Universal measure for flashes?

[andytakeone]

So flash photography is generally measured by fractions of a flash's full power, ie., 1/2, 1/4, 1/8 power.

But what confuses me is that if you take different flashes, and set them all at full power, each flash is going to output a different amount of light, no?

Which leads me to think that there's no way of knowing how much light a flash will output if you just bought it. Is there a universal measure for flashes? So that you can, say, compare 3 different flashes outputting at 1/1 full power, yet still know how much output you're getting from each.


Thanks,
let me know if that was unclear.

 

[480sparky]

It's called the Guide Number.

 

[zombiesniper]

From B&H
"Guide numbers are the standardized, numerical way of determining the power of a flash, with a higher guide number representing a more powerful flash. A guide number is the product of multiplying the f/stop of an exposure with a given distance, at ISO 100; or GN = f/number x distance."


Edit: 480sparky beat me to it. lol

 

[Alexr25]

Guide numbers are used by speedlights to indicate the total light output of the flash tube/reflector combination.
Studio flash units are are designed to be used with many different reflectors and light modifiers so are generally not rated by guide number but rather by the energy output capacity of the flash head. The unit of energy is the Joule or the Watt second, both terms are used to interchangeably to rate studio flash units since 1 Joule = 1 Watt second.

 

[andytakeone]

I see, thanks everyone!

I'm surprised I haven't heard that term before, you'd think people would mention something like that a bit more in their tutorials. I'll definitely read up on it.

Thanks.

 

[photo1x1.com]

That is a highly interesting topic. I always wanted to compare the light output of speedlights to the light output of "studio" flash and see if I can come up with some principles - because it is said they are not to compare. I really need to make a video about that in my photography course. So thanks - I´ve already written down some notes right now.

Back to your questions:

Is there a universal measure for flashes?

You already received answers above. There is a the guide number for speedlights and Watt seconds (WS) for "studio" flash.

So that you can, say, compare 3 different flashes outputting at 1/1 full power, yet still know how much output you're getting from each.

Not really. Most speedlights have zoom settings. If you use a longer focal length on your camera, the speedlights modify their reflectors automatically to "focus" the light and get a longer reach. So all you could do is compare them at the same focal length settings. Most manufacturers specify their Guide Number for a certain zoom setting (for example: GN40 [at 105mm zoom head position] or GN27 [at 35mm zoom head position])

What the guide number actually tells you is how far the light of your flash will reach (when the camera is set to ISO 100!!!).
A guide number (GN) of 100 means that the flash will light your subject well in 100m distance at an aperture setting of f1.0 (!!!)
The forumla is: GN = distance × f-number
So you would get correct flash exposure for:

GN100 = 50m x f2.0
GN50 = 25m x f2.0
GN100 = 10m x f10
GN50 = 5m x f10

guide numbers can be given in feet or meter!!!

Conclusion: you can compare the max output of different speedlights if you find a source with the GN at the same zoom settings. BUT: not even the manufacturers list the same zoom settings for different products on their websites:
Canon 600EX II has GN60 (at 200mm)
Canon 430EX III has GN43 (at 105mm)
Nikon SB-910 AF has GN34 (at 35mm)
Nikon SB-5000 AF has GN34.5 (at 35mm) AND they also list GN55 (at 200mm)
Nikon SB-500 AF has GN24 (they don´t even state the zoom on their website!!!)
Nissin i60A has GN 60 (at 200mm)
Nissin Di866II GN 60 (at 105mm)

On a sidenote: I myself am always a little puzzled by the guide number formula, because an aperture setting of f2.0 actually only lets 1/4 of the light onto your sensor compared to f1.0 (the guide number formula would assume it is 1/2 instead). Maybe somebody else can clear this up for me.

 

[tirediron]

Guide numbers are used by speedlights to indicate the total light output of the flash tube/reflector combination.
Studio flash units are are designed to be used with many different reflectors and light modifiers so are generally not rated by guide number but rather by the energy output capacity of the flash head. The unit of energy is the Joule or the Watt second, both terms are used to interchangeably to rate studio flash units since 1 Joule = 1 Watt second.

