Security cameras utilize two basic lens types based on focusing parameters. These are called fixed lenses and varifocal lenses. Fixed lenses have a “fixed” focal length while varifocal lenses have a focal length that can vary. A more familiar term used for electric powered varifocal lenses is a “Zoom” lens; we will “focus” (pun intended) on the fixed type in the following article.
In addition to the focusing characteristics of the lens, light requirements must also be considered. The light that is allowed to penetrate through the lens and strike the electronic sensor chip is normally controlled by a mechanical device known as an “iris.” The iris may be fixed, manual, or automatic. This refers to how the iris makes the light opening, called the aperture, smaller or larger. The aperture is equivalent to the human eyeball’s pupil. In very low light settings the aperture needs to be open wide to allow enough light to strike the sensor chip. However, in bright light conditions, the aperture is very small to prevent too much light from flooding the sensor chip.
If a security camera is used indoors in a setting that has a constant intensity of adequate light (i.e., a store, a warehouse, or a well lit room), then a “Fixed Iris Lens” should perform adequately to serve your needs.
In settings where the light conditions can vary from dark to bright (i.e., outside or a camera aimed at an outside window) an “Auto Iris Lens” will be needed to compensate and adjust for the varying light levels.
Finally, in light conditions that are constant for long periods of time but change from dark to light or light to dark, a “Manual Iris Lens” can be used. These lenses’ apertures are changed by hand to provide the optimum light exposure.
Lenses are created using a combination of Fixed and Varifocal parameters with Fixed Iris, Auto Iris, or Manual Iris apertures. Keeping the aperture requirements in mind let’s return to the issue of focal length. Most security camera lenses’ focal lengths vary from 3.6 mm to 16 mm for fixed focal length lenses, to well over 70mm for zoom lenses. A short focal length will yield a wide angle of view and a long focal length will yield a narrow angle of view
The actual mathematical formula used to calculate the width and height of the area covered is Width = Camera Horizontal Format X Distance/Focal length and Height = Camera Vertical Format X Distance/Focal length.
In this formula, values for the Camera’s Horizontal and Vertical Format are the size of the CCD or CMOS chip expressed in MILLIMETERS, not inches, with the pictures aspect ratio of 4:3. For example, for a 1/3 inch CMOS chip the vertical size of the chip in mm is 3.7 mm and the horizontal size is 4.9 mm. Use the vertical format value of 3.7 mm to calculate height and the horizontal format value of 4.9 mm to calculate width. Here is a list of some common sensor sizes and their corresponding camera vertical/horizontal format values:
• 1/3 inch 4.9 mm horizontal 3.7 mm vertical
• ½ inch 6.4 mm horizontal 4.8 mm vertical
• 2/3 inch 8.8 mm horizontal 6.6 mm vertical
• 1 inch 12.7 mm horizontal 9.6 mm vertical
For example, 2/3 inch CCD using a 25 mm lens that is 30 feet from the area being monitored can be calculated the following way:
Horizontal: (8.8 x 30)/25 = 264/25 = 10.56 feet horizontal
Vertical: (6.6 x 30)/25 = 198/25 = 7.92 feet vertical
To determine the lens requirement, just switch around the values in the formula to solve for focal length so Focal Length = Vertical Format x Distance/Height of Picture. For example, the focal lens requirement for a camera using a ½ inch CCD that is 20 feet from the area being monitored covering a height of 12 feet can be calculated as:
Focal length = (4.8 x 20)/12 = 96/12 = 8 mm
To sum it all up, let’s say you need a camera to monitor an outdoor area, all day and evening long. The camera has a ½ inch CCD chip, the area being monitored is 20 feet from the camera and it needs to cover an area 12 feet high. Your camera lens requirements would most likely be a fixed focal lens with either a manual or automatic iris, and a lens with a focal length of 8mm.