1. Field of the Invention
The present invention relates to a lens barrel and motion compensation device to compensate for motion causing image blur in a camera, such as a silver salt camera, digital camera, or the like. More particularly, the present invention relates to a motion compensation device having a motion compensation optical system position detection device to detect a position of a motion compensation optical system relative to the lens barrel.
2. Description of the Related Art
A conventional motion compensation device in a camera compensates for blur of an image projected onto an image plane which occurs during photography as a result of vibration of the camera caused by, for example, hand shake of the photographer, or the like. The conventional motion compensation device includes a motion detection unit to detect vibration caused by hand shake or the like, and to output a motion detection signal; a motion compensation optical system, constituting at least a portion of the photographic optical system, which moves relative to the lens barrel in a direction at right angles to the photographic optical axis (referred to as "optical axis" hereinbelow), and causes the optical axis to change to compensate for the motion in the image plane causing image blur; a position detection unit to detect the relative position of the motion compensation optical system with respect to the lens barrel and to output a position detection signal; a calculating unit to calculate an amount of optical axis change necessary to compensate for the motion causing image blur; and a drive unit to drive the motion compensation optical system according to the correction amount.
FIG. 12 illustrates a conventional position detection unit of a motion compensation device. The conventional position detection unit includes a position sensing device (PSD) and a light emitting diode (LED) to detect the position of the motion compensation optical system. More particularly, FIG. 12 is a schematic diagram showing the change of position of a light beam incident on the PSD 54 when the relative position of the LED 51 and the PSD 54 changes. The relationship between the detection position of a light beam incident on the position sensing device (PSD) and the relative position of the light emitting diode (LED) will now be described in detail below with reference to FIG. 12.
As shown in FIG. 12, a distance D1.sub.L (D1.sub.S) is the length between the LED 51 positioned at point A (point B) and a light receiving surface 54a of the PSD 54. The distance D1.sub.L is longer than the distance D1.sub.S. Moreover, a light beam L.sub.L (L.sub.S) emitted from the LED 51 positioned at point A (point B) passes through a slit 52a and is incident on the light receiving surface 54a. A point O is a center position of the detection direction of the PSD 54. The slit position X is the distance the center position of the slit 52a has moved from the point O. The detection position P.sub.L (P.sub.S) is the distance from the point O to the centroid of the light beam L.sub.L (L.sub.S) on the light receiving surface 54a of the PSD 54. A perpendicular N is an imaginary perpendicular to the light receiving surface 54a of the slit plate 52, and is a straight line through the center of the slit 52a. The angle of incidence .alpha..sub.L (.alpha..sub.S) is an angle a light beam incident on the center of the slit 52a makes with the imaginary perpendicular N.
As shown in FIG. 12, the detection position P.sub.L and the detection position P.sub.S do not coincide with each other, and the slit position X does not coincide with the respective detection positions P.sub.L, P.sub.S. In particular, when the positional relationship of the LED 51 and the PSD 54 becomes closer, the angle of incidence .alpha..sub.S becomes greater than the angle of incidence .alpha..sub.L. The error of the detection position P.sub.L and the slit position X is marked. When the slit position X is to be found by a position detection device 54 as shown in FIG. 12, the actual slit position X and the detection positions P.sub.L, P.sub.S do not coincide. Accordingly, as the slit position X becomes larger, the error of the position X and the detection positions P.sub.L, P.sub.S becomes large in proportion to the amount of movement of the slit plate 52. Moreover, the error between the actual slit position X and the detection positions P.sub.L, P.sub.S becomes larger as the distance between the LED 51 and the PSD 54 becomes shorter.
FIG. 13 is a graph showing the centroid position of the light incident on the PSD 54 from the LED 51 with respect to the slit position X. As shown in FIG. 13, the abscissa represents the slit position X and the ordinate represents the result when the centroid position of the light beam incident on the PSD 54 is calculated based on the output signal of the PSD 54. The full line P.sub.L represents the result of calculation of the centroid position when the LED 51 is positioned at the point A shown in FIG. 12; the full line P.sub.S represents the result of calculation of the centroid position when the LED 51 is positioned at the point B shown in FIG. 12. Furthermore, the origin O is the center position of the detection position of the PSD 54; and, the broken line represents the center position of the slit 52a. As shown in FIG. 13, according to the output of the PSD 54, the respective detection positions P.sub.L, P.sub.S and the slit position X do not coincide. Thus, the greater the slit position X with respect to the center of the PSD 54, and the shorter the distance between the LED 51 and the PSD 54, the greater the error of the respective detection points P.sub.L, P.sub.S and the slit position X.
