how does a slave flash work?

Discussion in '35mm Cameras' started by Francesco Spegni, May 1, 2005.

  1. hallo newsgroup,

    i'm quite new to photography so i just hope these questions are not too
    stupid :p

    take an slr camera where it's possible to attach a flash to it. there are
    special contacts on top of the camera for plugging it, and through those
    contacts the flash can synchronize itself with the camera, so it actually
    enlights the space at the right moment (i.e. when we shoot the picture).

    but then take a so-called "slave" flash: it plugs _under_ the camera, on
    the same plug we use for the tripod. how can it synchronize the light
    with the moment you actually shoot the picture?

    i hope you can explain this to me :) by the way, i don't like the enbodied
    flash of my camera (eos 300), would you suggest me to take a slave flash
    or a regular flash to be plugged on top of the camera? i also knew that a
    decentralized flash would avoid the nasty red-eye effect, so i thought a
    slave flash would be a good idea... again, can you guide me through this
    decision? :)

    thanx in advance

    Francesco Spegni, May 1, 2005
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  2. Francesco Spegni

    Peter Irwin Guest

    The usual kind of slave flash responds to the flash of light
    from a flash attached to your camera. It has a sensor which
    fires the flash as soon as it senses the light from another
    flash. Amazingly enough this whole process takes a millisecond
    or less, so for practical purposes the two flashes flash
    at the same time.

    It is possible to hook up two or more flashes to the sync
    connector on your camera, but having the extra flashes sync
    to the light of one camera-connected flash avoids the need
    for long wires. You don't have to buy a "slave flash", a
    standard flash plus a slave unit does the same thing.
    Using two flashes can greatly improve your lighting.
    For example, you could have a bounce flash mounted above
    your camera, and a second slave flash on a tripod aimed
    at your subject from a 45 degree angle. This could give
    you a nice combination of diffuse overhead light and
    some direct light hitting your subject from an angle.

    There are lots of books on photographic lighting to
    give you ideas.

    Peter Irwin, May 1, 2005
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  3. But when my camera gets enough light, it "quenches" the flash on its hotshoe
    with a separate signal that it sends through an extra terminal on the
    hotshoe. Would the slave flash quench also, or would I have to buy a special
    unit for this?
    William Graham, May 1, 2005
  4. Francesco Spegni

    Paul Rubin Guest

    That's TTL (thru the lens) flash metering, which is a fancy feature
    developed fairly recently in the scheme of things. Traditionally auto
    flash metering was done with a photocell on the flash unit, that told
    the flash to quench itself. For these multi-flash setups you'd
    probably set the flash intensity manually. Fancy modern SLR systems
    have all kinds of wireless communication between the main and slave
    flashes and are a big improvement over the old stuff. In the old days
    you'd use modelling lights, polaroids, etc. to see how the flash
    exposure was going to turn out.
    Paul Rubin, May 1, 2005
  5. Francesco Spegni

    Tony Guest

    There is sensor that trips the slave when another flash goes off. In
    order to use TTL metering with it the slave has to be compatible with the
    camera. I'm not sure which flashes are compatible with the EOS 300 as I tend
    to use a flash meter when using lights.
    Since I've never gotten into auto-flash I don't really know much about
    the auto-flash systems available for the 300, but I do know that if you use
    the built-in flash to trigger the slave you will still have a better than
    even chance of getting red-eye.
    The best thing to do is use a separate flash tall enough to put the light
    well above the lens - or better yet use a flash and bracket combination that
    will allow you to switch to vertical shots and still have the light above
    the lens. When you get the flash a foot or more above the lens there is
    little chance of red-eye at almost any range(with humans at least) unless
    you are sshooting in a coal mine. You will also get a better look than the
    blasted effect you get with the built-in flash.
    Flash is complicated enough that it is worth readng a few articles or a
    book on the subject. Your local library probably has at least one book on
    lighting in their photography section. A night of reading can save you
    dozens of rolls of film.
    Tony, May 1, 2005
  6. Francesco Spegni

    Alan Browne Guest

    That is TTL flash, and requires a flash unit that has a Thyristor (a
    very fast switching power transistor) to interrupt the flow of electrons
    from the capacitor(s) to the tube.
    Slaved units (and PC sync'd units) dump all of the power in the
    capacitor. (Some may charge part way to control power, such as studio

    Wireless TTL units receive a control signal from the camera's TTL logic,
    and hence power can be controlled. A terrible way to get consistent
    lighting in a series of photos.

    Alan Browne, May 1, 2005
  7. Francesco Spegni

    Peter Irwin Guest

    As others have replied, you would use manual flash in multiple
    flash setups. Even if you have a bunch of fancy electronics
    available, you still want to control lighting ratios by the
    power and distance of each flash.

    If you are ok with the low tech approach, you can do a practice
    set-up by using guide numbers, charts from books and guesstimates,
    make detailed notes and then when you see the developed film
    you can attach the notes to prints of the better pictures.
    You can then use your notes to do the same thing again.

    For a higher-tech approach you could use a flash meter and do
    some test shots with a Polaroid or a digital camera. I've
    only used flash with negative films where the lighting
    ratio is important, but the total exposure isn't (as long
    as it is enough). I suspect a flash meter would make things
    a lot easier if you shoot slides.

    For a quick and easy bounce flash plus low power slave at
    an angle, I have tried putting the bounce flash on auto
    and used guide numbers for the small flash on a tripod,
    and the combination seems to work pretty well.

    The tripod which holds the flash doesn't have to be very
    sturdy, so if you have a cheap tripod which is no good
    for holding a camera it may still have a good use.

    Peter Irwin, May 1, 2005
  8. Francesco Spegni

    Douglas Guest

    Most Canon speedlights can become a slave in their own right. They take
    their command to fire from the rapid rise in light the main flash give out.
    In day of old a flash set on auto with a single hot shoe contact, would give
    correct exposure when you set the camera at f8 or any predetermined
    aperture. The flash itself had a built in reflection meter to quench it's
    light based on the returning light.

    When you use a photodiode slave and a Electronic Thru The Lens ETTL flash,
    the camera will meter for the light as if it were a normal exposure and
    prematurely close the shutter (or delay the close) based on a combination of
    factors you can program into the primary flash.

    If you can afford the cost, two Canon speedlights are the best way to get
    good exposures all the time. Otherwise you may find the preflash emitted to
    aid focus on many non Canon flashes cannot be switched off and will trigger
    the slave before the shot.

    If you are working indoors in a controlled space, you may find using some
    cheap tungsten work lights from a building supply are very easy to work

    Douglas, May 1, 2005
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