1. Optical compensation zoom system: when several groups of lenses are used for zoom and compensation in the zoom lens, the movement of each lens group is carried out in the same direction and at the same speed, so it only needs a simple mechanical structure to connect the lens groups together to make linear motion. Because the optical compensation zoom system can't fully compensate the image plane displacement, the moving group must move to some special positions to get a stable and clear image plane. Its focal length is difficult to change continuously, but several discrete values, so its use is limited. But it simplifies the mechanical structure, which is conducive to control the accuracy of the optical axis; and only a group of electromechanical control system is used to realize zoom and focus, thus reducing the cost and weight of the system, but the design is relatively difficult.
2. Mechanical compensation zoom system: it refers to the relatively complex corresponding movement of each motion component according to different motion laws, so as to prevent the image plane from moving. Mechanical compensation zoom lens: one group of lenses makes linear movement (commonly known as zoom group) to change the focal length, and the other group of lenses (commonly known as compensation group) makes a small amount of nonlinear movement to compensate the image plane displacement, so as to achieve the requirements of both zoom and stable image plane position of the optical system. The zoom group is generally a negative lens group, and the compensation group is either a positive lens group or a negative lens group. The motion direction of compensation lens group is different from that of zoom lens group, but their relative motion has strict corresponding relationship. Each lens group realizes relative motion through a complex cam mechanism. The focal length of this kind of zoom lens changes continuously in a certain range.
2、 Optical structure
Optical structure.
The optical structure of mechanical compensation zoom lens is composed of front fixed group, zoom group, compensation group and rear fixed group.
1. Anterior fixation group: its function is to provide a fixed image for the system;
2. Zoom group: responsible for the zoom function of the system, do linear movement to change the focal length;
3. Compensation group: make nonlinear motion according to a certain curve track to compensate the image plane movement produced by the zoom group in the zoom process;
4. Rear fixed group: used to convert the image of compensation group into the final real image of the system, and adjust the synthetic focal length value of the system and the aperture of the equipment to ensure that the relative aperture of the system remains unchanged during the zoom movement.
3、 Guide mechanism of zoom group
1. A combined mechanism of a light screw guide rail and a ball screw. Because of the different trajectories of zooming and compensation moving at the same time, two sets of guiding and driving mechanisms are needed, which takes up a large space and makes the design of control system difficult.
2. Two cylindrical guide sliding mechanism. Because the sliding parts are two cylindrical guide rails, this structure has high magnification accuracy and carries more load than the first one. However, due to the super positioning structure, the optical aperture is too large, it is easy to cause the mechanism to jam, and the radial size of the mechanism is also large. Generally, it is suitable for the structure with the aperture of 30-80 mm.
3. Three cylindrical guide sliding mechanism. The advantage of this structure is that it is comfortable and stable to move, and it is not easy to get stuck. It can drive the optical components with large aperture. The disadvantage is that the movement accuracy is lower than the former two, and it is generally suitable for the structure with the aperture of 50-120mm.
