You will need to decide the following when choosing a cctv camera & lens for a digital cctv system:
The Digital Camera Location : Will the camera be situated Indoors / Outdoors?
The amount or variance of light conditions for the digital cctv camera : Will the camera have to record in low-light conditions such as at night?
The Field of View required for the camera : How wide or narrow an angle of vision is required ?
The Focal Length - How far away objects you wish to record will be from the camera ?