When specifying a fixed focal length lens  as opposed to a varifocal lens for a particular application, an installer will generally ask the client what they want to see, then they will measure the size of the target object (e.g a door and the area around it) and the minimum and maximum distances from the camera for which the scene must be in focus (i.e depth of field). The installer will then calculate the correct focal length for the lens to meet the client’s specifications.

The problem comes when the fixed focal length lens is installed. It is often difficult for the client to describe exactly what they want to see and after installation is complete, they may decide that they need more detail (a longer focal length) or a wider view ( a shorter focal length) than they have with the installed lens. It is not the client’s fault, nor of the installer. It occurs simply because video doesn’t work like the human eye and it’s difficult for the cilent to describe exactly what they see in their mind’s eye. They may not even realize what they want until they get the system up and running. 

With Fixed focal length lenses, a change of focal length is time consuming and expensive as a new lens must be ordered and shiped (if not in inventory) and then installed, configured and tested again.