CCTV Lens |
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A basic definition of a lens is "A device that collects light from an environment, narrowly focusing it onto either a cathod ray tube or solid-state target."
In the scheme of things, lenses are considered to be a camera peripheral rather than part of the camera itself. This is because it does not require a lens to produce a 1-Volt-peak-to-peak (Vpp) video signal at a camera's output. In fact, all that is necessary to create a video signal is to have light strike the target inside the camera. However, without the proper lens, video images cannot be focused onto the light-sensative target in a video camera to produce a "usable" video signal.
Among the many issues that pertain to a CCTV syste, the most important that should be considered before buying a lens are:
camera format,
whether the situation calls for a fixed-iris lens or one with an auto-iris,
whether the operator needs to zoom in on particular parts of the scene,
and the size of the viewing area involved.
Format
Lenses and cameras now come in four formats: 1/3-, 1/2-, 2/3-, and 1-in. Lens format is actually determined by the size of the opening in a camera where the lens attaches. When the diameter of this opening measures 1/3 in., the camera is said to be a 1/3-in. formated camera. On the other hand, when the diameter is 2/3 in., the camera is said to have a 2/3-in. format.
For proper viewing, experts say to use only a lens that's formatted the same or higher than the camera used. For example, if the opening in a camera (camera format) is 1/2 in., a 1/2- to 1- in. formatted lens should be used for optimum viewing. Or, if a camera with an opening of 1 in. is used, use a 1-in. formatted lens. To do otherwise will result in the projection of only part of the viewing area onto the target in the camera.
Fixed-Iris, Manual-Iris and Auto-Iris Lenses
Whether to use a fixed-iris lens or one with an automatically-operated iris depends largely on the light conditions where a camera and lens will be used. For example, a lens with an auto-iris feature should be used in environments where the light intensity can vary from minute by minute, hour by hour, or day by day. This is almost a necessity in outdoor applications and where lights are switched on during business hours and off after closing.
Fixed-iris and manual-iris lenses, on the other hand, can be used in situations where the light essentially stays the same all of the time. This type of application is often found inside of buildings where the lights never vary, no matter what time of day it may be.
The iris itself is actually a small aperture through which light must pass before it can strike the light-sensitive target inside a camera. In low-light, for example, the aperture is fully open and in bright-light, it will nearly be closed. In medium-lit conditions, the same aperture will physically be open somewhere between its fully-open and closed position.
To measure the size of an aperture opening, the lens industry adopted the F-stop measurement. The larger the F-stop number, for example, the smaller the opening. Thus, when a camera is set to an F-stop of f-1.4, the size of the aperture is larger than when the same lens on the same camera is adjusted to f-8 or f-22.
In a fixed-iris lens, the aperture is set and cannot be changed. In manual- and auto-iris lenses, however, the aperture can be changed to suit the amount of reflective lighting in the environment. The manual-iris model, for example, requires someone to change the aperture setting to suit the situation, where the auto-iris model can make these aperture adjustments automatically without intervention of any kind.
Manual-iris lenses are commonly adjusted using a thin ring at the base of the lens where it screws onto the camera. On most models the f-stop setting is clearly shown on the ring. To readjust the aperture of the lens, all the dealer has to do is turn the ring. A pointer on the stationary part of the lens points to the f-stop settings as the ring is turned. Common F-stops included on manual-iris lenses are 1.4, 2.0, 2.8, 4.0, 5.6, 8.0, 11, 16, and 22.
Generally, the higher the f/stop number, the less light that a camera lens can pass. In addition, the lower the f/stop number, the more light that is passed.
Auto-iris lenses, on the other hand, automatically adjust the amount of light that passes through them. This is done with a motorized aperture that adjusts itself according to the amount of light that's available in the environment. To perform this task, an auto-iris lens is equipped with either a tiny motor and gears or a magneto-array ring. An electronic controller board or video sampling board are used to control the setting of the lens aperture, using the amount of video signal generated inside the camera as the adjustment criteria. In turn, the amount of video signal generated by a camera depends on the amount of available reflective light in the environment.
