There are two almost independent things going on in an OTF OM
The mechanical thing:
Mirror rises
Meter switches on to integrate off the first curtain
slight delay 1
First curtain is released
The electronic thing:
Meter sees enough light integrated and switches off
Causing the Solenoid to drop
slight delay 2
Thereby releasing the Second curtain
The only point of interface is that the mirror NOT the first curtain
switches on the meter. Evidently there are two delays in here which have to
be accurately aligned.
For a highest speed run, the second curtain is released before the first
starts to move. All the metering takes place during delay 1
At any speed the meter is switched off before the second curtain runs.
If it wasn't switched off, what would you like it to be doing? Making tea?
It has no power to control anything except the release time of the second
curtain.
The two delays are the reason why an OM can NEVER flash sync at 1/60th in
auto OTF. The shutter assembly is capable of it, as is the meter, but the
two windows of opportunity overlap for less time than 1//60th requires. The
flash sequences are:
Mirror up
Meter on - sees blackness
Delay 1
First curtain start
First curtain end
X sync contacts closed
Fire flash
Meter integrates
Solenoid drops - back emf is the quench signal
Short delay 2 - this is the one that prevents the 1/60th
Second curtain starts
X Sync contacts open
Second curtain finish
The real benefits of OTF are in flash control and long exposure. For short
exposure it makes not a jot of difference. the only real reason for doing
the metering OTF rather than direct is that direct metering the middle of
the light path would be bloody hard without casting a shadow or using a beam
splitter in front of the mirror. That would just be expense for no benefit.
The patterning on the first curtain is primarily for show. The centre
weighting on an OM2 is defined by the crossing of the two receptors, not by
the pattern. Single receptor OMs do it by simple trigonometry and a single
element lens in front of the SBC.
Julian
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