Iris drive mechanism

Abstract: The iris drive mechanism (1) has two diaphragms (13,15) which are opened and/or closed by a first lever (9) rotated by an ideal gyrator (IG) meter (5) and a second lever (17) in which an engagement hole (17A) with which an output shaft (5A) of the IG meter is engaged, and an engagement pin (17B) engaged into an oblong hole (9B) formed through the first lever, and a rotary shaft (11) to which the first lever is rotatably supported. The first lever (9) is rotated through the second lever (17) by the IG meter. Therefore, when an aperture is small, a resolution of the F number can be increased, and this can enhance the automatic exposure control performance.

Description: BACKGROUND OF THE INVENTION Field of the Invention The present invention relates to an iris drive mechanism which is suitably assembled into a lens block. Description of the Prior Art In an iris for adjusting exposure, two diaphragms are operated by an ideal gyrator (IG) meter (motor) and the moving distances of these diaphragms are substantially proportional to a rotational angle of the IG meter. Accordingly, even though the aperture is small and the rotational angle of the IG meter is small, the diaphragms are moved by the distances equal to those when the aperture is large, which considerably changes the F-number. As a consequence, resolution of the F-number when the aperture is small is lowered and an automatic exposure (AE) control performance is deteriorated. US-A-4 884 091 describes iris drive mechanisms designed to enable the diaphragm blades defining the aperture to move with little resistance. One of the described mechanisms has a link portion consisting of two levers for controlling displacement of the diaphragm blades. A first one of the levers is pivoted at its midpoint and has pins at its ends which engage in holes in the diaphragm blades. The second of the levers is mounted at one end on a motor drive shaft and has a driving pin at its other end engaging a long curved slot in the first lever, that slot being between the pivot point of the first lever and one of the pins driving a diaphragm blade. As the motor drive shaft rotates, the driving pin travels along the long curved slot in the first lever from a position near the end of the first lever to a position close to the pivot point of the first lever. OBJECTS AND SUMMARY OF THE INVENTION Therefore, it is an object of the present invention to provide an improved iris drive mechanism in which the aforesaid shortcomings and disadvantages encountered with the prior art can be eliminated. More specifically, it is an object of the present invention to provide an iris drive mechanism in which a resolution of F-number is high even in a small aperture. Another object of the present invention is to provide an iris drive mechanism in which an automatic exposure (AE) control performance can be improved. Still another object of the present invention is to provide an iris drive mechanism in which a space of a lens can be effectively utilized even when the lens is miniaturized. The present invention provides an iris drive mechanism having first and second diaphragm blades which are slideable in first and second opposed directions and define therebetween an aperture, and comprising: a housing; a first lever which is pivotally supported on the housing so as to be pivotal about a first axis which passes through substantially the midpoint of said first lever; first and second pins which are mounted on said first lever proximate the first and second ends of said first lever respectively, and at substantially equal distances from the first axis, said first and second pins being received in respective openings formed in said first and second diaphragm blades; an elongate opening formed in said first lever; a motor having a drive shaft; and a second lever which is mounted on and rotates with said drive shaft, said second lever being relatively short compared with the first lever and having a third pin which is received in said elongate opening of said first lever; characterized in that the elongate opening formed in said first lever is located between said first pin and said first end of the first lever so that, in use, the first and second levers co-operate to displace the first and second diaphragm blades: a) relatively slowly at a first a variable rate of displacement relative to the angular rotation of the second lever, through an initial stage wherein the aperture defined by said first and second diaphragm blades changes from a minimal opening degree to a first predetermined opening degree; b) relatively quickly at a second variable rate of displacement relative to the angular rotation of the second lever, through an intermediate stage wherein the aperture defined by said first and second diaphragm blades opens from the first predetermined opening degree to a second, larger predetermined opening degree; and