1. Field of the Invention:
This invention relates to improvements in a double light-blocking mechanism of a shutter.
2. Description of the Related Art:
Heretofore, in the field of focal plane shutters, there is known a shutter having an increased ability to block off light in which the aperture is blocked with a leading blade group and a trailing blade group, that is, a so-called double light-blocking shutter.
In one type of the double light-blocking shutter, just after the termination of the exposure, while the trailing blade group is covering the aperture, the leading blade group is moved to a position of covering the aperture, and the double light-blocking is cancelled when the film is wound.
Cameras in recent years generally have motor drive device incorporated therein, and the termination of the exposure is immediately followed by the film winding operation. Hence, the use of this type of double light-blocking shutter in such a camera lowers the significance of blocking light, as the double light-blocking is cancelled soon after the exposure has terminated.
Another type of double light-blocking shutter is adapted to the so-called motor drive device built-in camera, and is operated in such a manner that the termination of the exposure is followed by a charging operation which permits the trailing blade group to remain in the position of covering the aperture. From the time when the leading blade group gets fully charged, that is, when the charging is completed with the leading blade group, too, in the state of covering the aperture, actuation of the next camera release is waited for. When the camera release is actuated, the trailing blade group which has so far been held in the position of covering the aperture is then released. It is after the retracting of the trailing blade group from the aperture is completed that the leading blade group and the trailing blade group run at a predetermined timing to make an exposure.
However, even this latter type of shutter, when used in some photographic situations such as that described below, has a drawback.
That is, in such a camera, the interval from the moment at which the photographer has actuated a camera release to the moment at which the exposure starts is called the "release time lag". The shorther the release time lag, the closer the photograph can be taken to the moment the photographer has intended to shoot. Particularly, in sports photography or racing photography, the release time lag becomes a large obstacle in catching the decisive moment. With the above-described latter type of shutter, however, because the trailing blade group, after the release is actuated, is set to the retracted position, the sum of the time necessary for the trailing blade group to retract and the time it takes until the retracted trailing blade group becomes stable is unavoidably added to the release time lag. Therefore, such a device is unsuited to catch the decisive moment. Incidentally, to shorten the release time lag, it has been known that the reflex mirror is fixed so that the time it takes to move the mirror from the viewing position to the non-viewing position is romoved. In a camera of this kind, it is virtually impossible to incorporate the above-described latter type shutter.
Accordingly, it may be considered that the trailing blade group is made to retract before the actuation of the release in order to avoid the addition of the above-described time of retracting the trailing blade group to the release time lag. However, if, after the setting of the trailing blade group to the retracted position has been performed before the shutter release, the actuation of the camera release is interrupted, (it happens that the photographer, though having intended to shoot, gives it up because the scene has changed), the shutter will subsequently be operated to establish the double light-blocking again. For this purpose, a running completion state of the trailing blade group must be brought about. That is, the trailing blade group only is obliged to run as if another usual camera release were actuated, so that the running completion state of the trailing blade group is regained. And, after that, the charging operation must once more be carried out. Thus, the shutter is returned to the double light-blocking state. Such an action, because of its having, despite the interruption of a shooting prior to making of the exposure, to allow one of the blade groups to run, is accompanied with production of a sound as if the film were exposed after the shutter button is released from the half depression, so that the photographer is deceived. Further, because it is only the trailing blade group that has to run even when no exposure is made, as this is very often repeated, there will arise a possibility of occurrence of an accident due to the premature damage of the trailing blade group.