Lenses for network video cameras

Field of view

The field of view is the area of coverage and the degree of detail to view, as determined by the focal length of the lens and the size of the image sensor. The focal length is usually defined as the distance between the entrance lens 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 field of view can be classified in three types:

  • Wide angle (left image): a larger field of view with less detail than in normal view. A wide-angle lens generally provides a good depth of field and fair low-light performance.
  • Normal view (center image): offering the same field of view as the human eye.
  • Telephoto (right image): a narrower field of view, providing finer details than a human eye can deliver, to view small or distant objects.
Examples of different fields of view

Figure 2: Examples of different fields of view.

Depth of field

A criterion that may be important to a video surveillance application is depth of field, which refers to the distance in front of and beyond the point of focus where objects appear sharp simultaneously. Depth of field may be important, for example, in monitoring a parking lot, where you might need to read license plates at 20, 30 and 50 meters (60, 90 and 150 feet).

Depth of field is affected by three factors: focal length, iris diameter and distance of the camera to the subject. A long focal length, a large iris opening, or a short distance between the camera and the subject will limit the depth of field.

Depth of field

Figure 3: In this line of people, the focus is on person B, but at the same time, it would also be possible to identify persons A and C. This range is the depth of field.

F-number

A camera’s light-gathering ability is specified by the lens’ f-number (also known as the f stop). The f-number defines how much light can pass through a lens, and is the ratio of the lens’ focal length to the diameter of the aperture or iris diameter; that is, f-number = focal length/aperture.

The smaller the f-number, the better the light gathering ability; i.e. more light can pass to the image sensor. In low-light situations, a lower f-number generally produces better image quality, while a higher f-number increases the depth of field. A lens with a low f-number is normally more expensive than a lens with a higher f-number.

Automatically adjustable lenses (DC-iris) have a range of f-numbers, with often only the maximum light gathering end of the range (smallest f-number) being specified.