Problems with external flash synch for digital camera

Discussion in 'Photography' started by Jan Roland Eriksson, Sep 4, 2004.

  1. I have a studio/umbrella flash unit that can be triggered through a
    standard synch cable but it can also act as a wireless slave to another
    flash unit, through the built in photocell at the back end of it.

    This "wireless" trigger works excellent as long as I use any one of my
    standard film cameras (Hasselblad 500/C and Nikon AF-501's) along with a
    cable triggered master flash.

    As of late I have done a lot of experiments to try to get this slave
    trigger system to work correctly with my only one year old digital
    camera (HP-850) but to no avail. My HP-850 does not have a synch cable
    contact but relies only on its built in "mini flash" which when used in
    studio creates a very flat and bland light, with sharp thin shadows
    behind a model.

    What I have tried so far is to put a piece of solid black plastic in
    front of the "mini flash" to prohibit the light from that flash to go
    directly on the model, but still allow some of the light to reflect
    backwards, enough to trigger the slave unit.

    In spite of the fact that the slave do trigger on the backward light
    from the camera, I only get underexposed pictures in storage that has
    not got any light at all on them from the slave.

    I have systematically worked myself through every possible setting that
    this camera offers but none of them seems to be fit for to pick up light
    from an external slave flash unit.

    All my experimenting leads me to think that the concept of "shutter
    speed" does not really have a meaning for a digital camera. My camera
    allows me in one of its menus to adjust shutter speed but I have been
    down to a setting of 1/8 of a second and still it refuse to accept that
    the slave flash "hits it" anywhere within that rather long time frame.

    The built in flash unit works wonders though, in outdoor shooting to
    balance the foreground, as can be seen here (all of these are taken
    close to one year ago with my then almost brand new HP-850)...

    <http://www.fantasywoman.nu/terra/terra01.html>

    ....and it also gives good exposures when used directly on the model in
    studio, as it is probably designed to be used (but with the drawback of
    "flat, bland and sharp shadowed" pictures as a result).

    I'm at ways end on this but I still do not really want to believe that
    the very short delay (must be less than 1/1000 of a second) from when
    the camera flashes and until the slave triggers, should have an impact
    on what gets captured from the CCD cell.

    I will gladly accept any input on this problem.

    All the best...
     
    Jan Roland Eriksson, Sep 4, 2004
    #1
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  2. Jan Roland Eriksson

    Jack Guest

    Hi Jan:

    Looks like you're a pro, and I'm not. I just want to share with you
    some research that I did in buying some equipment for in door portrait
    taking with my Canon Digital Rebel and 429 EX.

    I bought 2 420EX's and a wireless transmitter ST-E2 for the set up. I
    then wanted to light up the background with one of the Sunpak flashes
    that I already own. Since there is not a syncord outlet on the
    digital rebel, I needed to find a wireless way to trip off the Sunpak.
    I spoke to the Sunpak people who referred me to the Wein. The rep
    there explained that:

    Unlike film camera flashes, the digital flashes have pre-flashes.
    Apparently the main burst of flash comes AFTER 1, 2 or even 3
    preflashes. If one uses a film flash sensor to trigger another flash
    (such as your unbrella), then the slave will go off on the preflash
    and is out of sync with the main flash, resulting in underexposed
    shots.

    I spoke with a guy in Adorama, and bought a DIGITAL SLAVE HOTSHOE
    which ignors the preflash but senses the main flash. Now the Sunpak
    on this HOTSHOE works flawlessly with the 420EX's.

    Hope this helps.
     
    Jack, Sep 4, 2004
    #2
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  3. Jan Roland Eriksson

    BillB Guest

    That would explain why the slave flashes fired early, but are you
    sure about the film vs. digital cameras difference? Red lights are
    used both for redeye reduction and to help autofocus. They're less
    likely to have pre-triggered your slave flashes than the more
    powerful AF assist sometimes provided by a camera's regular flash,
    which is one of the Digital Rebel's features. I see no reason why
    film cameras wouldn't be able to do the same. It would seem to be a
    feature more readily available to cameras having built-in flashes
    than to any difference between digital and film cameras. The only
    requirement is that the camera have one or more computer chips
    inside and be designed to use the flash to assist focusing, and its
    the rare film camera that have a CPU somewhere inside.

    Can the Digital Rebel disable its autofocus assist using the flash
    while still using the flash to take pictures? An online review only
    mentioned that if the flash wasn't used to take pictures then it
    wouldn't be able to use the flash as an AF aid. Could it do the
    opposite? That is, can you disable the AF assist while still using
    the flash? If not, perhaps using the AF lock would do it. The
    review made no mention of the camera having the ability to focus
    manually, which if possible would be likely to disable the AF
    assist. It's immaterial at this point though, since you already
    have a useful solution. :)
     
    BillB, Sep 4, 2004
    #3
  4. Thanks for spending time on my problem Jack :)
    No, not a "pro" just a long time amateur (since 1967 basically)
    My daytime work is in electrical industrial automation as a fact.
    I have suspected something like that to happen.

    What I do know is if I set my built in flash to "red-eye" compensation
    it "bursts" a series of flashes before it fires for the main exposure.
    The initial burst comes at an interval that is visible to the naked eye.

    If the same thing happens when I set the flash to "steady straight on"
    the initial flash (maybe sent for exposure measurement purpose) must
    come at an extremely short interval before the main flash because I can
    not "see" it as a double flash. To my eye it appears as "one shot" only.
    You have sort of confirmed my suspicion, it seems I will have to search
    Google for a suitable source of additional components.

