Using Flash Beyond The Guide Number Distance.

Discussion in 'Digital Cameras' started by Mardon, Feb 2, 2007.

  1. Mardon

    Mardon Guest

    At 105mm & ISO100 my Canon Speedlite 580EX has a guide number of 58
    (meters). At ISO3200 (maximum ISO of the Canon 20D) the 'effective' guide
    number becomes 328. My lens has a maximum aperture of f/2.8. This
    indicates that in darkness, the maximum distance from the flash, at ISO3200
    and f/2.8, is 117 meters for a 'properly' exposed photo. My question is
    how far beyond this distance is a flash useful at night? The other night I
    photographed a piece of snow removal equipment at 200 meters, f/2.8,
    ISO3200, but it was badly underexposed. Even though the equipment was
    still well beyond the guide number distance of my 580EX, would the use of
    my flash have helped?.

    Related question for the 'oldies'. I seem to recall that the large
    flashbulbs I used back in the '60s really packed a wallop. I can't recall
    the guide numbers of those things. Does anyone know? I suspect they were
    well over 100 at ISO100 (expressed in the equivalent ASA in those days,of
    Mardon, Feb 2, 2007
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  2. Mardon

    Steve Wolfe Guest

    At 105mm & ISO100 my Canon Speedlite 580EX has a guide number of 58
    A little, but not a lot. Double the distance, quarter the power, so your
    flash would have been roughly two stops less than needed. If you *really*
    need to reach out and touch someone, get a "better beamer".

    Steve Wolfe, Feb 2, 2007
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  3. Mardon

    HEMI-Powered Guest

    Today, Mardon made these interesting comments ...
    The light power of flash (or any light source) varies as the
    square of the increased distance, so if you double the distance,
    you'd get 1/4 the light from your external flash. In photographic
    terms, each increase or decrease of 2x results in one f/stop, so
    doubling distance at night would reduce your exposure by 2
    f/stops. Whether that is or is not significant depends on ambient
    lighting of the subject you are shooting, surrounding areas, and
    your own definition of "good" and "bad".

    I would expect the noise in your photos to increase
    significantly, however. ISO 3200 may be noisy even when properly
    exposed, I don't know. I have a Canon Rebel XT and its noise
    becomes quite apparent at ISO 400 and its max ISO 1600 is pretty
    much useless to me as it is so noisy that it is virtually
    impossible to fix post-processing in PSP 9.
    HEMI-Powered, Feb 2, 2007
  4. Mardon

    Mardon Guest

    Thanks for your comments. The night photo that I mentioned in my OP was
    pretty poor; both under-exposed and noisy at ISO3200 with my 20D. In case
    you're curious about how bad it really was, I've put a copy here:
    The metadata is embedded.
    Mardon, Feb 2, 2007
  5. Mardon

    Neil Ellwood Guest

    Neil Ellwood, Feb 2, 2007
  6. What I find interesting is that the only thing that *appears to be
    illuminated by the flash is the top pedestrian sign- or is that

    What about trying same shot, wide open, at say 1/10th, with and without

    It's almost certain the flash wasn't going off full. Try also full
    manual on the flash.

    *knowing appearances are deceiving.....<s>
    John McWilliams, Feb 2, 2007
  7. Mardon

    Mardon Guest

    Thanks for the comments. You're right in your what you say but wrong in
    your assumption. I did not have any flash with me, so that's why you can't
    detect any effects of one being used for the image. The sign was self-
    I wasn't out looking to take any photos, so I didn't have a tripod with me
    either. I hand-held the camera at 1/160 which is about as slow as I
    figured I could go with a 200mm focal length and a 1.6x crop, even with the
    IS lens.
    Mardon, Feb 2, 2007
  8. I see now where I missed the sentence in the o.p., but was blinded by
    O.K., again, I was focussed on the questions of flash, etc. but you did
    have a couple more stops available.
    John McWilliams, Feb 2, 2007
  9. Mardon

    HEMI-Powered Guest

    Today, Mardon made these interesting comments ...
    It is said that "beauty is in the eyes of the beholder", and so is
    picture quality. You are the best - and only real - judge of how
    good your night shots are. I imagine they are noisy and have other
    defects, but the only other way for you to get the shot would be to
    mount the camera on a tripod and use a long exposure, perhaps
    multiple seconds. Tripod night shots are particuarly problematic,
    as I'm sure you're aware.
    HEMI-Powered, Feb 2, 2007
  10. Tripod, beanbag, or use self-timer to trip shutter with least camera shake.

    Morton Linder, Feb 3, 2007
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