Specifications
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Many manufacturers specify the minimum level of illumination needed for a network camera to
produce an acceptable image. While such specications are helpful in making light sensitivity
comparisons for cameras produced by the same manufacturer, it may not be helpful to use such
numbers to compare cameras from dierent manufacturers. This is because dierent manufac-
turers use dierent methods and have dierent criteria for what is an acceptable image.
To properly compare the low light performance of two dierent cameras, the cameras should be
placed side by side and be viewing a moving object in low light.
3.2 Lens elements
A lens or lens assembly on a network camera performs several functions. They include:
> Dening the eld of view; that is, dening 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.
3.2.1 Field of view
A consideration to take into account when selecting a camera is the eld of view required; that
is, the area of coverage and the degree of detail to be viewed. The eld of view is determined by
the focal length of the lens and the size of the image sensor; both are specied in a network
camera’s datasheet.
A len’s focal length is dened as the distance between the entrance lens (or a specic point in
a complicated lens assembly) and the point where all the light rays converge to a point (nor-
mally the camera’s image sensor). The longer the focal length, the narrower the eld of view.
The fastest way to nd out what focal length lens is required for a desired eld of view is to use
a rotating lens calculator or an online lens calculator (www.axis.com/tools), both of which are
available from Axis. The size of a network 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.)
CHAPTER 3 - CAMERA ELEMENTS