Optical Slaves in Strobes

Discussion in '35mm Cameras' started by Robert Meyers, Oct 13, 2004.

  1. Hello all,

    I was wondering what the synch speed is for most optical slaves integrated
    into studio flashes. I was considering using a on camera dedicated flash as
    a fill, and to set off strobes. The flashes synch much faster than the
    traditional synch (1/4000 or 1/8000 depending on camera flash) versus
    1/180th of a second.


    Robert Meyers
    Robert Meyers, Oct 13, 2004
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  2. I was wondering if I could use the high speed synch for propretary flashes
    to cheat
    the max x-synch of the camera. This sounds like the answer is yes?

    Robert Meyers, Oct 13, 2004
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  3. Robert Meyers

    McLeod Guest

    I'm not quite sure I understand your question. If you are shooting
    with 35 mm you will have to shoot at your maximum sync speed of your
    camera or lower.

    Your on camera flash will trigger your slaved flashes instantaneously
    (but not quite at the speed of light!).

    Some older flash units have quite slow sync speeds. I have used some
    older Bowens Monolights that recommend a shutter speed (with medium
    format) of no more than 1/400 of a second.

    I don't know if this helps.
    McLeod, Oct 13, 2004
  4. Robert Meyers

    McLeod Guest

    No. You cannot cheat the max x-sync of your camera with studio
    flashes. HSS is a different beast altogether.
    McLeod, Oct 13, 2004

  5. Depends what you're trying to do, but for the most part, the answer
    is "no".

    Maximum sync speed on most cameras is determined by the shutter
    mechanism, and has nothing to do with the flash. The shutter curtains (that
    open to expose the film) can only move so fast physically.

    There are two shutter curtains in most SLRs, and they both move the
    same direction across the film - one opens, say moving left to right, and
    the other closes in the same direction, following it. For high shutter
    speeds, the first one doesn't even open all the way before the second one
    starts to close, and they both move across the film at the same time. The
    gap between them is the actual shutter speed, but the film never gets
    exposed all at once - it's actually exposed by a slit between the curtains
    that passes across the film.

    Flash durations are usually extremely brief, much shorter than the
    shutter speeds. So when they go off, they only expose the portion of the
    film currently uncovered by the shutter curtains, and what you get is a
    narrow band on the film that's properly lit - the rest is underexposed.

    So maximum sync speeds are actually the highest speed the camera can
    manage where both curtains are open all the way at the same time - it
    pretty much tells you how fast the curtains can move.

    High Speed Sync flashes are made to go off several times in rapid
    succession, carefully timed to illuminate each patch of film as that slit
    travels. This is pretty hard to get around - it requires precision
    communication between camera and flash.

    However, this would not stop you from locking the shutter open for a
    longer period and using the flash as the sole illumination for the image -
    for instance, in a very dark room. How you might accomplish this is up to
    you, but the method of flash metering has to be taken into consideration.

    Hope this helps. Good luck!

    - Al.
    Al Denelsbeck, Oct 13, 2004
  6. No. What HSS does is keep the flash on _longer_, so that it continues
    to illuminate the subject while the "slit" between the front and rear
    curtains travels across the sensor (digital or film).

    If you were to use HSS to trigger a typical monolight, you'd end up
    with a stripe (probably horizontal) that's fully exposed; the rest
    of the shot would show varying degrees of underexposure.

    What's the shooting situation where you envision requiring high
    shutter speeds?

    Different systems offer various options for off-camera HSS, but they
    all require dedicated flash units. To be honest, I played with HSS
    when I first got my F100 and haven't used it since. If a situation
    requiring a high shutter speeds and mix of natural and off-camera
    lighting arose tomorrow, I'd either rent a continuous fluorescent
    light or use a medium format camera with a leaf shutter lens.
    Michael Benveniste, Oct 13, 2004
  7. Robert Meyers

    Alan Browne Guest

    1) Studio strobes 'dump' all of the power in the caps when triggered. The
    power level chosen sets the amount of cap charging. At low power settings, a
    studio strobe dumps in about 1/10,000 to 1/1,000 of a second, at high power
    settings, it might be as much as 2 ms. Leaf shutter lenses typically go up to
    1/500 so it is a convenient fit. (Some leaf shutters go even faster). Needless
    to say, the shutter needs to be fully open when the strobe fires.

