Slave hotshoe question

Discussion in '35mm Cameras' started by Jim Hutchison, Mar 28, 2005.

  1. I have a couple optical hot-shoe slaves that work great - except on a
    particular brand of flash.

    Sigma EF-500 ST. Great on-camera flash units, but they won't fire
    every time when mounted on a slave. Any idea why not?

    I have 2: one will fire 3 out of 5 times, and the other will fire 1
    out 5 times. Doesn't matter which slave they're on... I suspect the
    voltage necessary to fire them is just a bit over what my slaves can
    supply. Just a thought.

    Because of this, I'm having to re-think my whole lighting set-up.

    Jim Hutchison, Mar 28, 2005
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  2. Jim Hutchison

    Alan Browne Guest

    No. The slave cells use the voltage from the flash sync to power the
    detector circuit. When they detect a flash they short the line to fire
    the flash. eg: voltage should not matter unless it's simply not enough
    to power up the slave-cell. A few volts is more than enough.
    In some places the walls, furniture, etc., suck up the light and little
    is left to trigger the slave. The very little amount of light required
    for a slave to trip can be enhanced by the placement of white paper or
    aluminum foil to reflect the trigger light onto the slave sensor. I do
    this with my studio lights when I put them in a gym for sports... simply
    tape a piece of crumpled and then flattened alu foil to the wall behind
    the monolight (where the detector is located). This is more than enough
    to relfect the trigger flash onto the sensor.

    Alan Browne, Mar 28, 2005
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  3. Thanks Alan,

    That's not the issue - I get the results described above with a
    flash pointed directly at the slave just inches away, so it's not a
    distance/brightness problem. The *only* explanation that makes sense
    is the 2 Sigma flashes require a voltage that is not supplied
    adequately by the slaves.
    Jim Hutchison, Mar 28, 2005
  4. Jim Hutchison

    rcochran Guest

    But slaves don't EVER supply voltage to trigger a flash! Instead,
    the flash delivers voltage to the slave. A flash is triggered by
    closing the contacts - i.e. shorting them together. It's the same
    mechanism that has been used for many decades, dating from the
    time when all cameras were fully mechanical and had no battery,
    and thus, no means of delivering voltage to a flash. Inside
    the camera were two pieces of metal that would touch one another
    when the shutter fired, thus triggering the flash.

    You can trigger any flash with a paper clip or any other small bit
    of wire. If it's a hot shoe flash, bend the paper clip so that one
    end touches the center contact, and let the other end of the paper
    clip touch the metal contact on the edge, usually located in the
    slot ABOVE the floor of the flash foot. If the flash uses a PC
    cord instead of a hot shoe, then just touch both the center and
    ring contacts with the paper clip.

    The paper clip delivers 0 volts, and you'll note that 0 volts will
    easily trigger your sigma flashes. The problem isn't that the slave
    can't deliver enough voltage to trigger your flash.

    The problem may be that the flash doesn't give enough voltage to the
    slave, or perhaps the slave may require voltage of the opposite
    polarity from what the Sigma flashes provide.

    rcochran, Mar 29, 2005
  5. Jim Hutchison

    Alan Browne Guest

    Well then I have no idea what the problem is ... but I can tell you that
    the slaves do not 'supply' a voltage, they use the voltage from the sync
    signal of the flash. That powers the detector (slave) and is also the
    signal to the flash to fire (when it is shorted by the detector / slave).

    The caveat here is if we're not talking about old fashioned slave/sync
    circuits but newer 'wireless' schemes such as in some more recent (last
    20 years) flash systems (Minolta, Canon, Pentax, Nikon (perhaps Oly, I'm
    not sure)). In such systems compatibility is an issue and your Sigma
    flashes might not be compatible with your SLR.

    Good luck.
    Alan Browne, Mar 29, 2005

  6. Thanks Alan,

    Funny thing is, they both work fine on my Pentax bodies - they were
    purchased to be compatible with them.

    So, obviously the slaves aren't always closing the connection long
    enough for the flashes to fire. If they show up on eBay you'll know
    why. ;-)

    Thanks everyone...
    Jim Hutchison, Mar 29, 2005
  7. Jim Hutchison

    Alan Browne Guest

    How much ya want?
    Alan Browne, Mar 29, 2005
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