When designing a system where a range of fixed lenses is just not suitable, one has to turn to manual zoom lenses to get the best field of view. By manual zoom lenses, I mean as opposed to motorised zoom lenses.
Most “manual zoom” lenses are marketed as varifocal but there is a difference between these and a true manual zoom lens. A verifocal, as the name suggests, shifts its focus with every zoom adjustment. Whereas a true manual zoom lens adjusts the optical array so that the focus is fixed to an acceptable sharpness throughout the zoom range.
One element of the optical array moves either closer together or further apart as the zoom is adjusted another element also adjusts to compensate for the focal shift. A true zoom lens uses between three and five moving groups comprising of up to twenty individual optics.
In a varifocal lens there is no attempt to maintain a sharp focus through mechanical or optical compensation. This produces a cheaper zoom lens with the disadvantage of not being able to maintain a constant focus throughout the zoom range.
In most applications, a varifocal lens will be adjusted and “locked off” at the best setting to provide a permanent fixed field of view. However, in a system where each camera interacts with the next and fine adjustment of the field of view is necessary to get the optimum overlapping views, a manual zoom lens is a far more useful tool.
These applications are probably not encountered often but large high quality systems such as one covering the perimeter of a high security establishment, for example may need fixed focal length lenses that can only be achieved using manual zooms being finely adjusted and then locked in to position. To set up such a system with varifocal lenses would be unnecessarily difficult.
In any CCTV system, the preference from a quality point of view would be to get the survey right, do the calculations and provide a CCTV system with fixed focus lenses.