    The optical signal from the camera strobe to the slave travels about 1 foot per
    nanosecond. So negligible. The trigger time from receiving the pulse to
    closing the circuit at the slave is less than a microsecond. Negligible.

    In short, to use the studio strobes, you must operate your camera at its sync
    speed or slower regardless of the sync speed capability of the attachment flashes.

    2) 1/4000 (1/8000) synch is really not sync'd at all. Often called High Speed
    Sync (HSS), the flash is modulated to output a long series of tiny bursts while
    the shutter 'slit' is in movement. The flash actually begins firing before the
    first shutter begins moving and is still firing after the rear curtain gets to
    the other side. During travel, tiny bursts of light are fired.

    This is highly inefficient as part (sometimes most) of the 'return' light is
    falling on the shutter curtains not film, making a GN 56 m flash into a GN 3 m
    flash at 1/8,000.

    Again, you cannot use the HSS mode as fill if the shutter speed is above the
    sync speed of the shutter if you are triggering studio lights as well.

    3) Wireless HSS
    On Minolta cameras and flashes, off camera HSS is available (Maxxum 7,
    3600HS/5600HS flash).

    In the end, HSS is not that useful that often.

    Alan Browne, Oct 13, 2004
  8. Robert Meyers

    Deathwalker Guest

    I have often seen cameras bost rear curtain sync. What is this and how does
    it differ from normal sync?
    Deathwalker, Oct 13, 2004
  9. Robert Meyers

    Alan Browne Guest

    Normal sync: The exposure begins by the 'first' (front) curtain opening up
    completely. The entire frame is exposed. At that point in time the flash
    contact is fired and the strobe goes off. Some time later (on the order of 1 or
    2 msec, or much longer if that is what the photog set), the rear curtain begines
    to close.
    Advantage: catches action very near the time the flash fired
    Disadvantage: if motion is exposed by ambient light, the effect 'leads' the
    motion and it looks unnatural.

    Rear curtain sync: As above, except that the flash does not fire until the rear
    curtain is about to travel. Therefore regardless of the duration of the
    exposure, the flash comes at the end. (about 2 ms before the rear curtain begins
    to cover the frame.
    Advantage: motion streaks appear to trail the action.
    Disadvantage: on longer exposures, hard to judge the desired "frozen" by flash

    Alan Browne, Oct 13, 2004
  10. Robert Meyers

    Mr Jessop Guest

    so its not a technique for increasing the maximum flash sync speed. That is
    actually the multi firing gun and nothing more.
    Mr Jessop, Oct 13, 2004
  11. Robert Meyers

    Alan Browne Guest

    Rear sync simply means the flash fires at the end of the full frame exposure,
    not at the start of the full frame exposure. It is always at the sync speed or

    By "multi firing gun" I assume you mean HSS, which is the continuous firing of
    the flash as the slit crosses the frame (the frame is never full frame exposed
    in this case. See: http://www.photozone.de/bindex3.html HSS is usually at
    speeds above the sync speed.

    Alan Browne, Oct 13, 2004
  12. Robert Meyers

    Mr Jessop Guest

    my 300d flash sync speed is something like a 60th. there are claims with
    550ex it is 250th. I suspect this is hss.
    Mr Jessop, Oct 13, 2004
  13. Robert Meyers

    Bandicoot Guest

    Maybe, just maybe, you could fix it up with a Strobex Polikon and a lot of
    flash heads - but then you're talking a lot of money, a lot of effort, a lot
    of weight and bulk, and probably a lot of experimentation before you got it
    right. Certainly not a practical answer.

    Otherwise the studio heads would fire once when the first burst from the HSS
    head hit their slaves, and wouldn't be even close to being fully re-charged
    in time to fire again, never mind catching every one of the bursts, which is
    what they'd have to do to make this work.

    Bandicoot, Oct 14, 2004
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