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Picking the Proper Lens

You must calculate the lens size and the number of security cameras required to secure an area sufficiently.

Written By Darrel Hauk, CEO
This article was reposted with permission from Residential Systems magazine. To read more, visit www.residentialsystems.com

The ability to see the front porch, the backyard, or inside the home is beneficial to homeowners concerned about protecting their home and family. There’s no better way to do this than with a surveillance camera. Specified correctly, a camera can capture video of occurrences you might otherwise have missed. With the right components, it can see in the dark, transmit images to your smartphone, and let you control and view it from anywhere in the world.

Sounds simple enough, but what a professional integrator really needs to consider is the size of their security camera lens, which will determine everything else in the system. The basics of mathematics and a few dimensions from your field survey will provide you the answers of the lens size and the number of cameras required to secure an area sufficiently. In addition, you will articulate the information precisely to your customer because of your superior surveillance knowledge.

Calculate the required lens as follows:

  1. Measure the distance from your camera to the area you would like to view. Take into consideration the height of the camera and the furthest distance.
  2. Measure the width of the area you would like to view.
  3. Divide the width measurement into the distance measurement. For example, if the distance from the camera to the area you want to view is 20 feet, and you would like to have a 5-foot-wide view at 20 feet, divide 20 by 5 and your answer is four.
  4. The size of the camera’s imaging sensor also affects the angle of view. Using a smaller image sensor creates a narrower angle of view when used with the same lens. The format of the lens, however, is irrelevant to the angle of view. It merely needs to project an image that will cover the device, i.e.; the same format of the camera or a larger lens. This also means that one-third-inch cameras can utilize the entire range of lenses from one-third of an inch or twothirds of an inch; a one-third-inch 8mm lens gives the same angle of view as a two-thirds-inch 8mm lens. The latter combination also provides increased resolution and picture quality as only the center of the lens is being utilized, where the optics are manufactured more accurately.
  5. Look up the size for your camera image sensor in this list and multiply the answer in Step 3 by the given multiplier: 0.5-inch image sensor multiplier = 6.4; 0.33-inch image sensor multiplier = 4.8; 0.25-inch image sensor multiplier = 3.2. If you have a third-inch image sensor, following the above example you would multiply 4.8 by 4, which equals 19.2, so the lens size you would need is a 19.2 mm lens. There actually is not a fixed lens on the market that is 19.2mm, so you have several options. These include using a varifocal lens or zoom lens, or you will need to locate the camera closer to the subject, or settle on a standard fixed lens with a wider angle of view.
  6. Determine the picture height by calculating the horizontal field-of-view by 0.75 to get the vertical field-of-view. Using the example in Step 3, the horizontal field of view is 5 feet times 0.75, so your vertical field of view is 3.75 feet. This is a good field of view for being able to clearly see the details of a person’s face and upper torso at 20 feet away.
 

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