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The more you know about lenses, the greater will be your enjoyment of photography.
The basics of lenses and lens technology — which play a key role in the creative process — are explained herein.

LENS KNOWLEDGE

• Floating System

The floating system is used to control the
focus. This system moves the different
lens groups in the optical system to
different positions, thereby minimizing the
telescoping distance and the fluctuation of
aberration at different shooting distances.
This system is particularly effective for
macro lenses (which encompass a wide
range of shooting distances) and wide-angle
lenses (for Single-Lens Reflex cameras)
whose lens composition is asymmetric.
SIGMA uses the floating system for the
MACRO 50 mm f/2.8 EX DG lens and the
large-aperture wide-angle 28 mm f/1.8 EX DG ASPHERICAL MACRO lenses.

• OS (Optical Stabilizer) Function

The OS function uses two sensors inside the lens to detect vertical and horizontal
movement of the camera and works by moving an optical image stabilizing lens
group to effectively compensate for camera shake. The OS function offers the
equivalent of using a shutter speed 4 stops faster making it suitable for telephoto
and low light photography. Sony and Pentax mount lenses equipped with OS,
such as 18-250mm F3.5-6.3 DC OS
HSM and 18-50/2.8-4.5 DC OS HSM,
can be used even if the camera has
a built-in camera shake reduction
system.* As the stabilizing feature
is built into the lens, it can be
designed specifically to suit the
characteristics of that lens. This
provides precise compensation for
camera shake throughout the entire
zoom range. This also enables the
correctly stabilized image to appear
in the viewfinder aiding autofocus
and composition.

*For Sony mount, the stabilizer
function (OS) will function with all
Konica Minolta and Sony Digital SLR
cameras. It is not possible to use the
OS function with film SLR cameras.
*For Pentax, the OS will function
with Digital SLR cameras except *ist
Series and K100D. It is not possible to use the OS function with film SLR cameras.
*When the OS function of lens body is used, please turn the stabilizer function of
camera body off.

• Depth of Field

When you focus on an object, a certain area in
front of and behind the object is also in focus;
depth of field refers to the size of this area
that is in focus. The depth of field or the range
of focus becomes larger when you stop down
(decrease the size of the aperture), or smaller
when you open up (increase the size of the
aperture). The depth of field is smaller at smaller
shooting distances even when the aperture
size remains unchanged, and is larger at larger
shooting distances. The depth of field is also
dependent on the focal length of the lens; it is
larger for lenses of smaller focal lengths or wider
angles, and smaller for lenses of larger focal
lengths or telephoto lenses, if aperture and the
distance camera to subject remain the same.

• Angle of View

The focal length determines the area in which objects can be reproduced on the
image sensor surface. The angle of view is the area that the lens can photograph
and is expressed in degrees. The angle of view indicated in the brochure is the

• Aspherical Lens

This lens provides high optical performance while maintaining a compact size.
For example, the 12-24 mm f/4.5-5.6 EX DG ASPHERICAL lens widens the
range of wide-angle lenses, and it provides distortion-free images with image
reproduction performance equivalent to that of a single-focal length lens.
Aspherical lenses allow the production of high-quality images from compact,
lightweight telephoto zoom lenses.

• APO (APO Lens)

SIGMA's APO zoom lenses minimize color aberration. As the refractive index of
glass depends on the wavelength of light, color aberration occurs when different
colors form images at different
points. This problem often occurs with
telephoto lenses, but the Special Low-
Dispersion (SLD) glass, Extraordinary
Low Dispersion (ELD) and "F" Low
Dispersion (FLD) used in SIGMA's APO
lenses helps to compensate for color
aberration, thereby allowing them to
produce of sharp images.

• APO MACRO

Although telephoto zoom lenses can
be used closer to the object than fixed
focal length telephoto lenses, there
is still a minimum shooting distance.
SIGMA has made this minimum
distance smaller and developed the
zoom MACRO lens for taking close-
up photographs of the same quality
as those taken with a regular MACRO lens, while maintaining the performance
specific to an APO lens. Rather than carrying around the cumbersome accessories
required for close-up work, the photographer can now take photographs at a
magnification of 1:2 (one half lifesize) using a telephoto lens, by quickly shifting
from the normal setting to the full macro setting.

• Inner and Rear Focus

Conventional focusing has normally
been performed by moving either
all lens groups as a fixed unit or only
the first lens group. AF cameras are
now widely used, even for close-up
photography. Consequently, demand
has arisen for a focusing system
that will keep the length of the lens
unchanged while showing little
fluctuation of aberration. In response
to this demand, SIGMA has developed
a new inner focus system that moves
two lens groups inside the telephoto
and telephoto MACRO lenses. This
system has floating elements that
substantially improve the close-up
capability of the lens. The super wide
angle lens having a large front-lens
uses a rear focusing system to move
the rear-lens apparatus and enhance
the floating effect, and the 18-200mm
F3.5-6.3 DC features an inner focusing
system to move the secondary lens group during focusing. This lens has a
minimum focusing distance of 45 cm / 17.7 inch throughout entire zoom range.
The rear focus system ensures high-speed focusing with the wide lens 20mm F1.8
EX DG ASPHERICAL RF.

• DF (Dual Focus) System

The DF (Dual Focus) system disengages the linkage between the internal focusing
mechanism and outer focusing ring when the focusing ring is moved to the AF
position. This system provides easy and precise handling of the lens, since the
focusing ring does not rotate during autofocusing. The wide focusing ring also
enables easy and accurate manual focusing.

PRINCIPLES OF THE LENS

LENS TECHNOLOGY

Camera shake correction mechanism OFF

F2.8

F22

Special Low Dispersion (SLD)

Lens Element

Ordinary

Glass Element

Amount of Secondary

Spectrum

Amount of Secondary