1. Field of the Invention
The present invention is directed to a lens barrel which minimizes an image shifting phenomenon caused by a shifting of the center of a focusing lens group from an optical axis of a lens system.
2. Description of Related Art
A helicoid mechanism has been widely adopted as a focusing mechanism of a lens barrel for moving a focusing lens group along an optical axis thereof. In general, one of two annular members (e.g., a stationary lens barrel) is provided with a male helicoid and the other of the two annular members (e.g., a focusing ring provided with the focusing lens group) is provided with a female helicoid. The male and female helicoids are rotatably engaged with each other. When the focusing ring is rotated with respect to the stationary lens barrel, the focusing ring (i.e., the focusing lens group) moves along the optical axis in accordance with a leading angle of the helicoid to effect a focusing operation.
Another type of focusing mechanism is known in which a helicoid mechanism is not used. In this mechanism, a focusing ring is provided with a leading groove having a predetermined leading angle with which a roller provided on the stationary barrel is engaged.
The above-mentioned focusing mechanisms are similar in that, in both cases, one of the two annular members moves relative to the other along an optical axis when a relative rotational movement occurs between the two annular members, and further in that a focusing lens group is connected to one of the two annular members.
In the focusing mechanisms, a space (clearance) always exists between the engaged annular members. As a result, there is a possibility that the focusing lens group will shift by a small amount in a direction normal to the optical axis in the existing space. There is also a possibility that the optical axis of the focusing lens group will tilt to a certain degree, depending upon the breadth of the space. In particular, in the case of a surveillance camera in which a motor is used for a focusing operation, when a rotational driving power is applied to one of the two annular members through a gear or a gear train, it is impossible to avoid tilting of the focusing lens group and occurrence of deviation of the center of the focusing lens group from an optical axis of the lens system (hereinafter the center of the focusing lens will be referred to as a "principal point", and the deviation of the focusing lens group from the optical axis of the lens system will be referred to as a "shift of principal point"). The shift of principal point tends to occur when the direction of the driving motor is reversed.
The shift of principal point can be neglected in a still camera, as long as the amount of the shift is small. However, in the case of a surveillance camera in which an image formed by a photographing optical system is focused on a solid-state image sensor such as a charge coupled image sensor, and the image is observed on a monitor, when a shift of principal point takes place while an observer is watching the monitor, he or she will notice an "an image shifting phenomenon" on the monitor. The image shifting phenomenon makes the image on the monitor difficult to observe, resulting in low reliability of the system. The image shifting phenomenon tends to occur when the direction of movement of the focusing lens group is changed, i.e., when the direction of the motor is reversed.
Recently, surveillance cameras have become smaller. Originally, the size of a surveillance camera's solid-state image sensor was 1 inch (i.e., approximately 15.9 mm of its diagonal line). Since then, the size has been reduced to 2/3 inches (i.e., approximately 11.0 mm), 1/2 inch (i.e., approximately 8.0 mm), and 1/3 inch (i.e., approximately 6.0 mm). Since the image formed by a small-sized solid-state image sensor is observed while being enlarged, a slight shift of the principal point results in a large shift on the monitor. For instance, a 32 .mu.m image shift on a 1 inch solid-state image sensor results in a 1 mm image shift on a 20 inch (i.e., approximately 500 mm) monitor. An image shift of 22 .mu.m on a 2/3 inch solid-state image sensor, 16 .mu.m on a 1/2 inch solid-state image sensor, or 12 .mu.m on a 1/3 inch solid-state image sensor will also result in a 1 mm image shift on a 20 inch monitor.
In the table below, the amount of image shift on the above-noted types of solid-state image sensors is shown in terms of the distance which the principal point of the focusing lens group shifts from the center of the lens system optical axis. In the present embodiment, a lens in which f=56.3 mm and f.sub.1 =41.5 mm was used for determination of the values in the table, wherein "f" is equal to a composite focal length on the long focal length side of a lens system, and "f.sub.1 " is equal to a focal length of the focusing lens group. An image focusing plane is moved by a ratio of f/f.sub.1. In the above example, this ratio is equal to 1.3566. The shift amount of the principal point of the focusing lens group is calculated by dividing the image shift amount by the above coefficient of 1.3566.
______________________________________ amount of image shift on shift of principal size of solid-state solid-state point of focusing image sensor image sensor lens group ______________________________________ 1" 32 .mu.m 24 .mu.m 2/3" 22 .mu.m 16 .mu.m 1/2" 16 .mu.m 12 .mu.m 1/3" 12 .mu.m 9 .mu.m ______________________________________
The above result implies that the space between the two annular members which are engaged to each other must be made less than 9 .mu.m in order to make the image shift on a monitor less than 1 mm, in the case that a 1/3 inch solid-state image sensor and a 20 inch-sized monitor are used. However, even with the latest manufacturing technology, it would be quite difficult to attain this result.
On the other hand, with the present manufacturing technology, it is possible to reduce the space to about 30 to 60 .mu.m, in the case that two annular members are engaged by helicoid. When the space is reduced by this amount, if a high-viscosity lubricating oil, such as grease, is introduced into the space, the image shifting phenomenon can be substantially eliminated. However, if a high-viscosity lubricating oil is used, the temperature range within which the system can be used is narrow. Accordingly, under severe temperature conditions, a high-viscous lubricating oil cannot be used for eliminating the image shifting phenomenon.