Chap.2 Lens and Image Sensor Technology
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Iris is measured in F-number, which is the ratio of the focal length to the lens diameter. Iris size is inversely
proportional to F-number. Every time the F-number increases towards a larger number, the exposure ratio will
reduce by half (Figure 2.6).
Iris includes the following types in terms of control methods:
Manual-iris
Manual-iris is adjusted with a ring on the lens. It is used when light sources are steady.
Auto-iris
Auto-iris can automatically adjust the amount of light entering with a mechanism to have a camera stay in an
optimal light level. As a result, it is mainly required for outdoor applications or the places where lights change
frequently.
There are two types of auto-iris: Video-drive iris and DC-drive iris.
• Video-drive iris:
video signal is transmitted to a drive circuit in the lens and converted to currents to control the
iris motor. Because the amplifier circuit is built in the lens, a Video-drive iris lens is more expensive.
• DC-drive iris:
the iris is controlled by DC currents. Due to the drive circuit is integrated in the camera instead of in
the lens, DC-drive iris lens features lower costs.
2.1.3 Lens Mount Types
C- and CS-mount are two major lens mount standards developed for the purpose of changing lens. The main
difference between C/CS-mount lenses lies in the flange focal distance. The flange focal distance for a CS-mount
lens is 12.5mm while 17.526mm for a C-mount lens (Figure 2.7). A CS-mount lens has higher cost efficiency and
smaller size because fewer glass components are used.
f/1.4
f/2
f/2.8
f/4
f/5.6 f/8
F-number
Exposure ratio
1.4
32
2
16
2.8
8
4
4
5.6
2
8
1
Figure 2.5 Relationship between F-number and aperture
Figure 2.6 F-number and corresponding exposure ratio
Summary of Contents for IP Surveillance
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