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  1. #1
    raucousimages's Avatar
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    Watt/second:actual vs advertised?

    I just purchased two small portable flash units; a JTL mobilight-300 and a Elinchrome D-light 2. The JTL is advertised at 300WS the Elinchrome at 200WS. Using these lights, they seem to have much less output than advertised. To verify their output, I tested them against a Photogenic 2500, a 1250, and a Norman 400. These three lights were all within 2/10ths of a stop at 50, 100, 200, 300, and 400WS. Comparing the JTL and Elinchrome against these, the JTL tested at 104WS max output and the Elinchrome tested at at 51WS max output. Test was done with a Sekonic L-508 meter, at 10 feet, bare bulb in studio conditions. I realize that manufacturers are generous in their advertising. Has anyone had any experience with these? Is there a better way of determining the WS of a strobe?
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    You're testing the wrong thing. Watt seconds is the amount of power used. Not the output.

    But even if it was output everything from reflector design on would change those numbers.

  3. #3
    David A. Goldfarb's Avatar
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    Watt-seconds is just a measure of the power going into the head. The output depends on the efficiency of the head, bulb, and reflector or diffuser. Measuring the light output in the way that you are doing won't tell you the input in Watt-seconds. The fact that your Norman and Photogenic heads are close to each other in this regard is just a coincidence.

    It's best to use the Watt-seconds rating of the system as a kind of ballpark figure to give you an idea of what it will do, but it doesn't correlate very precisely with exposure.
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  4. #4
    raucousimages's Avatar
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    I expected the Photogenic and Norman 400 to be close due to them being designed by the same person but I was disapointed in the light output of the others. I wanted somthing closer norman 400. The norman is advertised as 400WS but is a full 3 stops brighter than the 300WS JTL.

    I guess this is like how car makers rate horse power. My Porsche 911 has 28 fewer horses than a Subaru WRX due to the different rating methods but in a head to head dyno test I beat it by about 60 horse. Just advertising.

    The JTL 300 is a good little light, AC/DC, 150 shots on a charge, fast recycle. I just wish it had more kick. At one time I saw a Watt Second meter, I think it was for calibrating industrial strobes. It might be interesting to see what the output of our lights realy is.
    DIGITAL IS FOR THOSE AFRAID OF THE DARK.

  5. #5

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    Quote Originally Posted by raucousimages View Post
    ... At one time I saw a Watt Second meter, I think it was for calibrating industrial strobes. It might be interesting to see what the output of our lights realy is.
    g'day raucous, in practice WS figures don't come into it, all you need to know is how much light is output in f stops

    when using studio strobes/flash use a flash meter to adjust each light in regard to every other light, this takes into account any light modifiers being used

  6. #6
    Helen B's Avatar
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    Quote Originally Posted by raucousimages View Post
    ***
    At one time I saw a Watt Second meter, I think it was for calibrating industrial strobes. It might be interesting to see what the output of our lights realy is.
    As already explained, a watt-second (Ws, equal to a joule, J) is a measure of the amount of electrical energy coming out of the capacitor. It isn't a measure of the amount of light energy coming out of the tube. Something that measures watt-seconds would generally be measuring electrical energy - like a standard domestic kilowatt-hour meter*.

    Some manufacturers, like Paul C Buff/White Lightning, quote 'effective watt seconds' because they want us to believe that their tubes are more efficient than other manufacturers' tubes in converting electricity into light (more lumens per watt). In fact they are about the same as most - around 40 lumens per watt. The total amount of light energy emitted by a flash tube is measured in lumen-seconds.

    If you are measuring flash with a hand-held meter, the best you are doing is measuring lux-seconds at a certain point. The relationship between the output in lux-seconds at that certain point and the total amount of lumen-seconds depends on how those lumen-seconds are used: ie the shape and surface of the diffuser etc as well as how far away you are from the source.

    Fortunately the most important thing to us is the measurement of lux-seconds, because that's what we are using whether we call it lux-seconds or not**. That's what a flash meter tells us, in the form we need (ie f-stop at a certain film speed).

    If flash manufacturers wanted to give us a piece of useful information about how much useful light energy their gear spat out, independently of which reflector was used, they could just quote lumen-seconds.

    *There is such a thing as a watt-second of radiant energy, but it isn't much use to us because it includes radiant energy other than light - it has no spectral weighting. Light energy cannot be measured in watt-seconds, because a watt is not a measure of light power.

