Lens has a fixed view. This field angle with fixed height and width is the so called viewing angle. Viewing angle is affected by f (focal length) and target surface size h/v (horizontal size and vertical dimension).
ah=2arctg（h/2f）
av=2arctg（v/2f）(ah means "angle of horizontal", av means "angle of vertical")
We can learn from the above that the shorter the f (focal length) is, the wider the viewing angle is; likewise, the wider the h/v (horizontal size/vertical size) is, the viewing angle is. If viewing angle of lens is too narrow, there may appear dead angle for monitoring. If viewing angle is too wide, there may be too small objects which are difficult to recognize and have distortion at edge of image. So, only choosing proper lens for actual environments can not only avoid dead angle but also keep image as large and clear as possible.
