What is the Ws in a camera flash?

Discussion in 'Lighting Equipment' started by paco_rosso, Dec 7, 2013.

  1. Each day I think I know less than the day before.
    Profoto says its new B1 (500ws) provide 10 times more light than a camera flash.

    I have made this measurements: At 2 meters, a profoto D1 500 (500ws) provide 700 lxs (lux second). So the BCPS is 700 x 4 = 2800 js (jules second)... but my metz 45 claim to have a BCPS 4.500 js...
    I measure a Bowens gemini 500 at 2 meters with the normal reflector (S) and have 1100 lxs, so it is a BCPS 4400 js.
    ¿What is the ws in a camera flash?
    ¿What is the photometric render in a camera flash? ¿35lm/w? ¿11lm/w? The broncolor data in their catalogues seems to be around 40lm/w.
    ¿Can a camera flash (a nikon SB900 for example) equal the light power of a 500ws studio strobe?
     
  2. Jules second means nothing. Watt second is jules.
    Hot shoe flash has a capacitor of about 1600uF which stores 80 jules of energy, or 80 watt second.
     
  3. Dehuan Xin is right.
    A Nikon SB900 is 76Ws so close to 80Ws. http://www.photo.net/photography-lighting-equipment-techniques-forum/00ZARK
    Smaller flash units like the SB600 and likely the SB700 are 54Ws.
    So it's not a lie to say that the Profoto B1 is ten times as powerful as a camera flash - of course depending on exactly what flash you are talking about.
    If we compare to the SB900, the Profoto B1 is 6.5 times more powerful, which equals 2.7 stop. So no, the SB900 can never equal a 500Ws strobe.

    BTW, ten times more powerful is the same as 3.3 stops, but ten times sounds like a lot more...
    How much light you get where you measure depends on the reflector and I guess to a small degree the efficiency of the flash tube (but I think that effect is negligible).
     
  4. Keep in mind that whether you call them joules or watt-seconds , they are only a measure of the amount of electric
    charge that can be discharged to a flash tube. Most small flashes like a Nikon SB-910 or similar large hot shoe mount
    flashes use a combination of a fixed fresnel type lens plus a movable internal parabolic reflector to change the beam angle
    from a wide angle flood to a semi-narrow spot. Usually the range is given in equivalents to focal lengths on a "full frame"
    24x36mm format camera.

    The wider the beam angle the more the energy (in the form of light) is dispersed, and the narrower the beam angle the
    more efficient a light appears because the energy is concentrated into a smaller area.

    Beyond these raw numbers what can be very important is evenness in fall off from center (on axis light) to the edge of the
    beam.

    The only way to make an honest comparison in actual light output is to test the lights you want to compare in identical circumstances.

    As it happens right now I have a pair of Profoto B1's in house to review for a national photographic magazine.

    The B1 uses a fixed internal reflector., which appears to be optimized for a very even wide angle coverage. The beam
    pattern can be altered with Profoto's range of reflectors or any other modifier with a Profoto mount.
     
  5. The most powerful hotshoe speedlights (or speedlites) have a flash storage capacitor rated at 1400uF and 330 volts*, which works out to around 75 Joules or Watt-seconds. The measured GN of these is around 28 in metres @ 100 ISO with an 'average' zoom setting. I get only a slightly greater measured GN from any of my Metz 45 Cx series hammerheads, and a measured GN of 45 from a 60 CT-4.
    [* The repair manual for the SB-900 shows this value; yielding a theoretical Watt-second rating of 76 at absolute maximum rated voltage.]
    In short, all flash makers' Guide Numbers are just plain lies IME. They're nearly all at least one stop short of the maker's claim when measured in any real-life situation, and I'm really not sure how they've got away with it for so many years. Rant now over - thanks for reading.
    Really my point is that you can't compare the highly focused and efficient fresnel beam from a little portable flash with the inefficient wide-angled reflector of a studio monolight. From empirical measurement, it appears that you have to have at least a 200 W/s monolight to equal the 75 W/s output of a Nikon SB-900. But all that changes when you place a diffusing modifier like a softbox in front of the two. Then it's easy to see that the wider and more even beam spread from the monolight results in a brighter illumination at the subject.
    ¿Can a camera flash (a nikon SB900 for example) equal the light power of a 500ws studio strobe?​
    No! But it can equal a 200 W/s studio strobe under some conditions. Just try it for yourself. Forget your calculations and do a practical test: Point an SB-900 or equivalent at a large matte-white reflector and measure what exposure you get from the reflected light at, say, 3 metres from the reflector. Then replace the speedlight with a studio strobe set to 200 W/s output. If my experience is anything to go by you'll get the same exposure within 1/3rd of a stop. With the proviso that in both cases the reflector must be large enough to catch most of the beam from each type of flash.
    Alternatively use both types of flash to try and illuminate a building outdoors at night. The studio strobe will almost certainly provide a much more even and possibly slightly brighter coverage. Although obviously you need to fit the studio strobe with a standard 7" reflector or similar.
     
  6. Ok but ¿What happens with the BCPS? Tonight I have measure the BCPS of a Profoto B1 without any lightshaper and have read a 230lxs at 3,16m, so the BCPS is 2300 js.
    Tonight I have illuminated the front of an hotel in a place bouncing the B1 (nude, without lightshaper) in the wall in front off the hotel. It is: I have illuminated one side of the place bounthing the light in the opposite wall. I was almost in 1/3 of the lenght of the place, near the bouncing wall. So it was about 15 meters from the B1 to the wall and about 50 meters fom this wall to the opposite. In this, I get a f:2,8 for a ISO 640/29.
    With a 1/1000 emission time the illuminance provided by the bouncing in the final wall was about 3300lx.
    BUT: if the BCPS of the B1 is 2300js ¿Why I cannot get the same light with a Metz 45 with a BCPS of 4400js?

    I am not talking about the allyouknowwattssecondsisnottheamountoflight. I am talking about BCPS wich is the intensity of the light (candles) in the exposimeter system (per second).


    My thinking is I: BCPS in the fabricants data is a marketing number not a physic measurement. Maybe the angle of emmision has things to say in the actual intensity and we do not must be confident with the numbers in the specifications table.
     

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