This type of lens is ideal for use in environments where the ambient light changes regularly. Automatic iris lenses are able to adjust the amount of light that passes through them using a built- in optical sensor that tells the lens what the light level is in the environment. In low-light conditions, for example, the optical sensor prompts the lens to open up, which allow more light to fall on the target in the camera.
Field of View
An important aspect that must be considered before choosing a camera lens is the field of view (FOV), or the actual area that the camera is expected to "see." When all else is the same, the expected FOV of a particular lens will change when the lens size is either increased or decreased.
Using a FOV calculator, for example, a 2/3-in. 25 mm lens will allow a camera to see a FOV of 5 ft.(h) x 3.8 ft.(w) at a distance of 15 ft. Using a 1/2-in., 25 mm lens will change the field of view to 3.8 ft.(h) x 2.8 ft.(w).
The more accurate way to determine FOV is to do it mathematically using the following formulas and chart:
Scene Width = Horizontal Format x Distance)/Focal Length
Scene Height = Vertical Format x Distance/Focal Length
+--------------------------------------+
| Horizontal & Vertical Camera Formats |
+---------+--------------+-------------+
| | Horizontal | Vertical |
| Lens | Camera | Camera |
| Format | Format | Format |
+---------+--------------+-------------+
| 1/3 | | |
+---------+--------------+-------------+
| 1/2 | 6.2 mm | 4.65 mm |
+---------+--------------+-------------+
| 2/3 | 8.8 mm | 6.60 mm |
+---------+--------------+-------------+
| 1 | 12.7 mm | 9.50 mm |
+---------+--------------+-------------+
In the above example, using the formulas and chart, the exact height and width of the 2/3-in., 25 mm lens under the same conditions is 3.96 ft.(h) x 5.28 ft.(w). Although the dial-up Lens Calculator was a slight bit off, the degree of inaccuracy is hardly worth the mention.
Focal Length
Another very important aspect to be considered when choosing a lens for a particular application. The Focal Length (FL) of a lens is actually the distance from the center of the lens to the surface of the tube or solid-state target. A lens with a FL that's shorter than the FL of a standard lens--for a given format, is considered to be a wide-angle lens. A lens with a longer FL than a standard lens is considered as a telephoto lens.
Optical Speed
The optical speed of a camera lens actually refers to it's light-gathering ability. The more light that a lens is able to collect and convey to the target in a camera, the better the resulting picture will be. The larger the lens, the more light that the lens can collect and use to generate a picture. The optical speed of a lens is measured in f/#.
A slow lens, for example, has a higher f/number assigned to it than a fast lens. For example, an f/2 lens is slower than a model rated at f/1.6.
To determine the f/number of a lens, use the formula: f/# = FL/d
(FL=Focal Length, d=Diameter)
Fixed Focal Length Lens
One of the most common lenses in use today, the fixed focal length lens is relatively economical. However, it offers only one focal length, usually between f/1.3 to f/1.8, which cannot be changed. Because the optics in this type of camera is simple and to-the-point in nature, these lenses are usually considered faster than variable- focal-length models. Fixed focal length lenses come in standard, wide-angle, telephoto and variable zoom.
Standard lenses essentially mimic what the eye sees. In terms of magnification, a standard lens has a magnification factor of 1. In terms of a numeric measurement, stated in millimeters (mm), to be a standard lens, a 1-in. formatted lens must have a focal length (FL) of 25mm, a 2/3-in. must have a FL of 16mm, a 1/2-in. lens must have a FL of 12mm, and a 1/3-in. lens must have a FL of 8mm.
Wide-angle lenses enable more of a scene to be witnessed than when looking at it with the naked eye. To be considered a wide- angle lens, a 2/3-in. camera must have a FL of 4.8mm and have a 96-deg. horizontal and 72-deg. vertical angle of view. |
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