A
lens or lens assembly on an IP surveillance camera performs several functions.
They include:
- Defining the field of view; that is,
defining how much of a scene and level of detail are to be captured.
-
Controlling the amount of light
passing through to the image sensor so that an image is correctly exposed.
-
Focusing by adjusting either
elements within the lens assembly or the distance between the lens assembly and
the image sensor.
Field of view
A
consideration to take into account when selecting a camera is the field of view
required; that is, the area of coverage and the degree of detail to be viewed.
The field of view is determined by the focal length of the lens and the size of
the image sensor; both are specified in an IP surveillance camera’s datasheet.
A
Len’s focal length is defined as the distance between the entrance lens (or a
specific point in a complicated lens assembly) and the point where all the
light rays converge to a point (normally the camera’s image sensor).The longer
the focal length, the narrower the field of view.
The
size of an IP surveillance camera’s image sensor, typically 1/4”, 1/3”, 1/2”
and
2/3”
must also be used in the calculation. (The drawback of using a lens calculator
is that it does not take into account any possible geometrical distortion of a
lens.)
The
field of view can be classified into three types:
-
Normal view: offering the same
field of view as the human eye.
-
Telephoto: a narrower field of
view, providing, in general, finer details than a human eye can deliver. A
telephoto lens is used when the surveillance object is either small or located
far away from the camera. A telephoto lens generally has less light gathering
capability than a normal lens.
-
Wide angle: a larger field of view with less detail than in normal view.
A wide-angle lens generally provides good depth of field and fair, low-light
performance. Wide-angle lenses sometimes produce geometrical distortions such
as the “fish-eye” effect.
Different fields of view: wide-angle view (at left); normal view
(middle); telephoto (at right).
There
are three main types of lenses:
-
Fixed lens: Such a lens offers a focal length that is fixed; that is,
only one field of view (normal, telephoto or wide angle). A common focal length
of a fixed IP surveillance camera lens is 4 mm.
- Varifocal lens:
This type of lens offers a range of focal lengths, and hence, different fields
of view. The field of view can be manually adjusted. Whenever the field of view
is changed, the user has to manually refocus the lens. Varifocal lenses for IP
surveillance cameras often provide focal lengths that range from 3 mm to 8 mm.
-
Zoom lens: Zoom lenses are like varifocal lenses in that they enable the
user to select different fields of view. However, with zoom lenses, there is no
need to refocus the lens if the field of view is changed. Focus can be
maintained within a range of focal lengths, for example, 6 mm to 48 mm. Lens
adjustments can be either manual or motorized for remote control. When a lens
states, for example, 3 x-zoom capabilities, it is referring to the ratio
between the lens’ longest and shortest focal length.
Manual or automatic iris
In
indoor environments where light levels may be constant, a manual iris lens can
be used. This type of lens either provides a ring to adjust the iris, or the
iris is fixed at a certain f-number. A lens with automatically adjustable iris
is recommended for outdoor applications and where the scene illumination is
constantly changing. The iris aperture is controlled by the camera and is used to
maintain the optimum light level to the image sensor if exposure and gain
settings are not available or used in the IP surveillance camera. The iris can
also be used to control the depth of field
(explained
in the section below) and to obtain sharper images. Most automatic iris lenses
are controlled by the camera’s processor via a direct current (DC) and are,
therefore, called “DC-iris” lenses.
