Focal length and field of vision
The first specification to consider when choosing a camera lens is what is it you are trying to see. Is it and area or an object (faces, car, entry door ect.).
Then its focal length. The focal length of a lens determines both the range and distance of its field of vision. The basic rule to remember is that a smaller focal length will lead to a wider viewable range. Conversely, a larger focal length will have a more narrow field of vision, but it will also have a further range. A 2.8mm lens would be ideal for capturing images in a wide field within 20 feet or so, but a 12mm lens could grab a clearer image of an object farther away. A lens with a focal length around 8mm would split the difference between the two extremes.
Like security cameras, lenses come in many different varieties. When choosing a lens, it’s important to realize what its intended function is and what limitations it may not be able to overcome. Different surveillance conditions call for a different camera and lens. An informed shopper should know what lens is best suited for his or her needs before making a purchase.