Noob flash question

Discussion in 'Photography' started by Munk, May 21, 2005.

  1. Munk

    Munk Guest

    Hi guys, I am looking to take pictures at very high speeds to capture
    perfectly formed rain drops. I have a canon 20d, and when using the built in
    flash I can only use a shutter speed of 1/250 sec, it wont let you go faster
    I believe cause it would be out of sink?

    My question is, if I were to purchase an external flash (ive never had one
    before) will this allow for faster exposures? and if so how fast?, any
    recommendations of good flashes would be very much appreciated,

    thanks in advance

    J
     
    Munk, May 21, 2005
    #1
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  2. Munk

    Rob Novak Guest

    That's "synch", or synchronization.

    The problem you're experiencing is that 1/250 is the fastest speed at
    which the shutter is completely open at any one time. Meaning that it
    takes about that long for a shutter curtain to completely expose the
    sensor. Faster shutter speeds are accomplished by moving a slit
    across the film/sensor. At 1/1000 second, the second shutter starts
    to drop (close) 1/1000 second after the first one is released,
    resulting in a slit that covers about 1/4 of the frame traversing the
    sensor.
    Sorta.

    Since flash tube discharge durations can be much shorter than
    mechanical shutter speeds, you set to the X-sync speed (the fastest
    shutter speed that allows for full frame exposure), and use several
    flash units to provide ultra-short (between 1/5000th and 1/20,000th)
    discharge times. Since those short flashes don't produce a lot of
    light, you need several flash heads to compensate. The flashes you
    use also need to have a relatively high guide number to produce enough
    light at the short durations you'll require.
     
    Rob Novak, May 21, 2005
    #2
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  3. The the minimum synchronization speed with the shutter full open.
    Take a look at http://www.usefilm.com/image/798988.html (you'll see only
    a small version as a non-registered user, but might get an idea). This
    was taken at 1/5000 with an external Metz 54MZ3 flash.

    What you need is an external flash which is able to do high speed
    synchronization. Canon got some of these, Metz has them, I don't know
    who else. Be warned: they're not cheap.

    greetings
    P.J.
     
    Peter-Josef Meisch, May 23, 2005
    #3
  4. Munk

    Owamanga Guest

    You've got two options:

    1) Borrow a Nikon D70 for this project. With an external flash it can
    be forced to synch up to 1/8000 without using strange
    'high-speed-flash' canon technology that wouldn't be suitable for what
    you are trying to achieve.

    2) Photograph the rain in the dark. The shutter speed becomes
    irrelevant because there will be little or no ambient light. The
    duration of a flash pulse will be fast enough to freeze the water -
    easily between 1/1,000 and 1/10,000 sec, depending on power and
    technology used.

    Having said all that, to freeze a raindrop (depending on focal length)
    doesn't actually require that high of a shutter speed (1/500th or
    faster will definitely get it frozen) which might be achievable in
    bright sunlight (assuming you 'create' the rain) with a fast ISO and
    no flash.

    I'm not sure what you mean by perfectly formed raindrops - I'm sure
    after plummeting 3,000 ft, crashing into dirt and other raindrops on
    the way down, with wind etc, it'll be far from perfect just
    milliseconds before it's demise when you want to photograph it...
     
    Owamanga, May 23, 2005
    #4
  5. Munk

    Munk Guest

    I was planning on taking a shot of real rain, but your suggestion of
    creating it might just be the answer, (or at least the cheapest option) as
    this will allow me to do it in sun light, great plan thanks.
    j
     
    Munk, May 23, 2005
    #5
  6. Munk

    Munk Guest

    Thanks for the advice, that picture is just the thing i am aming for. Ive
    looked at a few flashes, but i dont see any rating to go on to let me know
    what speeds its capable off, do you know what i should be looking for?

    J
     
    Munk, May 23, 2005
    #6
  7. Munk

    dadiOH Guest

    Most all small flashes have/used to have very brief durations. Large,
    powerful studio flashes, OTOH, were often in the 1/300 - 1/500 range.

    With small, thyrister quenched flashes, the flash duration is shorter
    when it is quenched faster; i.e., if the flash has a range of f stops
    of, say, f2.8 - f11, the flash would be briefer when used at 2.8 instead
    of 11 assuming the same flash to subject distance. The duration is
    usually in the 1/1000 - 1/5000 range. Much briefer and you start to get
    into reciprocity failure.

    --
    dadiOH
    ____________________________

    dadiOH's dandies v3.06...
    ....a help file of info about MP3s, recording from
    LP/cassette and tips & tricks on this and that.
    Get it at http://mysite.verizon.net/xico
     
    dadiOH, May 23, 2005
    #7
  8. Munk

    Dirty Harry Guest

    A canon 420ex speedlite has high speed mode.
     
    Dirty Harry, May 24, 2005
    #8
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