hesitate to shoot "wide-open" when
necessary, especially in the 100-200mm range; my images do meet professional
standards in terms of sharpness, contrast, clarity, and definition of
intricate detail. By f/5.6, center and edge sharpness were very similar,
adequate for a fine 11x14 print or a double-page spread in a magazine.
I found no need to stop down to smaller apertures to improve image quality
at any focal length. This consistency at all f/stops is a sign of excellence
in optical design.
AF 28-70mm f/2.8 EX Aspherical
DF
This "standard" zoom bears a strong family resemblance to the longer model,
but employs a conventional focus system that's driven by the camera's
focus motor. It also incorporates the focus clutch mechanism described
earlier in the tele-zoom section, plus nearly identical mechanical characteristics.
However, this short zoom also boasts some impressive features: as of this
writing, it's the smallest of its type and offers the shortest minimum
focus distance.
The optical formula includes
two aspherical elements, with a non-spherical surface. This type effectively
corrects spherical distortion, causing all light rays to converge at a
common point, for higher edge sharpness at wide apertures. Such elements
also correct barrel distortion so there is less bowing outward of lines
near the edge of the frame, as well as reducing halo and comatic flare.
As a bonus, size and weight are reduced because fewer elements are required
for corrections in comparison to lenses of conventional design.
The maximum aperture of f/2.8
is very wide, making this lens useful for photojournalism in low-light
situations. I was able to get sharp pictures handheld inside a cathedral
with ISO 100 film, at 1/15 sec at 28mm (and even longer shutter speeds
with my elbows braced). The wide aperture is important whenever flash
or a tripod is prohibited or impractical, and also to increase the effective
range of flash. Serious photographers will certainly appreciate the ability
to shoot at f/2.8 instead of f/3.5-5.6, as they must with many "standard"
zooms.
Standard Zoom Evaluation
Mechanically and cosmetically, this is clearly a high-grade lens, as suggested
by the EX designation. Barrel distortion was minimal, so straight lines
are accurately rendered even at 28mm. Clarity, resolution, sharpness,
and definition of fine detail range from excellent to superlative. Slides
made at f/2.8 were surprisingly sharp under an 8x loupe. By f/4 edge sharpness
is excellent. In the mid range of apertures, the images appear to be etched
on the emulsion. At every f/stop from f/5.6 to f/11, edge sharpness matches
central sharpness at all focal lengths, confirming a well-designed optical
formula. My best slides would make for a fine 16x20 print; even those
made at f/2.8 would easily stand up to enlargement to 8x12" or a full-page
spread in a magazine. When focused at its shortest distance, the very
best results were produced in the f/11 to f/22 range. Since depth of field
is very shallow in close-up work, you'll generally want to shoot at such
f/stops in any event. This should render all-important parts of a three-dimensional
subject within the zone of acceptably sharp focus.
Although autofocus made the
usual sound, it was not too loud or objectionable. Overall, I found response
to be reliable with the SA-9 camera. Focus was always precise. Even in
low light around sunrise and sunset, focus was quickly acquired because
of the wide aperture that transmits a lot of light to the AF sensors.
Continuous focus was certainly acceptable when I photographed cyclists
approaching my position during a race; however, this may not be relevant
since a short zoom is not often used for sports photography.
Final Evaluation
Although the compact, ultralight, and inexpensive 28-80mm f/3.5-5.6 Sigma
zooms will far outsell the AF 28-70mm f/2.8 EX Aspherical lens, I would
recommend this "faster" model to photo enthusiasts. The superior optics,
rugged construction, impressive resistance to flare, and minimal barrel
distortion all make it highly desirable. When you first check the price
you'll believe that someone made a mistake, but it is accurate, making
this fine lens an incredible value. If you previously thought that you
could not afford a fast pro zoom, this model may just change your mind.
The APO 100-300mm f/4 EX IF
model is much larger, heavier, and more expensive, as you'd expect from
a wide/constant f/4 aperture tele-zoom with four low dispersion elements.
For those who prefer something more compact, Sigma offers an alternative:
the AF 70-300mm f/4-5.6 APO Macro Super with three low dispersion elements,
a more common 58mm filter size and much closer focusing for high magnification
up to 0.5x. (See the May 2000 issue for my test report of that fine lens.)
But if you need f/4 at longer focal lengths for action photography, better
results with a tele-converter, HSM focusing in some mounts, and tank-like
construction, check out the 100-300mm model instead. As a professional
tool, it's not intended for everyone, but this zoom offers exceptional
performance and it's available at a price that should not blow the limit
on your credit card.
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