Security Camera - Choosing The Best Lens (Focal Length)

This article assumes that you are choosing to buyfind out the CCD format size BEFORE you buy it.
a security camera with a FIXED focal length lensThe CCD sensor acts like the 'eye' of the camera.
as opposed to a VARIABLE focal length lens.The lens of the camera focuses incoming light
Variable focal length cameras are becoming moreonto the CCD chip which then converts it to a
commonly available. However, be aware that onvideo signal using some very fancy circuitry. The
average cameras with a variable focal length lensfocal length is the distance from the lens to the
cost significantly more than those with fixedsurface of the CCD sensor.
lenses. It costs more to manufacture a goodStep 4: Go to an online lens calculator (see
quality variable focal length lens and it's also awebsite at bottom of this article).
challenge to weatherproof a varifocal camera forStep 5: Enter the CCD format size (BE SURE TO
use outdoors due to the fact that the lens needsCHOOSE THE CORRECT CCD SIZE!), target
to turn to adjust the focal length. The largedistance and target area and click the 'Calculate'
commercial surveillance cameras that have abutton to get the focal length.
zoom lens are usually housed in a weatherproofStep 6: You'll notice that it's probably some odd
housing for outdoor use. This really isn't a practicalnumber that does not match the standard focal
security camera for the homeowner or smalllengths that are available in the security camera
business owner though, due to its appearance andmarket. That's ok. The idea is to choose the focal
cost.length that is closest to the one that you
Fixed lenses are commonly seen in the small,calculated. Some common standard focal lengths
easily concealed weatherproof 'bullet' camerasfor security cameras are 3.6 mm (wide angle), 6
that are sealed, and fairly inexpensive. Therefore,mm, and 16 mm. If the focal length that you
the fixed lens is still a mainstay of videocalculated is is in between a standard size then
surveillance systems for the general consumer.you need to choose the one that you think is
Choosing the best focal length for your particularbest for your particular application.
situation is important. You need to ask yourselfHere's where you need to make a decision as to
these key questions: Is the camera just going towhether you want more magnification or greater
be used for a wide coverage (say to cover thecoverage area. If you want more magnification
parking lot at my business or my front yard) tothen you would choose the longer focal length, if
monitor general activity with little detail? Is theyou want a wider coverage area then choose the
camera going to be used to cover a specificsmaller focal length.
narrow target area (say an entrance way orFor example, let's say that you have a 1/3 inch
doorway) where I want to identify the face ofCCD camera and your target distance is 25 ft.
the subject, and therefore need greaterand your desired target width is 15 ft. Go to the
magnification?lens calculator, enter the information, click
Here are some steps that may be useful in'calculate' and the answer is 8 mm. Perhaps the
determining the best focal length security camera:type of surveillance camera that you want to buy
Step 1: First, determine the target distance - thatdoesn't come with an 8 mm lens as an option.
is how far away your subject is likely to be. TheYou could use a 6 mm lens and it would most
idea here is to ascertain the target distance basedlikely be fine since the 6 mm lens will actually
on the situation. It's not an exact assessment, butcover a slightly wider area than the 8 mm. While
choose a target distance that is representative ofthe 6 mm lens does cover a wider area, you
the area you want to cover. For example, if youwon't get quite as much magnification with it as
want the security camera to keep watch overyou would with the 8 mm lens. Remember, there
your car parked in the driveway, choose a spotis a trade-off between magnification of detail and
to mount the camera and then measure thecoverage area!
distance from the camera to the far end of theIf you want a better chance of identifying your
car furthest from the camera. That way you'resubject at a distance you might want to go with
able to cover the car and the surrounding areathe stronger focal length of say 16 mm to give
that a perpetrator might have to cross in orderyou better detail. Keep in mind though, the field of
to access your car. The idea is to choose anview will be narrower which entails a smaller
average distance away from the camera that thecoverage area. For a 1/3 inch CCD, with the
perpetrator is likely to be.target distance at 25 ft., using a 16 mm lens gives
Step 2: Next, determine the width of the targetyou are coverage width of only 7.5 ft. This lens
area - that is the width of the area that youmay help you get a subject's face in detail, but
want the camera to cover. You want to chooseyou may just miss him if he doesn't walk through
a coverage area that is wide enough to capturethe narrow target area! (This high magnification
the action, but not so wide that the magnificationfocal length might be good for a doorway or
is diluted. Remember there is a trade off betweenentrance that is a narrow target area is or if your
coverage area and magnification, in that if youtarget is at a greater distance).
increase one you'll decrease the other (all otherWhat about the 'height' of the coverage area? For
things being equal).most surveillance cameras, the height of the
Step 3: Determine the CCD chip format size ofcoverage area is about 3/4 of the width (4 by 3
your camera. If you don't know this, then ask theaspect ratio). So just multiply the width by 0.75 to
vendor that you purchased it from. Better still,obtain the height of coverage area.