If you ever want to experience a distorted view of the world, try looking through someone else’s prescription eyeglasses.
To see things normally, you need lenses that compensate for the particular characteristics of your eyes and the particular conditions of your surroundings — whether for myopia or astigmatism, staring at a computer screen all day or driving at night facing glaring headlights.
In the world of video surveillance, your cameras face a similar challenge. A video camera’s image sensor is much like an eye, while the lens serves the same role as a pair of prescription eyeglasses. For your camera to attain 20/20 vision, it needs to be fitted with the right lens for the job.
The basics of lens selection have not changed much over the years. You still need to choose a lens mount, determine the appropriate focal length, and match the lens to the surveillance environment — whether indoors or outdoors. But with some of the newer network camera technologies, such as megapixel and HDTV resolution, lens selection becomes even more critical for optimum performance. The following are some of the key issues you need to consider when seeking the right lens for your surveillance application.
Matching Lens to Sensor
A camera "sees" when light passes through a lens and is then focused on the camera’s image sensor. The image sensor is made up of many tiny photosites, and each photosite corresponds to a picture element, more commonly known as a "pixel." Each pixel on an image sensor registers the amount of light it is exposed to and converts that light into a corresponding number of electrons. The brighter the light, the more electrons are generated.
Image sensors come in a range of sizes, most commonly one-quarter to one-third of an inch — so you need to make sure to choose the right sized lens for the sensor. If the lens is rated for a smaller image sensor than the one inside the camera, the field of view will have black corners. Conversely, if the lens is rated for a larger image sensor than the one inside the camera, you will lose some of the field of view outside the image sensor range (see figure 1, page 25). This situation creates a telephoto effect, making everything look zoomed in.
In the case of a megapixel or HDTV camera, you need a high-quality lens, since these sensors have pixels that are much smaller than those on a VGA sensor. The pixels must be smaller in order to fit that many more on the sensor. For instance, a 720p camera with 1280 x 720 pixels has three times as many pixels as a VGA camera, which has 640 x 480 pixels. To fully use the camera’s capability, the lens and camera resolutions should match.
Picking the Right Lens Mount
Network cameras use one of two standard mounts: CS-mount and C-mount. They both have 1-inch thread and look the same. Where they differ is in the distance from the lens to the sensor when fitted on the camera. If you find it impossible to focus the camera, it is likely that you attached the wrong type of lens mount.
In a CS-mount, the most common lens mount, the distance between the sensor and the lens should be 12.5 mm; in a C-mount, the distance between the sensor and the lens should be 17.526 mm.
Choosing the Right Focal Length
A lens’ focal length is defined as the distance between the entrance of light into the lens and the point where the light rays converge on the image sensor. The general rule is that the longer the focal length, the narrower the field of view. There are three types of fields of view:
• Normal — a view that most closely corresponds to how the human eye sees;
• Telephoto — a narrower field of view that generally provides finer details than can be detected by the human eye; and
• Wide angle — a larger field of view with less detail than seen by the human eye.
Correspondingly, there are also three main types of lenses: