convert guide number to watts

hi all,

I am seriously thinking of using many small hotshoe flash now for my future fashion shoots rather than portable studio lights with heavy battery packs.

the thing is, i am too used to gauging the power of lights using watts, does anyone knows how to convert guide number into watts?

example, the watts of a canon 580EX.

thanks!!!

It really cannot be done because each reflector and flash tube combo is different.
By the way most "shoe" units are about 50 w/s.

W/S or guild numbers are both very poor ways to measure a flash burst, if i measure a burst in a small white room my guild Number will be higher. If i put a small metal reflector on a on a flash head my f/stop or guild number will be higher. If I use a small metal reflector in a small white room my measurement will be much higher then outdoors on a beach, for example. I can put 100 W/S into a soft box and get a number, I can aim the same 100 w/s head at the same distance right at the meter and get a very different number. I could put 2 50 w/s "heads" into the same soft box and get a different guild number or f/stop then i would with a single larger head useing the same amount of "W/S".

You have to measure using the same light modifers under the same conditions.
hope this helps
william
http://www.wcmitchell.com

Estimated about 100 Ws. The GN is a result of the highly efficient reflector and a pretty narrow beam, compared to a typical studio flash (bare bulb type).

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cheers, Peter

Germany

only 100watts? thats pretty weak

what about the new canon 580EX? heard it is the strongest hot shoes flash available? what about the metz hammerheads?

actually i dun mind the narrow beam of lights as i seldom use soft lights now in my shoots, there was once i even zoom the flash in the hot shoes flash to 105mm and it works just like a studio flash with grids (some what)

Metz is up to 200Ws, Bowens Pioneer 300Ws. Others, as e.g. Norman, offer up to 400Ws.

If I were in your shoes, I'd meter the power that you're used to from your studio flashes and calculate it's guide number. This lets you compare them to battery flashes.

But you should be aware that battery flashes' refresh rate is usually slower than studio flashes'.

--
cheers, Peter

Germany

100 is pretty close, but it is watt seconds, and doesn't really transltae, and there are dozens of problems wiht the calculations.

Figure out some guide numbers and you'll get closer, but the variety of light modifiers on studio lights and the zoom heads of camera-makes flashes confuse things, too.

BAK

The only solution I found is to take some images spearately with strobe and flash and change the strobe output untill the exposure is the same as the flash.

I did this with my Sigma EF 500 Super DG and a studio strobe. Both used the same umbrella. The Sigma on full power and 35 mm. I got equal exposures when my studio strobe used 50 w/s.

-Pelle Piano
http://www.pellepiano.com
Gallery at http://www.studiobild.com