Auto FP High Speed Sync mode - Nikon F6 + SB-800 flash

Discussion in 'Nikon' started by Dave, Dec 31, 2004.

  1. Dave

    Dave Guest

    I stuck the following on a Nikon mailing list, but I thought I'd try
    here too.

    Hi,
    I have an F6 and in the instructions it says you can use high shutter
    speeds (up to 1/8000th sec) in "Auto FP High Speed Sync mode" if used
    with an SB-800 (perhaps some other speedlights too, but definitely the
    SB-800).

    Well I have the SB-800 and the F6. If I set the flash mode to 1/250FP as
    it says in the F6 manual, I can set the shutter speed to anything I
    want. But as soon as the SB-800 is attached, so the indicated speed
    drops to 1/250. The SB-800's manual says these high speed modes must be
    set on the camera, not on the flashgun.

    Also, reading the SB-800's manual, it says the duration of the flash is
    as follows:

    1/1050 at M 1/1 (full output)
    1/1100 sec at M 1/2 output.
    1/2700 sec at M 1/4 output
    1/5900 sec at M 1/8 output
    1/10900 sec at M 1/16 output
    ....
    1/41600 sec at M 1/128 output

    So it seems to me that unless the output is somewhere between 1/8th and
    1/16th of full output (or less), the length of the flash will be >
    1/8000th of second, so it seems unlikely you could ever use 1/8000
    except perhaps at very close distances. But I can't get the shutter
    speed to indicate anything faster than 1/250th, no matter what I do. I
    even changed the ISO of the film as if I was rating it at some huge ISO
    (whatever the maximum supported on the F6 is), but still no joy. I thied
    this on an 20mm f/2.8 and the 70-200 f/2.8 VR lenses.

    It's probably me reading the manual wrong, as my last Nikon was an F3
    which was ever so slightly (well quite a lot really) less complex than
    the F6. And the SB-800 is a pretty complex bit of kit too.

    BTW, does anyone know why the duration of the flash differs so little
    (1/1050th vs 1/1100th sec) going between full and half output? At
    anything under 1/2th output, the duration gets much shorter, but above
    that, the duration remains the same. I'm just interested in how they
    control the output level. (This is my science/engineering background
    being inquisitive).

    dave
     
    Dave, Dec 31, 2004
    #1
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  2. Dave

    [BnH] Guest

    For my D1H , you just set the mode to M [manual] set the desired shutter
    speed n aperture,
    set the SB-80DX speedlight mode to M ... then adjust the power til it
    indicates FP.

    Then flash away :)

    =bob=
     
    [BnH], Jan 1, 2005
    #2
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  3. Dave

    Alan Browne Guest

    High speed flash is spread out in little pulses (unlike normal flash which is an
    uncontrolled discharge up to the point where the TTL shuts it off.)

    High speed flash: http://www.photozone.de/bindex3.html

    For 'normal flash', the discharge curve goes from 0 to full power very quickly,
    so for the lowest power, the flash has to be stopped before full discharge rates
    occur. That is why a plot of the timings above is so skewed. Here is a link
    for both high speed and TTL flash.

    http://www.chem.helsinki.fi/~toomas/photo/flash-faq.html
    http://www.chem.helsinki.fi/~toomas/photo/flash-discharge.html



    See the link above. Will answer much.
     
    Alan Browne, Jan 1, 2005
    #3
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