I did some further experiments today.
I cut a remote-release extension cable in two (ouch) and used it and a
disassembled 60T3 to connect two EOS620s in parallel. If done properly, it
works perfectly. Either camera's main shutter release button, or a 60T3
connected to both (or other switch inserted into the cable that runs between
them) releases both cameras sufficiently synchronously. I determined the
degree of synchrony with an "optical AND" - I put one body in front of the
other, with no lenses on them and the backs open, and a bright target in front
of the forward body.
Important details:
To use a flash with a high shutter speed, or to take pictures of moving
subjects, you must connect all three contacts together, not just the common and
shutter-release (full press) contacts. Either that, or you will have to press
both cameras' shutter releases halfway to make both cameras "active". If you
fire the cameras by just using the full-press contact, enough variable delay is
added in both cameras that there is full overlap between the shutters only at
1/30s and longer exposures, indicating considerable variability in shutter lag.
If you press the release rapidly, the same thing happens. You need to make the
half-pressed contact for a moment, so both cameras are alert, and then complete
the press to fire the cameras. If you do this, there is full overlap between
the shutters (allowing use of a flash) at 1/125 and longer exposures. At
1/250, there is full overlap only about 80% of the time. This variability
occurs even with the cameras on manual, and even though they are identical
models. The 620 has a fast shutter; with other cameras, my guess is you will
need to use 1/60 or longer.
I also measured the current that flows between the bodies. My meter measures
down to 0.1uA, so <0.1uA just means less than my meter could measure.
Shutter release (full press) contacts:
<0.1uA flows between the cameras without the button pressed. With one camera's
button pressed, 700uA flows, as expected, since one camera is shorting the
contact detection signal presented across the contacts of the other camera.
Half-press contact:
<0.1uA flows between cameras with the cameras in the "idle" state. 0.2uA flows
with the cameras "active". 94uA flows with button pressed (again as expected).
Based on this, I'm no longer concerned with disconnecting the cameras when they
aren't being used.
The next experiment will be to AND together the signals presented on the
flash contacts and trigger the flash from them. That is neccessary because
the flash can only be triggered by the first curtain or the second curtain (at
the start or end of the shutter-fully-open period). With the cameras set to
the same exposure time, which shutter opens first and which begins to close
first will be random. You could avoid the problem by setting one camera to a
longer exposure time than the other, and set the flash to be triggered by the
second curtain of the camera set to the shorter period, but that would be an
annoying constraint. You could also set the flash for first-curtain sync and
introduce an electronic delay into the flash trigger, but I think it's simpler
and more reliable to just AND the signals together. Unfortunately, just
connecting the cameras' flash triggers in series won't work if the shutter
releases are tied together as described above, because the flash triggers don't
float.
John
--
John DuBois [EMAIL PROTECTED] KC6QKZ/AE http://www.armory.com./~spcecdt/
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