Security Camera – Choosing The Best Lens (Focal Length)


This article assumes that you are choosing to buy a camera camera with a FIXED focal length lens as opposed to a VARIABLE focal length lens. Variable focal length cameras are becoming more commonly available. However, be aware that on average cameras with a variable focal length lens cost significantly more than those with fixed lenses. It costs more to manufacture a good quality variable focal length lens and it's also a challenge to weatherproof a varifocal camera for use outdoors due to the fact that the lens needs to turn to adjust the focal length. The large commercial surveillance cameras that have a zoom lens are usually housed in a weatherproof housing for outdoor use. This really is not a practical security camera for the homeowner or small business owner though, due to its appearance and cost.

Fixed lenses are commonly seen in the small, easily disguised weatherproof 'bullet' cameras that are sealed, and fairly inexpensively. Therefore, the fixed lens is still a mainstay of video surveillance systems for the general consumer. Choosing the best focal length for your particular situation is important. You need to ask yourself these key questions: Is the camera just going to be used for a wide coverage (say to cover the parking lot at my business or my front yard) to monitor general activity with little detail? Is the camera going to be used to cover a specific narrow target area (say an entrance way or doorway) where I want to identify the face of the subject, and therefore need greater magnification?

Here are some steps that may be useful in determining the best focal length security camera:

Step 1: First, determine the target distance – that is how far away your subject is likely to be. The idea here is to ascertain the target distance based on the situation. It's not an exact assessment, but choose a target distance that is representative of the area you want to cover. For example, if you want the security camera to keep watch over your car parked in the driveway, choose a spot to mount the camera and then measure the distance from the camera to the far end of the car farthest from the camera. That way you're able to cover the car and the surrounding area that a perpetrator might have to cross in order to access your car. The idea is to choose an average distance away from the camera that the perpetrator is likely to be.

Step 2: Next, determine the width of the target area – that is the width of the area that you want the camera to cover. You want to choose a coverage area that is wide enough to capture the action, but not so wide that the magnification is diluted. Remember there is a trade off between coverage area and magnification, in that if you increase one you'll decrease the other (all other things being equal).

Step 3: Determine the CCD chip format size of your camera. If you do not know this, then ask the vendor that you purchased it from. Better still, find out the CCD format size BEFORE you buy it. The CCD sensor acts like the 'eye' of the camera. The lens of the camera focuses incoming light onto the CCD chip which then connects it to a video signal using some very fancy circuitry. The focal length is the distance from the lens to the surface of the CCD sensor.

Step 4: Go to an online lens calculator (see website at bottom of this article).

Step 5: Enter the CCD format size (BE SURE TO CHOOSE THE CORRECT CCD SIZE!), Target distance and target area and click the 'Calculate' button to get the focal length.

Step 6: You'll notice that it's probably some odd number that does not match the standard focal lengths that are available in the security camera market. That's ok. The idea is to choose the focal length that is closest to the one that you calculated. Some common standard focal lengths for security cameras are 3.6 mm (wide angle), 6 mm, and 16 mm. If the focal length that you calculated is in between a standard size then you need to choose the one that you think is best for your particular application.

Here's where you need to make a decision as to whether you want more magnification or greater coverage area. If you want more magnification then you would choose the longer focal length, if you want a wider coverage area then choose the smaller focal length.

For example, let's say that you have a 1/3 inch CCD camera and your target distance is 25 ft. and your desired target width is 15 ft. Go to the lens calculator, enter the information, click 'calculate' and the answer is 8 mm. Perhaps the type of surveillance camera that you want to buy does not come with an 8 mm lens as an option. You could use a 6 mm lens and it would most likely be fine since the 6 mm lens will actually cover a slightly wider area than the 8 mm. While the 6 mm lens does cover a wider area, you will not get quite as much magnification with it as you would with the 8 mm lens. Remember, there is a trade-off between magnification of detail and coverage area!

If you want a better chance of identifying your subject at a distance you might want to go with the stronger focal length of say 16 mm to give you better detail. Keep in mind though, the field of view will be narrower which includes a smaller coverage area. For a 1/3 inch CCD, with the target distance at 25 ft., Using a 16 mm lens gives you a coverage width of only 7.5 ft. This lens may help you get a subject's face in detail, but you may just miss him if he does not walk through the narrow target area! (This high magnification focal length might be good for a doorway or entrance that is a narrow target area is or if your target is at a greater distance).

What about the 'height' of the coverage area? For most surveillance cameras, the height of the coverage area is about 3/4 of the width (4 by 3 aspect ratio). So just multiply the width by 0.75 to obtain the height of coverage area.


About the author


I'm a Web designer. I'm that guy who loves to sit in front of his computer almost all the time. This is my blog, where I like to do stuff as writing tutorials and share blogger related stuff.

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