In the Crosshairs: Choosing the Right Camera Lens
Choosing the correct lens for a video camera destined for a surveillance application is one of the most critical steps in the CCTV system design process. Unfortunately, it is a practice too often misunderstood – and ill executed – by salespeople and technicians.
The hazards of deploying a flawed lens selection? Namely, poor system performance and a disappointed customer.
To help you avoid such a fate, this report – replete with easy-to-understand diagrams and instructions – can be considered your course of action to ensure sensible camera lens assessments for a range of applications and environmental concerns.
Target Size Imperative: Is General or Detailed Coverage Required?
Simply, the optical function of a lens is to collect light reflected from a scene and to focus it onto a camera’s image sensor.
A safe passage for the reflected light’s journey to the image sensor is determined by three key factors: target area size, object distance and the camera’s imager format. (See the handy tutorial with illustrated examples of measuring devices used in calculating lens selection on page 50).
Misapply any of one of these facets when deploying a lens and the light’s journey is sure to be botched; and so, too, the desired image.
The first order of business in selecting a lens for surveillance is to define the target size of the area to be viewed. Is the camera intended to provide general coverage or capture close-up, detailed images?
General coverage surveys a wide area with a single camera. This is particularly useful when viewing a large area for activity – not necessarily to identify people. Subject identification may occur from another camera or as the subject approaches the general coverage camera.
Detailed coverage captures a narrow area, also with a single camera. A smaller area is covered so there is good quality identification and more detail, but there is less coverage of the overall scene.
Detailed coverage identifies subjects and may be used in conjunction with general coverage cameras viewing a larger defined area. Detailed coverage cameras are usually placed where they have the greatest chance of achieving identification (see diagram below).
Object Distance: Be Prepared to Don Your Geometry Cap
Object distance is defined as the distance from the camera mounting position to the target area being viewed.
The necessity for an accurate measurement of this figure depends on the target area size. If the target area will be viewed in general coverage, this measurement can simply be estimated. Granted, the more accurate the measurement, the better the image results.
Importantly, if the target area is intended to be viewed in detail, the object distance should be measured as accurately as possible.
For applications with detailed target areas and lengthy stretches of object distance and/or high mounting heights, the Pythagorean theorem should be applied to calculate the actual viewing distance (see diagram above).
Example: for a camera positioned 30 feet high and viewing a target 100 feet away, the actual distance is 104.4 feet.
Imager Sensor Handles Video Processing Inside of Camera
Camera imagers, a.k.a. image sensors, come in different formats or sizes. Currently, the most common size is 1/3-inch. Some newer cameras are now utilizing a 1/4-inch format (older cameras typically use 1/2- or even 2/3-inch imagers).
The size of the imager directly affects the field of view that a lens will provide and is critical to proper lens selection. Keep in mind, the larger the imager, the wider the field of view a camera will capture with the same lens.
Mind the General Rules for Focal Length and Field of View
Lenses with a higher focal length number – the distance between the optical center of a lens and a video camera’s image sensor – provide narrower, telephoto fields of view. As the focal length number decreases, the field of view of the lens gets wider.
Conversely, as the focal length number increases, the field of view will narrow. As the focal length moves from lower numbers (wider view) to higher numbers (narrower view), the camera moves from general coverage to detail coverage (see diagram below).
Installers Have Options to Overcome Undefined Target Areas
Some surveillance applications have target areas that cannot be easily defined. Long object distances that require complete coverage and wide target coverage for general observation are two common problems.
Hallways are a good example of a long object distance application. A single camera placed at one end is expected to cover the entire distance and still capture first-rate identification.
For this situation, the installer/designer can opt for a narrow field of view. Although good identification is achieved at the end of the target area, it will be sacrificed closer to the camera because the narrower field of view will allow subjects closer to the camera to pass without being viewed (see above).
Widening the field of view will better identify subjects closer to the camera, but poorer identification of subjects farther away can be expected. A subject passing at a distance through a wide view, and not moving toward the camera, will probably not be identifiable
There are three possible solutions to this problem.
First, the installer/designer can choose a spot halfway down the hallway to calculate the object distance, and then size the lens according to the target area at that distance.
Second, the installer/designer can use a variable focal length lens with a range wide enough to give close-up and long-range identification, and then allow the customer to choose a preferred view.
Third, the installer/designer can install multiple cameras with overlapping coverage. For this option, cameras are placed at each end of the hallway facing each other or placed at intervals all along the hallway. Obviously this is the most expensive solution, but it will provide the best coverage.
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