First of all, it is the lens that determines the amount of light entering the camera. Besides the quality of materials and of manufacturing, usually reflected in the price, some of the lens characteristics have much bearing on the viewed picture.

Even if you do not want to become an expert in lenses, there are a few concepts that you should understand before you purchase your surveillance camera.

The subjects are well known from the field of still photography, so that the principles should not be completely unfamiliar for whoever has some practice of family picture taking even at a very basic level.

In most surveillance cameras have fixed, non-interchangeable lenses. Therefore you should decide what you are trying to view; depth of focus, the angle, and the amount of detail you want to distinguish.

Normally one should select the best overall picture for the purpose. Note that by changing one of the characteristics (i.e. aperture) one modifies other related properties (i.e. the depth of focus). 

Fixed focal lenses are a common selection: although limiting somewhat the dimension of the space where all objects result clearly perceived (depth of focus), it simplifies the mechanical construction and reduces the price.

These cameras come with a varifocal lens which means that you can adjust the
focal distance to your particular application.  These are commonly used if the focal distance needs to be changed periodically.

You may be familiar with autofocus feature found in modern digital cameras. There is no comparable capability in cameras because they must work unattended. A lens whose focal length can be manually adjusted is called a Varifocal lens but should only be considered for surveillance purposes, when focal distances need to be changed.  (Remember:  The focal distance can only be changed on the camera itself).

Much more sophisticated for an application would be a motorized pan, tilt, zoom (PTZ) lens, capable of being commanded from a distance to select the preferred level of detail between wide-angle and telephoto.

Fixed iris or aperture is another characteristic to be considered that simplifies construction and operation, albeit with some compromise on picture quality. It would probably be easier to provide sufficient illumination.

Camera lenses also have a field of view, measured in millimeters (mm).  For example, a 3.6mm Lens will give you a much larger field of view, but everything looks farther way.  a 12mm Lens will shore detail, and look closer, but you will not see things around you.  Below is a chart that describes the horizontal field of view, for common lenses.

 
The images below show the differences in lens sizes, and give a graphical display. The camera is placed about 30 feet from the door, about 5 feet off the ground.  These images are be useful in understanding what which lens is most appropriate an application. 

3.6mm Lens	6mm Lens

8mm Lens	12mm Lens

Going back to the Varifocal lens.  They can be adjusted from a minimum to a maximum lens size, set by the type of lens installed.  Examples are 3.5 - 8mm lenses (of which the 6mm sample above would be roughly in the middle of its range).  Telephoto lenses can range from around 5 to 50mm.

In conclusion, there area many options available.  Since most cameras are installed once, it is important to choose the lens that gives you the best field of view.  Remember, the wider angle lenses (lower millimeter number) provide a larger field of view, but less detail at a distance.