Moreover, the error between the respective detection points P.sub.L, P.sub.S and the slit position X becomes large in proportion to the amount of movement of the slit 52a when the range of the slit position X becomes large, and the light incident from the LED 51 does not impinge on the light receiving surface 54a of the PSD 54. Because of the error between the detection points P.sub.L, P.sub.S and the slit position X, when the slit position X exceeds the effective range of the slit position X, as shown in FIG. 13, the results of calculation of the centroid position based on the output of the PSD 54 with respect to the movement of the slit plate 52 become disproportionately distorted. More particularly, the range of the detection positions P.sub.L, P.sub.S detected by the PSD 54, which is in a proportional relationship with respect to the movement of the slit plate 52, becomes narrow. Further, the range in which position detection is possible becomes limited to the case in which the slit position X is small (i.e., a small amount of movement). The range in which position detection is possible tends to become narrower as the distance between the PSD 54 and the slit plate 52 becomes narrower. When position detection is performed in the above-described manner, a problem occurs in a position detection device which uses a slit plate 52 in that the effective stroke of the slit plate 52 in which position detection is possible changes according to the distance between the PSD 54 and the LED 51.
Furthermore, when the LED 51 and PSD 54 are positioned close to each other, the conventional position detection device receives many effects of the profile of the light emitting device, causing a fall in the linearity of the position calculation results. As a result of the fall in linearity of the position calculation results, when the relative positional relationship of the LED 51 and the PSD 54 is close, the effective range of the slit plate 52 becomes short, and the error between the actual position of the motion compensation lens and the detection positions P.sub.L, P.sub.S detected by the PSD 54 becomes large.
On the other hand, the problems described above with respect to the conventional position detection device can be reduced by making the distance between the LED 51 and the slit plate 52 large. However, when the distance between the LED 51 and the slit plate 52 is large, the position detection device becomes larger in size, resulting in a large size of a motion compensation unit, and, as a result, there is a problem of increased size of a camera. Compactness is strongly demanded in cameras, and therefore it is necessary to make the distance between the LED 51 and PSD 54 small in order achieve a compact design. More particularly, in a collapsible lens barrel type of camera in which the optical system is maintained in a collapsible lens barrel, the space between the LED 51 and the PSD 54 has to be set at the time of a motion compensation operation when the barrel is collapsed, resulting in a limitation on designing a camera of reduced size.
Moreover, when the light source is remote from the light receiving element, the amount of light incident on the light receiving element is of course insufficient. Because of the insufficient light, when performing signal processing, the signal to noise ratio S/N becomes poor, and, as a result, position detection errors occur.
The relationship between the drive amount (shift amount) of the motion compensation optical system and the compensation amount in the image plane of the conventional motion compensation device will now be described below. The drive amount of the motion compensation optical system and the movement amount of the image in the image plane are not necessarily in agreement. In order to correct motion in the image plane, when a drive amount d.sub.s of the motion compensation optical system results in a movement amount (motion compensation amount) d.sub.i of the image in the image plane, the following equation generally holds: EQU d.sub.i =d.sub.s.times.K,
where the constant K is an antivibration compensation coefficient. The antivibration compensation coefficient K is an optically determined numerical value, and for the same lens, or lens group, changes according to the focal distance. In a zoom lens, the value of the antivibration compensation coefficient K generally becomes larger toward the TELE side having a long focal distance, and becomes smaller toward the WIDE side having a shorter focal distance. In order to compensate for motion in the image plane, the motion compensation device shifts the motion compensation optical axis according to the focal distance.
In a zoom lens, assuming that the antivibration compensation coefficient K is 2.0 at the TELE end and 0.8 at the WIDE end, when it is desired to move the optical axis 50 .mu.m in the image plane, the following shift amounts (drive amounts) of the motion compensation optical system are necessary:
TELE end: 50/2.0=25.0 .mu.m PA1 WIDE end: 50/0.8=62.5 .mu.m.
Therefore, in order to compensate for the same motion of 50 .mu.m in the image plane at the TELE end and the WIDE end of the zoom lens, it is necessary to drive the motion compensation optical system by a smaller drive amount at the TELE end than at the WIDE end. As a result, it is necessary to drive the motion compensation optical system with greater accuracy when the lens is at the long focal distance (TELE) side than at the short focal distance (WIDE) side, and it is also necessary to detect the position of the motion compensation optical system with greater accuracy.
However, in the prior art motion compensation devices, drive control of the motion compensation optical system is at the same accuracy at both the TELE side and the WIDE side. Because of this, a problem occurs in that when accurate drive control of the motion compensation optical system is performed on the TELE side, the accuracy of drive control becomes excessive on the WIDE side. On the other hand, when drive control of the motion compensation optical system is matched to the accuracy of the WIDE side, a problem occurs in that the accuracy on the TELE side is insufficient.