PROGRAMMED SHUTTER AND APPARATUS FOR PREVENTING FOREIGN MATTER ON LENS FROM BEING PHOTOGRAPHED IN A LENS SHUTTER TYPE OF CAMERA

Abstract: A programmed shutter and an apparatus are provided for preventing material, such as dust, on camera lenses from being photographed. The shutter can be used alone, or can be used togther with a pivotable neutral density filter which can be selectively positioned along the optical axis of a lens system having a zoom lens or at least two focal length positions.

Description: The present invention generally relates to a camera as described in the generic part of the main claim. A camera of this kind is disclosed in US-A- 4 320 944. The exposure control in this camera, by controlling a combination of F-numbers and shutter speeds corresponding to said F-numbers, effects a reduction of hand blur. In conventional cameras, even when foreign material is located on a camera lens, there is no possibility that the foreign matter will be photographed in the picture, provided that such matter comprises fine material. This is because the area which is occupied by the fine foreign matter is extremely small with respect to the rays of light which define the picture (i.e., the image), such that any image of such foreign matter will be dimmed due to the diffraction of light. However, in a zoom lens system in which the position of the diaphragm of the camera varies, the exit pupil of the camera closely approaches the film plane at one of the focal length extremities of the camera. Accordingly, at a small value of the diaphragm, in which the diameter of the exit pupil is below a predetermined value, i.e., when the lens system closely approaches the film plane, foreign matter which is located on the rearmost lens adjacent the center of the lens will be photographed in the picture. Figs. 4A, 4B, and 4C all show how foreign matter on the rear surface of a lens is photographed when a picture is taken. Specifically, Figs. 4A and 4B illustrate a lens system L which closely approaches the film plane F, when the lens system is at its minimum focal length. In Fig. 4A, foreign matter cannot be photographed when taking a picture, and, in Fig. 4B, foreign matter will be photographed as a black point in the picture when the photograph is taken. Assuming that foreign matter Q, having a diameter of d , is applied to the center portion of the rearmost lens of lens system L, when exit pupil P of lens system L is small and when the distance between the rearmost lens and film plane F is also small, the quantity of light on the center of film plane F will be decreased by the presence of the foreign matter thereon. Such a decreased quantity of light will effect a phenomenon in which the foreign matter is photographed. When the diameter of the foreign matter is 0.3 mm , which is the most likely diameter of material which will be located on lens system L, the minimum exit pupil P which will not recognize such foreign matter is, e.g., F13.5 (see Fig. 4A). When the exit pupil P is smaller than F13.5, e.g., when the exit pupil P is F22, foreign matter can be clearly recognized and will be photographed, as shown in Fig. 4B. To the contrary, when lens system L moves forwardly towards a maximum focal length, as shown in Fig. 4C, no problem of decreased light resulting from the presence of foreign matter Q will occur, even if exit pupil P is F22, which is identical to the diaphragm aperture at the minimum focal length of the camera. Specifically, when the distance between the lens system and film plane F is above a predetermined value, no foreign matter can be photographed. Recently, with zoom lens being used having increased magnifications, and with minimization of the size of the lens system, it has occurred that the rearmost position of the lens system and of the exit pupil more closely approach the film plane. This results in an increased possibility that foreign matter will be photographed in pictures being taken with such cameras. In another known camera, in which a focus switching lens moves in and out of the optical path of the lens system in order to selectively provide maximum and minimal focal lengths for the lens system, such a problem (photographing of dirt and other particles) also occurs. The problem of the present invention is, therefore, to provide a camera having a programmed shutter which will eliminate the drawbacks caused by undesireable foreign matter being photographed in a programmed lens shutter type of camera having a zoom lens. In accordance with the present invention, the problem is solved by the features given in the main claim. Advantageous further developments are described in the subclaims. According to one aspect of the present invention, a specific range of diaphragm values in which foreign matter can be photographed will not be used. Theoretically, it is possible in a programmed shutter to open the shutter when the diaphragm achieves a specific value in order to increase the shutter speed and to thereby take a picture at the same EV (i.e., exposure value). However, when using a programmed shutter, there is an upper limit for the shutter speed, and, accordingly, the use of a solution to the problem which involves increases in camera shutter speed is impracticable and unadvisable. Generally speaking, in a variable power lens system, the mechanical diaphragm aperture can be varied so as to have a plurality of F values which correspond to, e.g., the minimum diaphragm aperture at the maximum focal length and to the maximum diaphragm aperture at the minimum focal length, in accordance with the change in the focal length of the lens system, respectively. In other words, when magnification is decreased towards the minimum focal length of the lens after setting the F number at the longest focal length, the F number will become relatively small, i.e., more light will be transmitted through the diaphragm aperture. It should be noted that even if the relative F number becomes small, it