c) relatively slowly at said first variable rate of displacement relative to the angular rotation of the second lever, through a final stage wherein the aperture defined by said first and second diaphragm blades changes from said second predetermined opening degree to a maximum opening degree. Therefore, in the mechanisms according to the invention, when an aperture is small, a resolution of the F-number can be increased, which can enhance the AE control performance. The present invention still further provides lens systems and lens apparatus incorporating drive mechanisms as specified above. BRIEF DESCRIPTION OF THE DRAWINGS A better understanding of other objects, features and advantages of the present invention can be gained from a consideration of the following detailed description of an illustrative embodiment thereof, in conjunction with the figures of the accompanying drawings, in which: FIG. 1 is a perspective view illustrating an embodiment of an iris drive mechanism according to the present invention in an exploded fashion; FIG. 2 is a schematic diagram used to explain operation in the assembly of the iris drive mechanism according to the present invention; FIG. 3 is a schematic diagram showing the condition such that the iris mechanism is assembled into a lens system; and FIG. 4 is a graph used to explain the correlation between the rotational angle of a lever and the moving distance of a diaphragm. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT An iris drive mechanism according to a preferred embodiment of the present invention will hereinafter be described with reference to the drawings. FIG. 1 of the accompanying drawings shows an overall arrangement of an iris drive mechanism 1 according to the present invention. As shown in FIG. 1, the iris drive mechanism 1 comprises a frame 3 through which an aperture portion 3A for providing an optical path is formed and an IG meter 5 secured to the frame 3. A Hall element 7 is mounted on the IG meter 5. The Hall element 7 is used to detect the opening and/or closing amount of the iris. The frame 3 has a support shaft 11 of a rod configuration projected thereon to which a first lever 9 is rotatably supported. In association with the support shaft 11, a cylindrical portion 9A into which the support shaft 11 is engaged with a pressure is formed on the first lever 9. As will be understood from FIG. 2, the first lever 9 is shaped as a long plate, and engagement pins 9B, 9C which are respectively engaged with oblong holes 13A, 15A formed through two diaphragm blades 13, 15 which open and/or close the aperture portion 3A are projected on the first lever 9 in both sides of the cylindrical portion 9A in the longitudinal direction. An oblong hole 9D is formed through the first lever 9 at its end portion at which the engagement pin 9B is formed. An engagement pin 17B, which is engaged with the oblong hole 9D, is projected on a second lever 17 of a long plate configuration. A through-hole 17A into which an output shaft 5A of the IG meter 5 is engaged with a pressure is formed through the second lever 17. A frame 19, which serves as a cover member, is attached to the frame 3. The frame 19 has formed therethrough a through-hole 19D to which a base side projection portion 9a of the cylindrical portion 9A is engaged, opening recesses 19B, 19C of arc configurations into which the engagement pins 9B, 9C are inserted so as to become movable about the shaft of the rotary shaft 11 and an aperture portion 19A corresponding to the aperture portion 3A. As will be understood from FIG. 3, the iris drive mechanism 1 is assembled into a lens block at its predetermined position. According to the above-mentioned arrangement, as shown in FIG. 2, when the IG meter 5 is driven, the second lever 17 is rotated and the rotational operation of the second lever 17 is transmitted through the engagement pin 17B and the oblong hole 9D to the first lever 9 (link mechanism). Accordingly, the first lever 9 is rotated about the rotary shaft 11. Then, the rotation of the first lever 9 is transmitted to the one diaphragm blade 13 through the engagement pin 9B and the oblong hole 13A and also transmitted to the other diaphragm blade 15 through the engagement pin 9C and the oblong hole 15A with the result that the diaphragm blades 13, 15 are moved in the directions shown by arrows X and Y in FIG. 2. Thus, when the second lever 17 is located on the opposite to L side shown by a broken line in FIG. 2, even if the rotational angle of the second lever 17 is a constant angle, the moving amount of the diaphragm blades 13, 15 is small as shown by a curve P of FIG. 4. When the rotational angle of the second lever 17 is 5 degrees on the small aperture side, for example, the moving amount of the diaphragm blades 13, 15 becomes 0.2 mm, which is half of the amount (0.4 mm) of the conventional iris drive mechanism (see curve Q in FIG. 4). The iris