3. Automatic by Memory
Closing the "Memory" switch in either automatic
mode instructs the CPU to store the brightness reading
used in the last exposure. That is, after the shutter has
run, the same speed will be used for all subsequent
exposures regardless of light conditions. Changes in
aperture and ASA will cause comparable changes in
shutter speed. The condition will remain for about one
hour or until the "Clear" switch is closed,
The first Auto-by-Average exposure in Memory mode
will be off-the-film using IC-103's integrator All other
exposures will be controlled by IC-102 in the same
manner as for Auto by Spot.
4. Manual by Average
Manual speeds are selected by turning the speed
selector ring behind the lens. A variable resistor
(TV-VR) under the Shutter Board is turned by a gear
coupled to the speed selector ring. In manual mode the
CPU instructs IC-102 to convert voltage changes from
the TV-VR to binary code along with voltage changes
from brightness, aperture, and ASA.
The CPU calculates proper exposure based upon ASA
and light. The difference between the correct exposure
and that resulting from aperture and speed settings is
indicated by the bar graph in the viewfinder display.
When the two are equal, the end of the bar graph is at the
index mark in the center.
Shutter speed is controlled by IC-102 in the same
manner as Auto by Spot. The current to charge C-104 is
proportional to the manual speed selected.
5. Manual by Spot
Exposure calculations by the CPU are based upon an
average of up to eight spot light readings. The bar graph
indicates the result of this average, Shutter speed control
is the same in all manual modes, since the speed selected
will be used regardless of exposure error.
6. Highlight and Shadow
In any "Spot" mode of operation, pressing the "Hi,
Light" or "Shadow" switches will change the indicated
exposure. "Shadow" shortens exposure time by about 2 1/2
Ev, while "Hi. Light" extends exposure time by 2 Ev. In
auto modes, the actual shutter speed is changed
accordingly,
7- Off-The-Film Flash Exposure Control
When a suitable flash, such as the T-32, is installed on
the camera, operation changes to Off-The-Film Flash
mode. All other modes are cleared by the flash operation.
The CPU gathers light, ASA, and aperture conditions,
then calculates an exposure. If shutter speed is less than
1/60, the flash will be triggered during exposure. Light
from the flash is integrated by IC-I03, the same as Auto
by Average mode. When a proper exposure has been
made, pin 48, IC-102 goes high signaling the flash
through its dedication contact. The flash quenches, the
shutter times out at 1 /60 second, and the system resets for
the next exposure.
After a flash exposure, the strobe LED (viewfinder)
will flicker if exposure was correct. For insufficient light,
the "Under" LCD will flicker and the "Over" LCD flickers
to indicate too much light.
The Sync Switch does not connect directly to the hot
shoe contact. When the opening curtain runs to close the
Sync Switch (X-Contact), pin 59, IC-102 is connected to
the chassis or B+, Pin 50, IC-102, then goes low to turn on
transistor Q-101. Q-101 switches B+ to the gate of an
SCR which turns on to connect the hot shoe contact to
the body. If the auto speed will be faster than 1/60, the
SCR is not switched on and the flash does not fire.
Connecting the positive side of a 2v power source to the
shoe and the negative side to the right-side dedication
contact should set flash mode operation. The red LED
indicating the flash symbol will light in the viewfinder
and shutter speed will be 1/60.
Readout
The LCD viewfinder indication is driven by IC-104
situated under the flex circuit between the prism and
rewind shaft. Capacitor C-302 between pins 3 and 4,
IC-104, acts as a voltage booster (+/- 3v to Vcc) to
provide the necessary 6 volts to operate the LCD, Power
(Vcc) is supplied through transistor Q-301 (PNP) which
is controlled by pin 54, IC-102.
As the CPU determines exposure and mode, that
information is transmitted to IC-104 for decoding and
display, Data transmission takes place about once every
100ms through a series line,
1. Data Sync signal is sent from pin 12, IC-101 to
pin l l
s
IC-104.
2. Timing clock signal is sent from pin 10, IC-101 to
pin 13, IC-104.
3. Series data is sent from pin 11, IC-104 to pin 12,
IC-104.
The display is made up of 41 horizontal rows and back
plates in three vertical rows. Voltage differences of 4.0v
between any horizontal and vertical will result in that
segment being darkened. There is a RAM included inside
IC-104,
There is a backlight lamp for viewing the display in
dark conditions. Pressing the "Lamp" switch sets the
light on for about 8 seconds.
OLYMPUS OM-4, PAGE 3
Summary of Contents for OM-4
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