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What Watts Seconds value do on-camera flashes have?
Old 07-16-2005, 05:13 PM   #1 (permalink)
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I am considering of buying 2 or three on-camera flashes (Sigma 500 DG Super http://www.sigma-photo.co.jp/english...500_super.htm) to use them outdoors and sometimes with my 'backpack studio'. They give only the Guide numbers for different angles.

Does anyone know - what is average W*S values for flashes like that? (with GN around 50 at 105mm coverage)
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Re: What Watts Seconds value do on-camera flashes have?
Old 07-16-2005, 05:19 PM   #2 (permalink)
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The guide numbers on these flashes vary depending on how much they're zoomed. So you're going to find flashes like this quite challenging compared to something like the Alien Bees which you could get for about the same price and the Bees are a lot more powerful and recycle in 1/2 second as opposed to 3-10 seconds for the one's you are considering.
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Re: What Watts Seconds value do on-camera flashes have?
Old 07-16-2005, 05:40 PM   #3 (permalink)
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Watt-seconds is a tricky way to measure flash output. It's mostly useful comparing flash units from the same manufacturer... much less useful comparing different manufacturers.

Think Guide number at a given ISO. Most studio flash manufacturers also provide guide numbers for their units. Reflectors and fresnels affect guide numbers.
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Re: What Watts Seconds value do on-camera flashes have?
Old 07-16-2005, 09:54 PM   #4 (permalink)
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[ QUOTE ]
Does anyone know - what is average W*S values for flashes like that? (with GN around 50 at 105mm coverage)

[/ QUOTE ]

Not sure, but that sounds kinda low... I just piked up a second SB-800 (nikon) for the truly minimalist's portable studio. Now I just need some time to see how well this 'creative lighting system' thing works.

As for guide numbers, I think they're each something like 125 or 145 @ 100iso.
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Re: What Watts Seconds value do on-camera flashes have?
Old 07-16-2005, 09:55 PM   #5 (permalink)
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AlienBees guide number info is at http://www.alienbees.com/specs.html

Scroll towards the bottom third of the page.
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Re: What Watts Seconds value do on-camera flashes have?
Old 07-16-2005, 10:35 PM   #6 (permalink)
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He's probably quoting the GN in meters. 50 meters at 105mm is a reasonable number.
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Re: What Watts Seconds value do on-camera flashes have?
Old 07-17-2005, 12:00 AM   #7 (permalink)
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First I'll attempt to clear up some confusion:

To simplify things, the number of watt-seconds a strobe is rated for is the amount of electrical energy it can pass through its internal circuitry in one second. So, when a 160 Ws strobe fires, it passes 160 joules of current through its internals in one second. It is more or less a number that tells you how much energy a strobe can store and release.

Comparing various strobes by watt-second rating doesn't really tell you much about the unit's actual ability to produce light, because you don't know how efficiently the strobe converts that stored energy into light. Some units are incredibly efficient, some are merely adequate. It is safe to say that the more efficient the strobe, the more expensive it is going to be, because better components cost more money.

A better way to evaluate units is to compare their guide numbers. But even here, you have to make sure you're making similar comparisons. Guide numbers are usually calculated assuming the following parameters: ISO 100 and placement of the measuring device (a light meter) at either 3 meters (if given in meters) or 10 feet (if measured in feet). For studio strobes, the standard reflector is placed on the unit. For an on-camera flash it is typically set at the 50mm zoom setting. Since the various manufacturers like to advertise the biggest number possible, you may have to do quite a bit of digging to find numbers that match the above parameters. To further confuse things, non-zoom heads typically cover a 28mm lens, so you really need to compare them to a zoom head set at 28mm.

To calculate proper exposure, take the guide number and divide it by the distance from the flash to your subject to determine the correct f-stop for your lens. For example, if you have a flash with a guide number of 80 (ft) and the flash is 10 feet from the subject, you should set your aperture to f8.0 to obtain proper exposure.

Now, looking at the Sigma flash you mentioned, it has a guide number of 132 feet at ISO 100 when set to the 50mm zoom setting. Placed 10 feet from your subject, you would need to set your aperture to f13.2 (call it f11-1/2) in order to achieve proper exposure.

Compare that unit to the AlienBees B400. With the standard reflector the B400, placed 10 feet from its subject, requires an f-stop of (more or less) f11 1/3). This translates into a guide number of about 125. But a little digging reveals that the standard reflector would allow you to fill the coverage angle of a 28mm lens. The optional 11" reflector covers a 50mm lens' coverage angle and changes the guide number to 220.

So after all is said and done, that Sigma flash produces about 60% of the light that the B400 does. This is above average power for an on-camera flash, on par with the top of the line Nikon, Canon, and Metz units.

Bottom line, Guide Number tells you more about a flash than the Ws rating does. Hope this helped.

-Chip
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Re: What Watts Seconds value do on-camera flashes have?
Old 07-17-2005, 09:47 AM   #8 (permalink)
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It's one thing to have a good photography education... it's entirely another to actually remember ALL of it!
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