Polaroid Joshua Vision Service Manual page 26

The microprocessor determines which zone to use, based on IR reflectance data resulting from the strobe
wink. (Although the electronics uses the wink mode to determine whether the subject is near or far from
the camera, the primary purpose of the wink mode is to predict the strobe fire aperture.)
When the Far Focus Lens is needed, it is pivoted into the optical path by Solenoid 2 and latched by the Lens
Latch and Latch Actuator (Figure 1-19).
Figure 1-19. Two-Zone Focus System Components
As exposure begins, the two blades are opened relatively slowly (about 40 ms from closed to fully open), to
maximize the amount of ambient light used in the exposure, and the time the blades are at smaller apertures,
to enhance depth of field. The rack and pinion (inertia link and inertia gear, respectively) of the inertia
system act as an effective multiplier of inertial forces, to allow the blades to begin opening slowly. (The
blades are coupled to the Inertia Link and Gear through the Walking Beam and Inertia Spring.)
Blade position is very accurately monitored and controlled by the Blade Encoder system. The photodiode
encoder pair (on the PC board) and horizontal rows of nine slits in the blades together act as a pulse
generator, as the slits successively overlap during blade opening. Because of the accuracy of the Encoder in
supplying blade position/aperture information, no proJoshua for blade speed adjustment is needed in the
Joshua shutter.
The strobe is fired at an aperture selected by the microprocessor from a lookup table, and quenched 2.5 ms
later or when the IR integrator reaches the strobe contribution value selected by the microprocessor.
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