    Thanks...
     
    Jan Roland Eriksson, Sep 4, 2004
    #4
  5. Jan Roland Eriksson

    Jack Guest

    Bill:

    I don't think the preflash has anything to so with that that allows
    auto focus in dim lights. The way it was explained to me was that
    those preflashe(s) are different and separate from the AF flashes.
    Indeed, even when I switched to manual focus on my EF lens, I still
    got underexposure if I did not have the digital slave hotshoe. What
    are the reasons or function of those preflashes? Don't know! I even
    called Canon tech support and they conformed the story.
     
    Jack, Sep 4, 2004
    #5
  6. Jan Roland Eriksson

    Jack Guest


    Glad to have help, Jan.

    Jack
     
    Jack, Sep 4, 2004
    #6
  7. Jan Roland Eriksson

    BillB Guest

    They were able to confirm the story but didn't know the purpose of
    the preflashes? Sounds like you spoke with the lowest, tech.
    trainee level and needed to have been bumped up to the next level of
    tech. support. Not being familiar with the camera I can only
    theorize (a fancy word for guess) so here are a few ideas. I'll
    assume that the anti-redeye flash isn't responsible, and that's easy
    to confirm since the Digital Rebel can disable that feature.

    The review I read implied that using the flash to help autofocus
    is not typical, and that if could only do so when using the flash.
    So if you're taking pictures without flash in low light levels it
    may use a different lamp, as other cameras do (even film cameras).
    If that other lamp isn't the source of the problem preflashes, it
    would be the main flash that's emitting the preflashes, and they may
    not even be intentional preflashes. Here's where the real guesses
    start . . .

    Electronic flashes normally fire one intense flash. But since the
    Digital Rebel's flash has already been designed to quickly fire many
    very weak flashes (to help autofocus), when used to provide light
    for the picture perhaps it works differently, firing not a single
    intense flash, but several very bright flashes instead - that may be
    so close together that they appear to be a single flash. One, or a
    small number of them may not be intended to be used for the normal
    exposure. The camera may analyse the light being returned from the
    preflash(es) to determine whether a proper exposure might need to
    have the flash boost or reduce the amount of light it produces.

    Or, the preflashes could simply be a problem caused by sloppy
    programming. If the camera is designed to produce multiple flashes
    during the window of time that the sensor is capturing light for its
    image, rather than have the flashes start and end at the precise
    beginning and end of the sensor's window while it is capturing
    light, it may start a bit early and end a bit late. The sloppiness
    may even be intentional, to avoid overly complex calculations that
    might arise from timing differences causes by different aperture /
    shutter / exposure combinations. If the flash's light always
    overlaps, or extends from before to after the sensor effectively
    uses the light, it would only waste a small fraction of its light
    energy, but guarantees that camera won't (in a very small number of
    cases) ruin the picture caused by firing the flashes too early or
    too late. A slave flash could compensate for this if it allows for
    you to add an adjustable delay before it's allowed to flash.

    Or . . . it could be part of a grand conspiracy, with the
    manufacturers soon to introduce a new line of much more expensive
    pro cameras that have an "I'm using slave flashes" option in their
    menus. :)
     
    BillB, Sep 4, 2004
    #7
  8. I went Googeling after I got the reply from Jack and found a page (IMMIC
    it was among the Wein pages at the Ritz Photo site) where they claimed
    that the only reason to send a minute pre flash was to establish the
    correct color temperature for the CCD to work on when the main flash was
    fired and that was the sole purpose for to send a "pre-flash".
    In my case, for the HP850, I can also disable the "red-eye compensation"
    in the flash unit but I will still have the underexposed problem.
    For my HP I can get the camera to focus and lock focus if I depress the
    trigger button halfway. The camera will turn on its "red-light beam" for
    a short time during the focusing process, if it decides that the
    contrast level in the area I have selected to focus is not enough.

    In my case, the flash is never involved with the focus process.

    All info I have found so far indicates that most built in flashes in
    digital cameras do send a "pre-flash" that is separated to the main
    flash by only few milliseconds, but strong enough to trigger a
    "wireless" slave so that when the main exposure flash comes along
    milliseconds later, the slave is already discharged and is out of energy
    to follow up.

    Side note:
    I did find what I think is the correct component (Wein), that I would
    need for my studio umbrella unit, at the Ritz site. Gosh; $100 for a
    device with the 1/4 inch "phone plug" that would fit straight into my
    umbrella unit.

    That's about 20% of what I originally paid for my HP digital camera at
    H.H.Gregg's in Cincinnati, January 2003. (Hmmm... :)

    All the best ...
     
    Jan Roland Eriksson, Sep 5, 2004
    #8
  9. Jan Roland Eriksson

    BillB Guest

    Thanks. That got me to go a-googling too. I didn't see that in
    any of Ritz's Wein pages that I found, but found several camera
    reviews or manufacturer's catalog pages that had something to say on
    the subject. It seems that all of the cameras that utilize
    preflashes use it to adjust the exposure on the fly, referring to it
    as preflash metering, which was one of the theories I guessed at in
    my last reply:

    But some - not all - camera manufacturers (such as Sony for some
    of their models) mention that preflashes are also used to adjust the
    white color balance. I think cameras that do this should be
    avoided, as they'd give you a feature taking control away from the
    photographer, as Point-and-Shoot cameras do, and that might result
    in messed up shots taken of objects not having typical coloring.
    Finally, here's a clip from one article I found that's interesting:

    http://www.pcmag.com/print_article/0,1761,a=29094,00.asp
     
    BillB, Sep 5, 2004
    #9
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