30
>
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 network 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, 3x-zoom capability, it is referring to the ratio
between the lens’ longest and shortest focal length.
3.2.2 Matching lens and sensor
If a network camera offers an exchangeable lens, it is important to select a lens suitable for the
camera. A lens made for a 1/2-inch image sensor will work with 1/2-inch, 1/3-inch and 1/4-inch
image sensors, but not with a 2/3-inch image sensor.
If a lens is made for a smaller image sensor than the one that is actually fitted inside the camera,
the image will have black corners (s
ee left-hand illustration in Figure 3.2c below
). If a lens is
made for a larger image sensor than the one that is actually fitted inside the camera, the field
of view will be smaller than the lens’ capability since part of the information will be “lost” out-
side the image sensor (
see right-hand illustration in Figure 3.2c
). This situation creates a tele-
photo effect as it makes everything look zoomed in.
Figure 3.2c Examples of different lenses mounted onto a 1/3-inch image sensor.
When replacing a lens on a megapixel camera, a high quality lens is required since megapixel sen-
sors have pixels that are much smaller than those on a VGA sensor (640x480 pixels). It is best to
match the lens resolution to the camera resolution in order to fully use the camera’s capability.
CHAPTER 3 - CAMERA ELEMENTS
