a. Field of the Invention:
The present invention relates to focal plane shutters, and more particularly, to improvements in a high speed exposure time control means adapted to focal plane shutters.
B. Description of the Prior Art:
Generally an exposure time control means for focal plane shutters comprises a high speed exposure time controlling mechanism for forming an exposure slit with shutter blades or shutter curtains and a low speed exposure time controlling mechanism for controlling the fully opened time of the shutter blades or shutter curtains.
Now, according to the conventional high speed exposure time controlling mechanism, in the relations of the front blade (curtain) actuating member, the rear blade (curtain) actuating member and the high speed exposure time controlling member releasing in turn both actuating members in the cocked state by its displacing motion, the time of releasing the front blade (curtain) actuating member is so set as to be always constant and the time of releasing the rear blade (curtain) actuating member is so made as to be varied with the setting of the exposure time.
That is to say, in FIG. 1, the reference symbol .gamma. shows an example of a variation characteristic of the time to rotation angle of a high speed exposure time controlling member (actuating cam). The angular velocity of the rotation of the actuating cam started by the shutter release is small in the initial period but becomes gradually larger.
The specific displacement position a of the actuating cam is made the front blade (curtain) actuating member releasing position, the positions b, c and d are made the rear blade (curtain) actuating member releasing positions and, for example, the time t.sub.3 shows 4 ms (1/250 sec.), the time t.sub.2 shows t.sub.3 /2 or 2 ms (1/500 sec.) and the time t.sub.1 shows t.sub.2 /2 or 1 ms (1/1000 sec.).
On the other hand, in case the rotation angle .theta..sub.3 of the actuating cam corresponding to the time t.sub.3 is made a basis, the rotation angle .theta..sub.2 will not be .theta..sub.3 /2 but .theta..sub.2 &lt;.theta..sub.3 /2. Needless to say, the rotation angle .theta..sub.1 will not be .theta..sub.2 /2 but .theta..sub.1 &lt;.theta..sub.2 /2 and .theta..sub.1 &lt;.theta..sub.3 /4.
This means that the displacement of the actuating cam from the time point of release of the front blade (curtain) actuating member at the time of the maximum speed, for example, of 1/1000 sec. to the time point of release of the rear blade (curtain) actuating member is small and has been a cause of making the maximum speed exposure time unstable in the conventional means.