ABSTRACT
In a purpose-designed zoom lens, the relay system can be used to correct aberrations of the preceding unit. A triplet configuration at least is required. The variator with one moving element is inadequate for a true zoom or pancratic lens where back focal distance is kept constant. There are two ways of achieving this, a process which is termed compensation. One is optical compensation, where an additional element or elements move axially in the variator, coupled to the zoom element. The other arrangement is mechanical compensation, where a compensating element moves in a non-linear manner relative to the linear motion of the zoom element. Early attempts to produce a lens of variable focal length included the ‘Casket’ lens, where various individual elements were assembled in different ways to vary equivalent focal length, and the primitive telephoto lenses with variable separation of front and rear groups.