will not be possible to solve the problem of photographing foreign matter at a short focal length, as long as no change in the mechanical diaphragm occurs. To solve this problem, in accordance with the present invention, the diaphragm aperture can be increased, at a short focal length in which the lens system closely approaches the film plane, in order that no foreign matter will be photographed; this can be achieved by increasing the diaphragm aperture beyond the F number which corresponds to the minimum diaphragm diameter which is set at the long focal length of the lens system. In accordance with one aspect of the invention, a lens shutter type of camera is provided having a zoom lens system or a variable power lens system with a maximum and a minimum focal length; in such cameras, the same mechanical diaphragm aperture will vary so as to be identical to the F number which corresponds to the small diaphragm aperture on the long focal length side and to the F number which corresponds to the large diaphragm aperture on the short focal length side, respectively. The diaphragm aperture changes in accordance with a change in focal lengths. A programmed shutter in accordance with the present invention is used in such structure; in this shutter, it is assumed that the F number of the short focal length side, which is obtained when the lens system focal length is changed, is set at Fa, which is larger than Fb, the minimum aperture value on the long focal length side of the lens system. In this manner, the programmed shutter program diagram will be determined as a combination of the F numbers and the shutter speed. In another aspect, a programmed shutter is used in a lens shutter type of camera as specified in the main claim. The lens system of the camera can be a variable power lens system comprising a zoom lens system, wherein variations in the focal length and F numbers of the zoom lens system are represented by a focal length-F-number program diagram. The diaphragm aperture includes a first area which is smaller than an area, on the focal length-F-number diagram, defined by a line which connects the minimum diaphragm value at the longest focal length of the zoom lens system and the minimum diaphragm value at a shorter focal length of the zoom lens system, the programmed shutter including means for non-use (or ignoring) the first area. The minimum diaphragm value at the shorter focal length side of the lens system is identical to the minimum diaphragm value at the shortest focal length side of the zoom lens system. The minimum diaphragm value at the shorter focal length side of the zoom lens system is identical to the minimum diaphragm value at an intermediate focal length of the zoom lens system. The camera can have a zoom lens system, or a variable power lens system having at least two focal lengths. A device is adapted for use in a lens shutter type of camera having either a zoom lens system or a variable power lens system having at least two focal lengths, the camera further including a mechanical diaphragm aperture with F numbers on long and short focal length sides of the lens system, which F numbers change in accordance with change in the focal length of the lens system, the device including means for preventing the camera from photographing undesirable material which is present on the lens system. The preventing means comprises means for controlling exposure in accordance with the F numbers and corresponding shutter speed, the device having a minimum F number Fa, on the short focal length side of the lens system. Fa is larger than the minimum F number Fb of the diaphgram aperture when the lens system closely approaches a camera film plane. The lens system can be a zoom lens, the device further comprising a program diagram for controlling a programmed shutter, the program diagram comprising an F number Fd at an intermediate focal length area. The program diagram includes eight focal length steps, the shutter comprising means for determining shutter speeds and diaphragm F numbers in accordance with the exposure value of a subject to be photographed. The minimum diaphragm value F is F 13.5 at a WIDE position of the lens system and F 42 at a TELE position of the lens system. The diagram comprises means for excluding said shutter from operating in all areas in which foreign matter on the lens can be photographed. The control circuit further comprises means for directing the program circuit to control exposure in accordance with a shifted program diagram which has been shifted to decrease exposure values from those exposure values on a first program diagram. The device further comprises a shutter control device, a code plate with lens system focal length data, a light measuring device, and a film sensitivity measuring device, the shutter control device comprising means for outputting a driving signal for driving a shutter in accordance with receipt of film sensitivity data, brightness data, and focal length data. The above-identified objects, features and advantages of the present invention will be more fully described with respect to the accompanying drawings, in which like reference numerals are used to illustrate similar parts throughout the several view, and wherein: Fig. 1 is a focal length-F number program diagram illustrating a relationship between the focal length of the zoom lens and the F number, representing a basic concept of the present invention; Fig. 2 is one example of a program diagram of a programmed shutter used in accordance with the present invention; Fig. 3 is a program diagram of a programmed shutter used in accordance with the present invention in which no problem of foreign matter being photographed occurs; and Figs. 4A, 4B and 4C, respectively, are schematic views illustrating the optical paths and distribution of light in combination with specific diaphragm values and different positions of the lens system. Fig. 1 is a focal length-F number diagram of a zoom lens system. In