Everything You Ever Wanted to Know About Lenses and Cameras
The objective of this article, the third installment of our special, six-part series on digital CCTV, is to educate dealers about many of the newer technologies found in today’s lenses and cameras.
Whether a camera is the size of a button or a small cannon, the industry is definitely changing. But in what direction is lens and camera technology heading? During the past year, the CCTV industry has turned its direction away from standard camera technologies and toward the age of advanced digital and networked cameras.
Lens and camera technologies are constantly changing. However, the basic function as well as selection parameters aim to reproduce an image that is acceptable for different applications. These applications may range from a camera at your front door connected to your home television to a system that consists of more than 1,400 cameras designed to protect a modern-day casino.
Cameras Are Only as Good as Their Lenses
Images on a monitor are determined by three factors: the lens, the camera and the monitor. If one of them has a poor quality, your picture becomes unacceptable. The lens is a main ingredient of any surveillance system, and it plays a very important role in producing a quality image.
The main purpose of the lens is to focus the desired image onto the faceplate of the charge-coupled device (CCD) sensor. Enhancements in lens designs and features, especially in areas such as viewing angles (measured in degrees), drive assemblies of the auto-iris (video or DC), and the speed or F-stop rating of the lenses, have also entered the security market.
The size of the image focused on the camera’s sensor is determined by the focal length of the lens and is measured in millimeters (mm), while the amount of light allowed to reach the camera’s sensor is determined by the lens opening (iris) and is measured in F-stops.
Challenges Include Long Distances, Variable Light
Focal length of a lens is the distance between the image plane (camera sensor) and the focus point of the lens measured in air. As the focal length increases, the viewing angle decreases or becomes more telephoto. As different focal lengths change the viewing angle of an application, the image format of the camera’s sensor will also affect the angle of view.
The most common image formats found in today’s cameras vary from 1/2 inch to 1/3 inch to 1/4 inch. As the camera sensor decreases in size, the viewing angle of lenses becomes narrower at the same focal length. New varifocal lens designs now offer lenses as narrow as 75mm, producing angle coverage of 3.5 degrees to 4.5 degrees depending on the camera’s sensor size.
Not only are manufacturers working on narrowing the angle of coverage and making the lens telephoto, but they are also working on the speed of the lens, or how well the lens can pass the available light. These ratings are measured in F-stops.
F-Stop Ratings Affect Lenses’ Light Levels
The formula for determining an F-stop can be stated as F-stop = f (focal length in mm)/D (effective aperture in mm). The lower the F-stop of the lens, the “faster” (greater) the light-passing capability of the lens. A normal minimum F-stop of a lens is usually between F=1.2 to F=1.8.
Just as digital signal processing (DSP) camera technology has improved the performance of standard analog CCD cameras, so too has the aspherical design of CCTV lenses.
Varifocal Lenses Have Wide To Normal Views
Selecting the proper lens angle of view for different applications can be very challenging to say the least. It always seems that, once the camera system is installed, your customer is not satisfied with the lens viewing angles or wants to make changes to the system design. This usually causes many headaches for both your installing technicians as well as the customer.
Research shows that about 65 percent of all indoor camera applications usually incorporate lenses that are listed as either normal or wide in nature. To help enhance or, in others words, reduce frustration, the industry has developed what is known as a varifocal lens assembly.
The demand for telephoto varifocal lenses is getting larger and larger, especially after the Sept. 11 attacks. The most common focal length is 5.5mm to 55mm for the 1/3-inch image format. Even a 7.5mm to 75mm for the 1/2-inch image format varifocal lens is available in the market, and it is equivalent to 10mm to 100mm when used with 1/3-inch cameras.
Manual Iris Suited for Indoors, Stable Lighting
Manual iris lenses operate just as they do in photography. The greater the light levels, the higher the F-stop setting of the lens.
The normal operating range of any manual iris lenses is between F=1.4 to F=22. Lenses that have a lower minimum F-stop, such as F=1.2 or F=1.0, are known as fast lenses because they are capable of passing more light than lenses with higher F-stops, thus increasing the amount of light reaching the camera’s sensor.
Auto-Iris Lenses Adjust According to Lighting
DC or video-controlled lenses are used in varying lighting conditions and automatically control the opening and closing of the iris, allowing the proper amount of light to reach the camera’s sensor. The difference between the two lenses is how they accomplish iris control.
In video-controlled auto-iris lenses, the lens requires two inputs. The first input is a positive DC voltage that operates the electronics and the iris motor and the second is a video signal. This video input, along with the internal lens electronics, controls the opening and closing of the iris. Excessive video input causes the iris to close, while insufficient video causes it to open.
Zoom Lenses Favored for Pan/Tilt Applications
The last of the three basic lens groups are the zoom lenses. As with fixed focal length and varifocal length lenses, zoom lenses have the same overall characteristics for iris control and mounting, but with one exception—the focal length of the lens can be zoomed while maintaining focus.
Zoom lenses are normally associated with systems that incorporate a pan/tilt unit and will offer a wider flexibly to the CCTV operator. With this increased flexibility comes an increase in system set-up time and installation costs.
Range Calculators Help Simplify Lens Selection
The market has a large variety of different focal length lenses available. The most common way to choose the correct lens is the range calculator.
This tool is a round slide rule designed to help CCTV installers select the appropriate focal length lens for specific applications. The range calculator is a very simple device and usually has instructions written on its backside.
2 Main Camera Categories Are DSP, Remote Set-up
Let’s take a look at cameras by dividing them into two categories. The first is the digital-signal processor (DSP) type. This category meets all EIA standards for analog or composite video output but with digital enhancements. The second category is the remote set-up type, which is now being incorporated into some cameras.
For the most part, DSP technology can be found in both black-and-white and color cameras. However, with color cameras outselling black and white by a ratio of four to one, I believe that the DSP technology is incorporated to enhance color applications more than that of black and white. DSP technology has improved color image reproduction and has created better low-light response, which is of great importance to all outdoor applications.
Lighting Compensation Boosts Imaging
Since DSP cameras allow the electronics to be digitally separated and evaluated, no longer does video processing have to rely on just averaging the signals found in a normal video signal. New technology allows the DSP camera to sample two images in 1/60th of a second rather th
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