Mechanical compensation zoom system: It means that each movement component is relatively complex and different corresponding movement according to different motion laws, and finally achieves the purpose of preventing image surface movement.The ir lenses : one set of lenses makes a linear movement (known as a zoom group) to change the focal length, and the other set of lenses (commonly called the compensation set) makes a small amount of nonlinear motion to compensate for the image plane displacement to achieve optical system zooming. The image position is stable.