Choosing the Right Lens

The most common thing that can impair the performance of a Video Camera is an incorrect or poorly chosen lens. Lens choice depends upon several factors such as the physical position of the camera, the available scene illumination and the type of view of the scene that is required.

Specifications of Lens

Focal Length
The focal length of a lens determines its field of view at a given distance. A wide angle lens as its name suggests has a wide field of view at a given distance. This means that it can capture a wide area of the scene in both the horizontal and vertical planes. A longer focal length lens can capture a distant image with magnification. Because of this, objects in the scene will appear far away and show little detail. Broadly speaking, the focal length of a lens falls into two categories: fixed or variable.

Focal Length and Angle of the Field of View
Lens with a shorter focal length monitor a nearer object more clear than that with longer focal length. In contrast, lens with longer focal length can take a more distinct image than that with shorter focal length. However, the angle of the Field of View is greater with a short focal length and will show a wider picture. The angle of the Field of View is smaller with a long focal length and will show a narrower picture. Here are the example of lens with different focal length.

Depth of View
The depth of field refers to the area within the field of view which is in focus. A large depth of field means that a large percentage of the field of view is in focus, from objects close to the lens often to infinity. A shallow depth of field has only a small section of the field of view in focus. The depth of field is influenced by several factors. A wide angle lens generally has a larger depth of field than a longer focal length lens.
 
Camera Lens Illustration

Fixed Focal Length
A lens having a fixed focal length is often the least expensive. Since the focal length is fixed, so is the field or angle of view. This means that accurate calculations will have to be performed in order to correctly select a lens for a given application. A change in the requirements of the application will often result in a changing of lens.

Variable Focal Length
Although more expensive, these lenses are easier to use, set up and change. It is much simpler to obtain the correct view of a scene when it is possible to vary the focal length [and therefore the angle of view] of the lens. Variable focal length lenses should not be confused with zoom lenses which have a much larger adjustment range.

Lens Mount
The larger format cameras such as 1" and 2/3" have used the C-mount type lens system to physically couple the lens to the camera. With the advent of smaller CCDs such as 1/2" and 1/3", the CCTV industry has adopted the CS-mount. However, the unique back-focusing mechanism on Video Cameras allows both types of lens mount to be used. This is because the CCD assembly can be physically moved backwards and forwards in relation to the back of the lens. If this were not the case, the flange of a C-mount lens would  mechanically interfere with the CCD causing damage. CS-mount lenses are often less expensive and, in general terms, for a given focal length, a CS-mount lens is physically smaller than an equivalent C-mount lens.

Lens Iris
The amount of light that falls on the surface of the CCD sensor needs to be within certain limits for optimum performance. Too much light and the image is overexposed or washed out. Too little, and the resulting image is dark losing details in the shadow areas of the scene. The lens iris us used to control the amount of light falling on the sensor. This iris consists of a number of thin metal plates arranged in such a way that they produce a circular opening at their center. This opening, called the iris or aperture, can be made smaller or larger usually in fixed increments called f-stops.

Lens Selection

Fixed Iris
Fixed iris lenses cannot be adjusted for different lighting conditions. These lenses are most situated to indoor conditions where the lighting level will remain constant. However, the Electronic Iris and Automatic Gain Control features of Video Camera can make this lens much more flexible in use.

Manual Iris
The iris on a manual iris lens is usually set up when the camera is installed to suit the prevailing lighting conditions. These lenses cannot react to changes in scene illumination and are best suited to indoor applications where the ambient light will remain constant. The Electronic Iris and Automatic Gain Control features of Video Cameras can allow this type of lens to be used in a wider range of application areas.

Automatic Iris
For external conditions, and where the scene illumination is constantly changing, a lens with some sort of automatically adjustable iris is preferred. The iris aperture is controlled by the camera and is constantly changed to maintain the optimum light level to the CCD.

Zoom Lenses
Zoom lenses is the from of variable focal length lenses and offer the greatest functionality. They can be continuously adjusted throughout their range, usually remotely, to vary the focal length and field of view. Note that because the depth of field is also dependent upon the focal length of the lens, it will continuously vary throughout the zoom range being at its greatest when the lens is zoomed fully out [wide angle]. Remotely controlled zoom lenses are often used by the operator to closely examine critical areas of the scene.

Infrared Lenses
In some cases, Video Cameras have to be used in a very dark conditions. Operations of Video Camera are based on light, however, we can install infrared light that can illuminate the areas. Image produced by Infrared is B/W. And human eyes cannot see Infrared light, as a result, the area illuminated by infrared remains darkness to human.

For more information:
Video Camera Selection Guide - White Paper