opening and/or closing amount detected by the Hall element 7 is converted from an angle into a voltage and then output. In this case, an output voltage becomes high as the angle is increased. According to this embodiment, even when the moving amount of the diaphragm blades 13, 15 is small, the rotational angle is increased and therefore the output voltage from the Hall element 7 is increased. Therefore, the resolution of the iris opening and/or closing amount when the cutput voltage is converted in the form of analog to digital signal can be increased considerably. As a consequence, the AE control performance at small apertures is enhanced and the opening and/or closing operation of the iris can be controlled satisfactorily. As will be understood from the above-mentioned description, according to the iris drive mechanism of the present invention, the rotary shaft to which the first lever is rotatably supported is provided separately and the first lever is rotated by the IG meter via the second lever. Since the first and second levers are linked by means of the oblong holes and engagement pins, the two diaphragm blades are opened and/or closed by the link mechanism. Consequently, the moving distance of the diaphragm blades relative to the rotational angle of the IG meter is reduced and therefore, when the F number is relatively small, resolution of the F number is increased particularly at small apertures which can improve the AE control performance. Furthermore, since the link mechanism is employed, a space of the lens can be effectively utilized even when the lens is miniaturized.

Claims: An iris drive mechanism (1) having first and second diaphragm blades (13,15) which are slideable in first and second opposed directions (x, y) and define therebetween an aperture, and comprising: a housing (3); a first lever (9) which is pivottally supported on the housing (3) so as to be pivotal about a first axis (11) which passes through substantially the midpoint of said first lever; first and second pins (9B,9C) which are mounted on said first lever proximate the first and second ends of said first lever, respectively, and at subtsantially equal distances from the first axis (11), said first and second pins being received in respective openings (13A,15A) formed in said first and second diaphragm blades; an elongate opening (9D) formed in said first lever; a motor (5) having a drive shaft (5A); and a second lever (17) which is mounted on and rotates with said drive shaft (5A), said second lever being relatively short compared with the first lever and having a third pin (17B) which is received in said elongate opening (9D) of said first lever; characterised in that the elongate opening formed in said first lever is located between said first pin and said first end of the first lever so that, in use, the first and second levers co-operate to displace the first and second diaphragm blades: a) relatively slowly at a first variable rate of displacement relative to the angular rotation of the second lever, through an initial stage wherein the aperture defined by said first and second diaphragm blades changes from a minimal opening degree to a first predetermined opening degree; b) relatively quickly at a second variable rate of displacement relative to the angular rotation of the second lever, through an intermediate stage wherein the aperture defined by said first and second diaphragm blades opens from the first predetermined opening degree to a second, larger predetermined opening degree; and c) relatively slowly at said first variable rate of displacement relative to the angular rotation of the second lever, through a final stage wherein the aperture defined by said first and second diaphragm blades changes from said second predetermined opening degree to a maximum opening degree. The iris drive mechanism according to claim 1, wherein the first and second levers are arranged to be in line with each other at a time during said intermediate stage of displacement of the diaphragm blades and to be inclined relative to one another during said initial and final stages. The iris drive mechanism according to claim 1 or 2, further comprising a Hall element (7) disposed on the motor (5) to detect rotation of the motor drive shaft (5A) and output a signal indicative of the opening and/or closing amount of the iris. A lens system comprising an iris drive mechanism according to claim 1, 2 or 3. A lens apparatus comprising the lens system of claim 4 and comprising means for detecting the brightness of an object image from an optical image formed by said lens system and for controlling the opening amount of the iris by controlling the motor (5).

Inventors: MORO SHUUJI, NISHIDA YASUO, OHSHIMA EIJI, TADA KANEHIRO, MORO, SHUUJI, NISHIDA, YASUO, OHSHIMA, EIJI, TADA, KANEHIRO
Applicants: SONY CORP, SONY CORPORATION
Application year: 1992
Main classification: G03B9
IPC codes: G03B9