The Building Blocks of CCTV

From veri-focal to megapixel, lens selection is essential to a properly functioning surveillance system


Lighting Issues
Lighting is very important in selecting a camera lens. There are three critical issues with lighting: the variations of light; the amount of light; and the color characteristics of light.
The problem of light variation is exacerbated when viewing exterior scenes. Exterior applications will often range from bright sunlight to starlight. The ambient light level will be about 10,000 foot candles (FC) during the day and about 1FC during a moonless/starless night. To properly handle such extremes, a lens with a variable iris is often used. The variable iris is normally automatic and adjusts to the changes in light level by sensing the electronic circuitry in the camera. A small circular area in the center of the lens is often covered with a filter that limits the amount of light that can pass through the lens at the maximum FC level during the day. This spot filter is called a neutral density filter.

If the light level is more consistent, such as what you would find in an interior application, the lens can use a fixed or a manually adjustable iris. Even in interior applications, there can be significant variations in light levels. For example, when an interior camera views an exterior window where the sun shines in during part of the day.

Another issue with lighting is the amount of light available to the camera. The lens chosen will affect the amount of light that is focused on the camera-imaging device. A common mistake is to assume that the existing ambient light level is the actual light level available when selecting a camera. For example, if the camera views a scene during the nighttime with an ambient light level of 1 FC, the camera must be much more sensitive than the 1 FC level, because the ambient light must reflect off the object being viewed and pass through the lens onto the camera’s imaging device. Consider an exterior camera viewing a scene where there is fresh snow. The light reflected by snow will be around 95 percent of the ambient light level. This means that 5 percent of the light is lost before it enters the lens. If the area being viewed is a new black asphalt parking lot, the ambient light reflected might be 5 percent — meaning that 95 percent of the light is lost before it enters the lens.

To help select the proper level of light transmission through the lens, a property of the lens referred to as lens speed (measured in an F-stop) is used. The aperture of the lens, as well as the quality of the lens material, affects the F-stop — which can range from F1.2 to F22. The smaller the F-stop number, the greater the amount of light that can pass through the lens. For each F-stop increase, the light level passing through the lens decreases by 50 percent. For low-light applications, a lens with a low F-stop is desirable. In the earlier example of viewing a new asphalt parking lot at night, there is only 5 percent of the reflected light reaching the lens. Using an F1.4 lens over a F1.2 lens reduces the light to 2.5 percent of the ambient light level. The original 1 FC is now .025FC on the camera’s imaging device. For most indoor applications, however, the F-stop is not as critical, and plastic lenses are often used to reduce costs.

The final issue with light passing through the lens is the characteristic of light itself. Light is actually composed of different colors. This is obvious when we see a rainbow or if you remember using a prism in science class. Each color will focus on a slightly different point on the surface of the camera-imaging device, because each color is a different wavelength. A coating can be added to the lens to allow the various colors to focus on the same point, correcting what is called “chromatic aberration.” The coating is applied to lenses that are used with color cameras.

A special coating can also be added to the lens to address the varying focal points on a camera’s imaging device which exists when viewing both visible light and infrared (IR) with the same camera. They have a slightly different focal point on the camera’s imaging device, because they have different wavelengths. When the ambient light level reaches a preset low point, the camera switches from visible light to IR. At night, IR technology can provide more detail in low-light conditions than visible light to enhance nighttime video. When the camera switches to the IR mode, the focal point on the imaging device is different from visible light and can cause the IR images to appear fuzzy on a monitor, if the coating is not present. Theses types of lenses are referred to as day/night lenses.

Field of View
Focusing a camera on a given field-of-view must be addressed to select the correct lens. In all situations, we want to see a certain target at a given distance from the camera. The viewing angle for a lens will depend on the desired distance to the target. As the distance from the camera increases, the viewed area decreases and becomes narrower.

Lenses are measured in millimeters (mm) — the larger the mm number of the lens, the further away the focused field-of-view will be from the camera. A 75mm lens will focus much further away than a 12mm lens. As the distance increases away from the camera, the actual field-of-view will shrink. If you think of a field-of-view as a vertical and horizontal area that is a given distance from the camera, the horizontal and vertical dimensions get smaller as the distance from the camera increases. For this reason, a small mm lens (6mm) could be used on a camera for a building lobby where your goal is to cover as wide a view of the lobby as possible. There are field-of-view calculators or charts available from lens manufacturers to help in the selection of the desired horizontal and vertical area covered by a given lens.

A lens that has a specific mm size is refereed to as a fixed lens. A lens that can be adjusted within some range (3mm to 8mm) is a veri-focal lens. A veri-focal lens has a fixed range, but can be adjusted anywhere within that range. This type of lens allows adjustments to be made at the camera to provide the desired field-of-view. The desired field-of-view might not be possible with a fixed lens without moving the camera. The veri-focal lens can be either manually or electronically adjustable.

A zoom lens is similar in that it is adjustable within a specific range, but the range for a zoom lens is normally provided as a number multiple. For example, a zoom lens might be a X10 lens. This means that the lens can be adjusted from its minimum mm size up to 10-times that number. For this reason, the manufacturer also provides the range of adjustment — two examples would be 8mm to 80mm and 15mm to 150mm.
In some applications when large multiples (zoom range) are needed, the lens provides part of the zoom capability and the electronics provide the rest. Some IP cameras use a wide zoom range so that a single camera can be used in many applications without changing the lens.

Specialty Lenses
One of the more popular specialty lenses is the megapixel lens. These lenses are used on megapixel IP cameras to provide extremely high-resolution video images. Because a megapixel camera’s imaging device has inherently more detail, the lens used with this type of camera must be able to pass this detail to the imaging chip. This is accomplished with a very high quality low distortion glass in the lens that allows the desired detail to be processed by the image chip.

The details needed to design a megapixel lens include the image device size, the number of pixels in the device and the number of pixels needed to define a point on the device. More resolution is needed if it is a color point.
A high-resolution lens performs a similar function on analog cameras as the megapixel lens does for a digital camera. The image chip in a high-resolution camera application normally uses a larger chip format than the standard CCTV analog camera. Many CCTV analog cameras today use quarter-inch and 1/3-inch imaging devices. A high-resolution camera often uses a half-inch or a 2/3-inch format.
Another specialty lens is a wide-angle lens, sometimes called a fisheye lens, because the lens looks a great deal like the eye of a fish. The lens allows a wide-angle view that varies, but can be 180 degrees. The extra detail can provide a view of an area with one camera instead of several. The limitation of this type of lens is that there is distortion on the edges of the field of view. The light causes the distortion as it travels through the outer edges of the lens at an extreme angle. The negative impact of the distortion is minimal considering the extra viewing that is possible with this lens. There are manufacturers that make lens assemblies that can view wider than 180 degrees. Some of these lens assemblies cover 360 degrees.

Other specialty lenses include split-image/pinhole, preposition lenses and gain control.