this Fig., the F number varies in accordance with changes in the focal length from a wide angle extremity (i.e., WIDE) position, to a telephoto extremity (i.e., TELE) position. Since the F number is established by the ratio between the focal length and the aperture diaphragm (i.e., the focal length/aperture diaphragm), the F number will vary in accordance with the change in the focal length, as noted above, when the mechanical aperture does not change. Specifically, in the first embodiment of the invention which is illustrated in Fig. 1, the maximum F number F4 and the minimum F number F22 at the short focal length (35mm) correspond to a maximum F number F8 and the minimum F number F45 at the long focal length (70mm), respectively. The programmed shutter of the present invention is capable of preventing foreign matter from being photographed by controlling exposure in accordance with a diaphragm aperture which is larger than the minimum F number F22 and a corresponding shutter speed, on the short focal length side in which the lens system closely approaches the film plane. Specifically, in the example referred to above, when magnification is changed towards the short focal length after the minimum F number at the long focal length has been set to be F45, the F number will become F22. Assuming that F22 is Fb, the F number Fb is not used in the actual program of the programmed shutter. In other words, Fa (e.g., F=13.5) which is larger than Fb is determined to be the minimum F number at the short focal length, such that exposure can be controlled by such a minimum F number and the shutter speed which corresponds thereto. It is, thus, not practical to use minimum F numbers in areas a and b of the short focal length side in Fig. 1, in view of the distance which exists between the zoom lens and the film plane, which distance depends upon the variation in magnification of the zoom lens. Area a is determined as set forth hereinafter. It is assumed that the focal length range of the zoom lens system is divided into two areas, i.e., an area L which defines an area from the maximum (i.e., longest) focal length to an intermediate focal length, and area S, which covers an area from the intermediate focal length to the minimum (i.e., shortest) focal length. No change will be made in the program diagram for area L; to the contrary, for area S, the program diagram will be modified as set forth herein: specifically, assuming that the F number at the shortest focal length is Fb when the focal length varies and while the minimum diaphragm aperture Fc is maintained in the intermediate area, an F-value Fa, larger than Fb, will be selected as the minimum diaphragm value at the minimum focal length. The area a is thus defined by connecting Fa and Fc on the program diagram. Further, area b is defined by connecting F numbers Fa and Fo, which represents the minimum diaphragm aperture at the longest focal length of the zoom lens of the program diagram. As a result, in the intermediate area between the longest focal length and the shortest focal length of the zoom lens, where the focal length of the zoom lens system varies, the minimum diaphragm aperture is maintained at the longest focal length Fc; and an F number Fd, corresponding to a diaphragm aperture larger than Fc, will be set as the minimum diaphragm value in the intermediate area. A detailed discussion will follow, with specific reference to Figs. 2 and 3, which illustrate program diagrams of the programmed shutter of the present invention. The numerical values provided in Figs. 2 and 3 are slightly different from those in Fig. 1. Fig. 2 illustrates a program diagram of a programmed shutter according to the present invention, and Fig. 3 illustrates a program diagram in which no problem resulting from foreign matter being photographed will occur. In Fig. 3, the program diagram varies from T0′ to T7′ (i.e., eight steps), in accordance with the change of the F number, which varies in accordance with the change of the focal length from the short focal length (i.e., WIDE: 36mm) to the long focal length (i.e., TELE: 68mm). The programmed shutter determines the shutter speed and the diaphragm value, in accordance with a predetermined program when the EV value (i.e., the exposure value) of a subject to be photographed is well known. When there is no problem that foreign matter will be photographed, as noted above, it is possible to control the exposure from EV9 (WIDE extremity: T0′) to EV18.75 (TELE extremity: T7′). The minimum diaphragm apertures at the two extremities are, therefore, F22 and F42, respectively. In the above-noted example, when the diaphragm aperture at WIDE extremity T0′ is below F13.5, or the diaphragm aperture at TELE extremity T7′ is below F42 (corresponding to F22 at the WIDE extremity), a problem involving the potential photographing of foreign matter will arise. In order to solve such a problem, in the present invention, an area or line S, which is defined by connecting the two noted points (i.e., F13.5 at WIDE T0′ and F42 at TELE T7′) in Fig. 3, will be deemed to be an area in which foreign matter can be photographed, so that a modified program diagram T0-T7, which does not include the area S, will (need to) be established, as illustrated in Fig. 2. It should be noted that area S corresponds to area b in Fig. 1. Area S is determined by the maximum EV which provides a shutter speed identical to a reference shutter speed of about 1/190 seconds which is obtained by the representation T0=13.5 on the diagram, for the area S1 of T0∼T3 on the WIDE extremity side. Accordingly, the minimum diaphragm value decreases in a direction from T1 towards T2 and T3 (and the F number increases). Looking at T4∼T7 on the TELE extremity side, the maximum EV (the minimum diaphragm value F32 at 17.75 being located at T7) is determined on the basis of a reference EV which provides a shutter speed of about 1/220 seconds. This takes into account the associated EV on the