aa2000 Posted January 18, 2005 Share Posted January 18, 2005 Hi,<br> How many watts per second (Ws, or Joules) have a Canon 550EX speedlite at 105mm head zoom and full power (1/1)? According to Canon, the guide number in these conditions is 180.<br> I found on the WEB an AC salve specification in which is stated: "Guide number GN: 36 (80 watts per second)". If the conversion Ws <-> GN is linear, then 550EX would have... 400Ws, which seems a little bit too much to me. :)<br> Thanks for responses. Link to comment Share on other sites More sharing options...
ellis_vener_photography Posted January 18, 2005 Share Posted January 18, 2005 It isn't "watts per second' It is "watt-seconds". Watt-seconds or Joules is a measure of the potential energy stored in the units capacitors. According to the 550EX manual the guide number at full manual power and the 50mm zoom setting is 42. expand angle of coverage to the 17mm setting and GN drops to 15, Zoom the head to 105mm setting (narrow beam) and the GN goes up to 55. Link to comment Share on other sites More sharing options...
aa2000 Posted January 18, 2005 Author Share Posted January 18, 2005 Ellis, I specified the GN of 550EX in inches. 55m=180'.<br> You right about the watts-second. Thanks. Link to comment Share on other sites More sharing options...
aa2000 Posted January 18, 2005 Author Share Posted January 18, 2005 err... I mean "watt-seconds"... Seems that I need another coffee. Link to comment Share on other sites More sharing options...
christian_lutz Posted January 18, 2005 Share Posted January 18, 2005 Ws or Joules is a measurement mostly for professional studio flashes, because of the reflector changing you always receive different GN. Unfortunately you cannot always compare the indication Ws or Joule of the different flash manufacturers. More importantly than the Ws or Joules is the light output (f-stop), which one gets from the flash unit. Professional photographers mostly use therefore a Flashmeter. Chris Link to comment Share on other sites More sharing options...
tomweis Posted January 18, 2005 Share Posted January 18, 2005 <p><i>More importantly than the Ws or Joules is the light output (f-stop), which one gets from the flash unit.</i></p> <p>Yes. Watt-seconds and Joules are measurements of the amount of stored electrical energy in the flash unit's capacitors. It is only one factor to determine what f-stop you will get from the strobe. Effective output also depends on the unit's reflector efficiency, flashtube efficiency, whether the flashtube is being fully loaded when fired or not, and of course the addition of any light modifiers. I am thinking of studio strobe systems here.</p> <p>Still, comparing WS numbers from one manufacturer to another is a conveninent - if not entirely accurate - way of comparing strobe systems.</p> Link to comment Share on other sites More sharing options...
kelly_flanigan1 Posted January 18, 2005 Share Posted January 18, 2005 The Watt-Seconds rating/spec is often the energy stored in the flashes high voltage storage capacitor. This is one half the Capacitance; times the DC voltage squared. The flash tube is not 100 percent efficient; heat is released; UV is released; besides usefull visible light. The losses; bulb efficiency; type of reflectors (if any) vary with each flash made. Scientific strobes are often specified with curves for bare bulb; and several different reflector sizes. Here GN versus watt-seconds can vary 3 to 1 or more; because the reflector can be focusable; and make a spotlight. An absolute GN versus Watt-seconds is not possible. It is like an absolute miles per gallon; litres per kilometer number that works with all vehicles; army tank to moped. Link to comment Share on other sites More sharing options...
ellis_vener_photography Posted January 18, 2005 Share Posted January 18, 2005 Where did you spec a reading in inches?<P>As others have pointed out the connection between watt-seconds and GN isn't linear (well it might be if you are only comparing settings for just a 550EX that is performing as the manual says it should. Start comparing that one ideal 550EX to other flashes and you can't do a linear equation.).<P> According to the table on page 121 of the Canon 550EX manual <B>at a zoom setting of 105mm , the GN at ISO 100 in meters is 55</B>. I assume this means that with an ideal 550EX at 1 meter you get a guide number of 55 in the exact center of the beam pattern. convert the distance measurement to inches or feet or cubits and you'll still end up with the same guide number. The equation is "Guide Number / Distance in Feet = aperture setting" Still, the maximum stored electric energy in a 550EX is roughly 60 - 70 watt-seconds, no matter what zoom setting you use on the Canon Speedlite 550EX Link to comment Share on other sites More sharing options...
ptn4egl Posted January 18, 2005 Share Posted January 18, 2005 Completely not answering the original question, but... <P> For those of you NOT using the metric system, the GN of the Canon 550EX is 180 @ 100 ISO @ 10' @ 105mm. <P> Don't forget how the Guide Number changes with the ISO:<ul> <li>200 1.4x <li>400 2x <li>800 2.8x <li>1600 4x </ul> GN at new ISO = GN * sqrt(ISO / 100) Link to comment Share on other sites More sharing options...
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