A lens or lens assembly on a network camera performs several functions. They include:
- Defining the field of view; that is, defining 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.
Field of view
A consideration to take into account when selecting a camera is the field of view required; that is, the area of coverage and the degree of detail to be viewed. The field of view is determined by the focal length of the lens and the size of the image sensor; both are specified in a network camera’s datasheet.
A lens' focal length is defined as the distance between the entrance lens (or a specific point in a complicated lens assembly) 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 fastest way to find out what focal length lens is required for a desired field of view is to use a rotating lens calculator or an online lens calculator, 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.)
The field of view can be classified into three types:
- Normal view:offering the same field of view as the human eye.
- Telephoto:a narrower field of view, providing, in general, finer details than a human eye can deliver. A telephoto lens is used when the surveillance object is either small or located far away from the camera. A telephoto lens generally has less light gathering capability than a normal lens.
- Wide angle: a larger field of view with less detail than in normal view. A wide-angle lens generally provides good depth of field and fair, low-light performance. Wide-angle lenses sometimes produce geometrical distortions such as the “fish-eye” effect.
There are three main types of lenses:
- Fixed lens: Such a lens offers a focal length that is fixed; that is, only one field of view (either normal, telephoto or wide angle). A common focal length of a fixed network camera lens is 4 mm.
- 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.
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 (see left-hand illustration 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” outside the image sensor (see right-hand illustration). This situation creates a telephoto effect as it makes everything look zoomed in.
When replacing a lens on a megapixel camera, a high quality lens is required since megapixel sensors 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.
Lens mount standards
When changing a lens, it is also important to know what type of lens mount the network camera has. There are two main standards used on network cameras: CS-mount and C-mount. They both have a 1-inch thread and they look the same. What differs is the distance from the lenses to the sensor when fitted on the camera:
- CS-mount. The distance between the sensor and the lens should be 12.5 mm.
- C-mount. The distance between the sensor and the lens should be 17.526 mm.
It is possible to mount a C-mount lens to a CS-mount camera body by using a 5 mm spacer (C/CS adapter ring). If it is impossible to focus a camera, it is likely that the wrong type of lens is used.
See also: Lenses for thermal cameras using TA mounts.
F-number and exposure
In low-light situations, particularly in indoor environments, an important factor to look for in a network camera is the lens’ light-gathering ability. This can be determined by the lens’ f-number, also known as f-stop. An f-number defines how much light can pass through a lens.
An f-number 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 (either short focal length relative to the aperture, or large aperture relative to the focal length), the better the lens’ light gathering ability; i.e. more light can pass through the lens to the image sensor. In low-light situations, a smaller f-number generally produces a better image quality. (There may be some sensors, however, that may not be able to take advantage of a lower f-number in low-light situations due to the way they are designed.) A higher f-number, on the other hand, increases the depth of field, which is explained below. A lens with a lower f-number is normally more expensive than a lens with a higher f-number.
F-numbers are often written as F/x. The slash indicates division. An F/4 means the iris diameter is equal to the focal length divided by 4; so if a camera has an 8 mm lens, light must pass through an iris opening that is 2 mm in diameter.
While lenses with automatically adjustable iris (DC-iris) have a range of f-numbers, often only the maximum light gathering end of the range (smallest f-number) is specified.
A lens’ light-gathering ability or f-number, and the exposure time (i.e., the length of time an image sensor is exposed to light) are the two main elements that control how much light an image sensor receives. A third element, the gain, is an amplifier that is used to make the image brighter. However, increasing the gain also increases the level of noise (graininess) in an image, so adjusting the exposure time or iris opening is preferred.
Limits to the exposure time and gain can be set in some Axis cameras. The longer the exposure time, the more light an image sensor receives. Bright environments require shorter exposure time, while low-light conditions require longer exposure time. It is important to be aware that increasing the exposure time also increases motion blur, while increasing the iris opening has the downside of reducing the depth of field, which is explained in the section below.
When deciding upon the exposure, a shorter exposure time is recommended when rapid movement or when a high frame rate is required. A longer exposure time will improve image quality in poor lighting conditions, but it may increase motion blur and lower the total frame rate since a longer time is required to expose each frame. In some network cameras, an automatic exposure setting means the frame rate will increase or decrease with the amount of available light. It is only as the light level decreases that artificial light or prioritized frame rate or image quality is important to consider.
Fixed or adjustable iris
The ability to control a camera’s iris opening plays an important role in image quality. An iris is used to maintain the optimum light level to the image sensor so that images can be sharp, clear and correctly exposed with good contrast and resolution. The iris can also be used to control the depth of field. Iris control can be fixed or adjustable. Adjustable iris lenses can be manual or automatic (auto iris and P-Iris).
Depth of field
A criterion that may be important to a video surveillance application is depth of field. Depth of field refers to the distance in front of and beyond the point of focus where objects appear to be sharp simultaneously. Depth of field may be important, for instance, in monitoring a parking lot, where there may be a need to identify license plates of cars at 20, 30 and 50 meters (60, 90 and 150 feet) away.
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.