Great question - how long is a piece of string? There is a calculation for this.
a. There are many factors involved in choosing a suitable lens, as the lens size increases the left to right field-of-view narrows and the distance increases.
A variable focus 1.8mm ~ 3.6mm lens is ideal for monitoring hallways, doorways, and average size rooms
A 5.0mm ~ 40mm lens has the ability to be used for monitoring hallways and doorways and can be easily adjusted to monitor long driveways or areas several hundred feet away.
Beware of many online lens calculators which produce inaccurate angle measures for these focal lengths. Also, most 1/4" CCD cameras produce a significantly narrower field of view with the same focal length lens. Due to the physical nature of lens optics, the numbers listed below should be considered accurate to within ± 3° and therefore a few feet / meters as well. They are intended as approximate guidelines to help select the appropriate lens, while each camera in fact has its own unique field of view with each specific lens. In addition, C- and CS-mount lenses typically deliver a slightly narrower field of view than our comparable board lenses.
Lenses wider than roughly 5mm produce "artificially" wider fields of view than more telephoto lenses. In order to do so, the video image is slightly curved at the edges of the screen. While this artificial distortion is nominal for focal lengths between 4.3 and 3.6mm, curvature increases with wider angle lenses.
Each size lens is available with a manual or auto-iris lens.
The manual-iris lens is more affordable but should only be used in indoor applications where the light source remains constant The auto-iris lens can be used for indoor areas with varying light conditions as well as outdoor installations.