Axis Network video - 3.2 Lens elements

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Many manufacturers specify the minimum level of illumination needed for a network camera to
produce an acceptable image. While such specications 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 dierent manufacturers. This is because dierent manufac-
turers use dierent methods and have dierent criteria for what is an acceptable image.
To properly compare the low light performance of two dierent 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:
> Dening the eld of view; that is, dening 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 specied in a network
camera’s datasheet.
A len’s focal length is dened as the distance between the entrance lens (or a specic 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