    **Not 100% accurate, but good enough for the purposes of this discussion, I hope. We humans might respond to illumination in lux, but film can have a different spectral sensitivity. Light (lux, lumens etc) is measured in terms of human spectral sensitivity. Photographic light meters try to take this difference into account, at least to some degree.

    Best,
    Helen
    Last edited by Helen B; 01-27-2007 at 04:48 PM. Click to view previous post history.

  7. #7
    Paul Sorensen's Avatar
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    Quote Originally Posted by raucousimages View Post
    I guess this is like how car makers rate horse power. My Porsche 911 has 28 fewer horses than a Subaru WRX due to the different rating methods but in a head to head dyno test I beat it by about 60 horse. Just advertising.
    To continue this analogy, comparing watt seconds to horsepower is very apt. Horsepower influences the speed of your car, but is not the only factor, and watt seconds influence the light output of a strobe, but is not the only factor.

    My big problem, and I think that this has really come up for me because of the Alien Bees/White Lightening advertising, is that manufacturers don't seem to want to advertise numbers that relate to their actual light output. If Paul Buff's stuff was really that much more efficient, there would be a better defense of their "effective watt second" calculations than they provide. I think that this nugget that is one the White Lightening web site really says it all. (from http://www.white-lightning.com/specs.html)
    The Effective Wattseconds rating, however, is rather arbitrary and cannot be easily proven true or untrue, as it is merely used as a basis for inflated comparison of different flash systems.
    If this wasn't basically hidden on their site, I would applaud them for their honesty.

  8. #8
    Jim Jones's Avatar
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    The White Lightning and Alien Bees sites do provide guide numbers for a variety of light modifiers. They even give the details of how the measurements were made. It doesn't get much better than this.

  9. #9
    Paul Sorensen's Avatar
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    Quote Originally Posted by Jim Jones View Post
    The White Lightning and Alien Bees sites do provide guide numbers for a variety of light modifiers. They even give the details of how the measurements were made. It doesn't get much better than this.
    That's true. My issue is that they don't provide any information that I have been able to use to compare their products to other companies' products. This is as much a problem of the competition as theirs, but I think that they really do beg the issue with their prominent use of a number (effective ws) that they admit on their site is for the purposes of creating an inflated comparison of their products.

  10. #10

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    Quote Originally Posted by Paul Sorensen View Post
    That's true. My issue is that they don't provide any information that I have been able to use to compare their products to other companies' products. This is as much a problem of the competition as theirs, but I think that they really do beg the issue with their prominent use of a number (effective ws) that they admit on their site is for the purposes of creating an inflated comparison of their products.
    That's an odd way to look at that. They do quote real, measurable, useful figures that they have measured for their own equipment--Lumen-seconds, which is right there on the web page right next to the effective and true watt-seconds, and elsewhere on their site is a guide number table which you can use to cross-check their comparisons. They did not invent the idea of effective watt-seconds, citing Inverse Square Systems in 1985. They've provided you all the data you really need to know about the output and also power consumption of the units, and also have an admittedly kludgy figure that they allege can be used to compare to other manufacturers.

    I am at a loss as to what you would like him to provide for you. It is not his responsibility to measure Ls coming out of his competitor's equipment and give it to you; that's really the other vendors problem. Yet because the industry is silly this way, Ls, while the best way to compare flash system output, is not an easy way to compare flash system output. So he has provided for you a figure that indexes the power output of his system to that of some nominal inefficient industry standard.

    Suppose someone comes up with something that releases 3500 lumens per watt input, through nuclear fission or something. Is it really sensible for them to say "well, these things are really 2 watt-second flashes, but they're really really efficient."? Photographers who don't know better will go "2 watt-seconds? That's nothing!" and demand a, say, 400 Ws flash, ignorant of the fact that the resulting 1,400,000 lumens-seconds of output will probably kill the model. If Paul Buff's efficiency claims are true (and I have no expertise nor experience one way or the other on this), demanding he make a flash that looks a third as powerful as its nearest true competitor is really unfair.

    This could readily be the same situation that has a 100 watt incandescent light bulb putting out as much real light as a 25 watt compact fluorescent.
    Last edited by gchpaco; 01-28-2007 at 06:53 AM. Click to view previous post history. Reason: Whoops, they also provide guide numbers.

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