They are essential for creating video functions. Lenses perform two main functions: First, it determines the scene that will be shown on the monitor. This is a function of the focal length. Second, it controls the amount of light reaching the sensor. This is a function of the iris. Focal length can be fixed or variable (e.g., a zoom lens). The iris may be manually adjusted or automatically controlled by the camera.
Fixed Focal Length Lenses
Fixed focus lenses are the simplest type of lens, and therefore the least expensive. Its preset focal length requires a precise calculation to select the lens most suitable for the location. This is based on the desired size of viewing area and its distance from the camera. Typical lens sizes offer either a narrow, 30-degree field of view to allow more detail at a given distance, or a wider, 60-degree field of view.
Varifocal lenses offer more flexibility, allowing you to adjust the field of view manually. Although slightly more expensive, these lenses are popular because you can get a more precise adjustment of the scene. They also simplify the specification process, because the flexible field of view means a single lens can often times be selected for all cameras in an entire system.
IR Corrected Lenses
The human eye sees in the "visible light" portion of the spectrum. Beyond visible light is a portion of the spectrum that includes infrared (IR). IR light negatively affects the accuracy of color reproduction: for this reason, all color cameras employ an IR cut-filter to minimize or eliminate IR light reaching the image sensor. This menas IR-corrected lenses are not necessary on standard color cameras.
Day/night and monochrome cameras can benefit from IR-corrected lenses. The CCD device inside a security camera can detect IR light and use it to help illuminate the area being viewed. In fact, using ordinary lenses on monochrome or day/night cameras typically results in a blurred, or worse-case, completely unfocused picture. This is because the wavelength of IR light differs from visible light, so the focus point of IR light is displaced compared to that of visible light.
Consequently, when using ordinary lenses and setting the focus at daytime, the picture will be unfocused or blurred at nighttime when using IR illumination, and vice versa. The problem can be rectified by using IR-corrected lenses, which focus both visible and infrared light in the same vertical plane.
IR-corrected lenses are not solely for use with IR illumination at nighttime. Many light sources include a portion of the IR light spectrum, so using IR-corrected lenses on a monochrome or day/night camera will provide a sharper picture because all the light is focused. You'll get a far more crisp picture compared to ordinary lenses.
Motorized Zoom Lenses
Zoom lenses are the most complex, but offer the greatest functionality. They can be adjusted remotely to allow variation of the focal length and maintain focus while tracking. This means that a single lens can be used to view a wide area, until an intruder is detected, whereupon you can zoom in to capture facial details. Generally, zoom lenses incorporate a motorized zoom, focus and auto-iris function to permit maximum usage.
Lenses are also categorized according to format size. The format of the lens (1/2-inch, 1/3-inch, 1/4-inch, etc.) is derived from the ratio of diameter to the viewing image produced. While it is often most cost-effective to match the lens format to the camera's sensor size, it is possible to use a larger lens on a smaller size (imager) camera, since the image only needs to be at least as large as the sensor.
Using a larger lens can often be advantageous, since it offers greater depth of field (the range of distance from the lense before objects too far away to be in focus). Larger lenses also mean that the image area used is taken entirely from the central, flatter part of the lens, which causes much less corner distortion and offers better focus.
CCTV lenses utilize either "C" or "CS" mounts that specify the thread type and dimensions. The difference between the two types is the distance form the back of the mounting flange to the face of the sensor. This is known as the "flange back length." With CS lenses, this distance is shorter, which allows the user of fewer and smaller glass elements, resulting in a more compact lens design.
Most cameras today use a CS-type lens mount. A CS lens may only be used on a camera with a CS format mounting. A C mount lens may be used on a CS mount camera by adding a 5mm adapter ring.
The thread type and its dimensions are identical for both types of enses, so either may be mounted to cameras with either type of mount without causing any damage. However, the lenses are not totally interchangeable: the wrong lens/mount combination will make it impossible to focus the camera.
Focal length is the distance between the center of the lens and the image sensor. Rays from infinitely distant objects are condensed internally in the lens at a common point on the optical axis. The point at which the CCTV camera's image sensor is positioned is called a focal point. By virtue of design, lenses have two principal points: a primary principal point and a secondary one. The distance between the secondary principal point and the focal point (image sensor) determines the lens' focal length.
Focal length measurement is expressed in millimeters. Lenses are defined as normal, wide-angle, or telephoto according to their focal length. For example, on a 1/3-inch format camera, an 8mm lens is a wide-angle lens because it is capable of capturing a very wide field of view. Conversely, a 125mm lens on the same camera in the same location sees a much narrower field of view, although the objects are significantly magnified.
The iris controls the amount, or quantity, of light striking the face of the image sensor. To provide optimum performance, it is critical that neither too much nor too little light fall on the camera sensor. If too much light hits the image sensor, the image is "washed out" (the image is all white or portions of the image are "too hot," where light-colored surfaces lose all detail). Closing the iris corrects this. At the other extreme, too little light hitting the image sensor results in a black image or one in which only the brightest objects are visible. Opening up the iris corrects this situation. Irises may be fixed, operate manually, or operate automatically.
A fixed iris lens offers no adjustment for different lighting conditions, so it is limited, and not suitable for applications where fine detail is consistently required.
Manual iris lenses are best suited to indoor applications, where the lighting level is controllable and consistent.
In cameras with automatic (electronic) iris control, the circuitry continuously samples the amount of light hitting the image sensor and opens or closes the iris accordingly. Auto iris is especially valuable in settings where light levels are constantly changing, such as exterior locations.
The aperture is the size of the opening in the iris, expressed in f-stops. A smaller f-stop means a larger opening, which results in a greater amount of light passing through the lens to the image sensor. This is also known as a faster lens. Conversely, a larger f-stop means a smaller opening, with less light transmitted through the lens.