WIDE extremity side as well. On the TELE side, when the diaphragm value is about F42 (corresponding to F22 at the WIDE extremity), the possibility that foreign matter will be photographed is quite small, because the rearwardmost lens of the zoom lens system usually moves forward on the TELE side. However, F42 is generally not used. Additionally, it is a quite important consideration to balance the EV between the TELE side and the WIDE side. Because of this balance, it is also preferred to make the maximum EV smaller along the TELE side. It should be appreciated that, only for the purposes of preventing foreign matter from being photographed, it is theoretically unnecessary to limit the maximum EV on the TELE side. Specifically, it is possible to set a maximum EV at a value which is as large as possible. As noted above, however, in view of the desirability of balancing the EV between the WIDE side and the TELE side, it is preferable to establish an area or section S′, which is obtained by connecting two points, i.e., T0=F13.5 (about 1/190 seconds) and T7=F42 (about 1/250 seconds) as an area in which foreign matter can be photographed in the picture, as illustrated in Fig. 2. However, this way of determining section S′ results in relatively complex programming of the program shutter, and thus no significant increase in the precision of exposure control can be expected to result. The above results in the minimum diaphragm aperture on the WIDE side being identical to the diaphragm aperture Fa (F13.5), which is smaller than the diaphragm aperture F22 (or F16, Fb), when zooming is effected towards the WIDE side after the minimum diaphragm aperture F22 (or F32) is set on the TELE side. Although the above discussion has been mainly directed to a zoom lens system having eight focal length steps, it should be easily understood that the number of steps for setting area S can be increased in accordance with an increase in the number of focal length steps. It should be noted that in a two focal length type of camera, the program diagram can be modified as noted above only when the switching lens is brought into the optical path of the lens system. Programmed shutters per se are well known. For example, one well known programmed shutter is illustrated in Japanese Printed Publication 60-235,126, the disclosure of which is expressly incorporated by reference herein. As noted from the above discussion, and in accordance with the present invention, the programmed shutter can be used to prevent foreign matter from being photographed in a picture. Consequently, when using such a programmed shutter in a lens shutter type of camera having either a zoom lens system or a variable power lens system, the freedom of design of the optical system can be increased, thereby resulting in a compact and effective zoom lens. Essentially, the present invention is directed to the avoidance of photographing dirt and other fine particles which are on the lens in a zoom lens ( or dual focal length) camera. When the camera is in its wide angle, short focal length position, there is a problem with such particles. This problem can be avoided by opening the diaphragm aperture; in such a situation, it is necessary to increase the speed of the shutter opening in order to compensate for the increased amount of light which is being permitted into the picture. In accordance with the basic process of the present invention, the aperture is automatically opened wider at the wide angle setting, and there is a need to operate the shutter automatically at higher speeds in order to compensate for overexposure resulting from such a wider aperture. For a given light measurement and focal length setting, the programmed shutter automatically sets the aperture opening and the shutter speed.

Claims: A camera having a programmed shutter, a lens system (L) with at least two different focal lengths and a variable mechanical diaphragm aperture means having F numbers, each F number corresponding to a larger diaphragm aperture for the longest focal length of said lens system (L), and to a smaller diaphragm aperture for the shortest focal length, in accordance with the changing focal length of said lens system (L), said shutter comprising means for controlling exposure in said camera by controlling a combination of said F numbers and shutter speeds corresponding to said F numbers, characterised in that, said shutter is constructed such that said diaphragm aperture means has a largest F number Fa for the shortest focal length, Fa being smaller than an F number Fb, said F number Fb being the F number obtained when the focal length of said lens system (L) is changed from the longest to the shortest focal length while maintaining the smallest diaphragm aperture of the longest focal length, said largest value Fa of the shortest focal length representing a greater diaphragm aperture than Fb. A camera in accordance with claim 1, wherein said lens system (L) is a variable power lens system comprising a zoom lens system. A camera in accordance with claim 2, wherein said shutter includes means for ignoring, for each focal length, F values which are larger than the F value on a line, as seen on a focal length-F-number diagram, said line being defined by the fact that it connects a point defined by the F number Fa at the shortest focal length and a point defined by an intermediate focal length, between the longest and the shortest focal lengths, and an F number, at said intermediate focal length, for an aperture of the same size as the smallest diaphragm aperture of the longest focal length. A camera in accordance with claim 2 or 3, wherein said smallest diaphragm aperture at the shortest focal length is identical to the smallest diaphragm aperture at an intermediate focal length.

Inventors: KOBAYASHI, TAKEO, KOHMOTO, SHINSUKE, KONDOH, SHIGERU, NISHIO, ETSUROH
Applicants: ASAHI KOGAKU KOGYO KABUSHIKI KAISHA
Application year: 1987
Main classification: G03B9
IPC codes: G03B9