There is really one question behind lens selection: can the framing be settled before the camera is mounted?

• Fixed lens: wider aperture, simpler construction, better cost and image quality. Ideal where the position and field of view are known, such as corridors, lift lobbies and assigned parking bays.
• Manual varifocal: adjust focal length and field of view during installation, then lock it off. Good where site conditions differ from the drawings and the view needs tuning by eye.
• Motorised varifocal: zoom and focus adjustable remotely from the client. A clear advantage at height or on poles where access is difficult, since later adjustment needs no second climb.
• Common mistakes: choosing a long lens purely for reach and ending up with a field of view so narrow it misses the surroundings; or choosing wide angle but expecting to identify faces at distance. Neither works.

Method: fix the distance and target size you need to cover, then derive the focal length. Where the requirement is uncertain, choose manual or motorised varifocal and keep room to adjust.