Lenses are either fixed or vari-focal length. Powered vari-focal lenses are called Zoom Camera Lenses. Focal lengths of most CCTV camera lenses vary from 3.6mm to 16mm for fixed focal length lenses, to well over 70mm for zoom lenses. This means that short focal length lenses have wide fields of view. This is good for close-ups or for seeing a large area. Objects appear smaller, rapidly, as distance from the camera increases. As lens focal length increases, the field of view narrows and more distant objects are easier to define. Please view the lens diagram below:

The Focal Lens Length Chart.

If you wanted to positively identify people, 7.5m (25') away from a camera, a short focal length lens (such as 3.6mm) would give such a large field of view that recognition would be uncertain. An 8mm or 12mm lens would be far better.

The Lens Iris
The "F Stop"