Sigma Full Line Catalog - page 25

25
PRINCIPLES OF THE LENS
Angle of view
Angle of view is determined by the focal length of the
lens and the size of the image (sensor or film format)
frame. With a given image size, changing the focal
length will change the area of the scene that appears
in the photographic image. Expressed in degrees,
this area of the scene is the angle of view, which in
this catalog is computed in reference to the diagonal
of image formats measuring 36mm x 24mm, 20.7mm
x 13.8mm, and 23.55mm x 15.7mm. The longer the
focal length, the smaller the angle of view and the
greater the image magnification.
F-number
The aperture controls how much light can be gath-
ered by the lens. The F-number (F2.8, F4, F5.6, etc.)
is the ratio of the focal length to the entrance pupil
diameter. The lower the F-number, the brighter the
lens; the higher the F-number, the darker the lens.
The benefits of a low F-number include the ability to
use higher shutter speeds, excellent bokeh effects,
and a bright viewfinder image.
Perspective
Changing the focal length of the lens changes the
apparent distance in an image between the subject
and its background. This optical effect is called per-
spective. For example, a wide-angle lens causes the
background to seem far away and vast, emphasizing
the distance between it and the subject. In contrast,
a telephoto lens with a long focal length will cause
the background to appear close to the subject, de-
emphasizing perspective. Further, a wide-angle lens
can bring the surroundings of the subject into the
shot, while a telephoto lens can effectively isolate
the subject. By leveraging the power of perspective
in this way, one may greatly increase the range of
photographic expression.
Depth of field
When you focus on a subject, some objects in front of
and behind the subject will also be in focus. “Depth
of field” refers to the depth of this foreground-back-
ground distance. A smaller lens aperture (higher
F-number) increases depth of field, bringing more
foreground and background into focus. A larger ap-
erture (lower F-number) isolates your subject with
a blurred bokeh foreground and background. Focal
length is also a factor. Telephoto lenses have less
depth of field, whereas wide-angle lenses have more.
Designed to optimize bokeh near
maximum aperture
Rounded diaphragm
The polygonal shape of a conventional iris dia-
phragm causes out-of-focus light points to appear
polygonal. A rounded diaphragm is designed to pro-
duce rounded out-of-focus light points when opened
to near maximum aperture. This creates attractive
bokeh effects in many situations, such as when pho-
tographing a subject against an out-of-focus surface
of water from which light is being reflected.
AF drive motor for rapid focusing
and quiet operation
Hypersonic motor (HSM)
The hypersonic motor (HSM) is an original Sigma
development that uses ultrasonic waves to drive the
autofocus mechanism. Its extremely quiet operation
helps avoid disturbing photographic subjects. High
torque and speed assure rapid autofocus response.
Sigma uses two types of HSM: ring HSM and micro
HSM. The Ring HSM configuration permits manual
fine tuning of focus (manual override) by turning the
focusing ring after autofocus is complete.
What you should know to choose the right lens for your needs.
84.1°
24mm
75.4°
28mm
46.8°
50mm
34.3°
70mm
23.3°
105mm
18.2°
135mm
12.3°
200mm
8.2°
300mm
500mm
3.1°
800mm
ANGLE OF VIEW AND FOCAL LENGTH
FISHEYE180°
8mm
FISHEYE180°
15mm
122°
12mm
103.7°
17mm
94.5°
20mm
1...,15,16,17,18,19,20,21,22,23,24 26,27,28,29,30,31,32
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