Guide numbers are used primarily as a means of determining exposure with strobed light, whether speedlight or studio light. You're right that studio lights (both monolights and pack & head systems) use a wide range of modifiers which can affect their output, but almost all are rated by both w/s and GN. If you read the fine print, they will express the GN [normally] using a standard reflector.

 

[photo1x1.com]

Guide numbers are used by speedlights to indicate the total light output of the flash tube/reflector combination.
Studio flash units are are designed to be used with many different reflectors and light modifiers so are generally not rated by guide number but rather by the energy output capacity of the flash head. The unit of energy is the Joule or the Watt second, both terms are used to interchangeably to rate studio flash units since 1 Joule = 1 Watt second.

Guide numbers are used primarily as a means of determining exposure with strobed light, whether speedlight or studio light. You're right that studio lights (both monolights and pack & head systems) use a wide range of modifiers which can affect their output, but almost all are rated by both w/s and GN. If you read the fine print, they will express the GN [normally] using a standard reflector.

I use mainly Elinchrom for studio flash gear, they rate my 500WS studioflash: F-Stop (1m, 100 ISO, reflector 48°) 64.8
According to the Guide Number Formula that would be a GN of 64.8

BUT: they rate a studioflash of the same series with half the power output (250WS) at 45.8

According to the formula for GN, the 250WS head has a reach of 45.8m (at f1.0) - so doubling that output would mean 91.6m. The weird thing is - the flashhead with double the light output (500WS) only has a reach of 64.8m.

So for my understanding (and according to the inverse square law) the formula for the guide number is just plain wrong. Maybe user Ysarex, or somebody else can correct me?

Sorry for hijacking your thread, but since you said you´re going to read up on it, I thought why not discuss that right here.

 

[andytakeone]

Yes!

Thanks for the continued discussion. This is very interesting, I'm glad you brought more information to the table beyond the basics.

 

[Alexr25]

I use mainly Elinchrom for studio flash gear, they rate my 500WS studioflash: F-Stop (1m, 100 ISO, reflector 48°) 64.8
According to the Guide Number Formula that would be a GN of 64.8

BUT: they rate a studioflash of the same series with half the power output (250WS) at 45.8

Yes that's right, halving the power reduces the output by 1 stop so the aperture needs to be open by 1 stop (a factor of 1/square_root of 2) and hence the guide number goes down to 45.8 (64.8/square-root2).

So for my understanding (and according to the inverse square law) the formula for the guide number is just plain wrong. Maybe user Ysarex, or somebody else can correct me?

It's the inverse SQUARE law, you are forgetting about the square part. The ratio of distance is proportional to the square root of the ratio of intensity.

 

[fmw]

In the film days we used flash meters to help determine exposure in the studio. These days you can make a guess, make an exposure and adjust based on the resultant image. With portable flash, we normally have automated TTL exposure. Hardly anyone actually uses the guide number any longer. The guide number can provide input for a buying decision but you aren't likely to use it to determine exposure.

 

[tirediron]

In the film days we used flash meters to help determine exposure in the studio. These days you can make a guess, make an exposure and adjust based on the resultant image. With portable flash, we normally have automated TTL exposure. Hardly anyone actually uses the guide number any longer. The guide number can provide input for a buying decision but you aren't likely to use it to determine exposure.

I must be old-school. I have GNs for all my lights and I used them to place the lights (roughly) and then a flash meter to dial in exposure. This saves me a lot of time and rarely, if ever do I do more than one or two test exposures.

 

[photo1x1.com]

I use mainly Elinchrom for studio flash gear, they rate my 500WS studioflash: F-Stop (1m, 100 ISO, reflector 48°) 64.8
According to the Guide Number Formula that would be a GN of 64.8

BUT: they rate a studioflash of the same series with half the power output (250WS) at 45.8

Yes that's right, halving the power reduces the output by 1 stop so the aperture needs to be open by 1 stop (a factor of 1/square_root of 2) and hence the guide number goes down to 45.8 (64.8/square-root2).

So for my understanding (and according to the inverse square law) the formula for the guide number is just plain wrong. Maybe user Ysarex, or somebody else can correct me?

It's the inverse SQUARE law, you are forgetting about the square part. The ratio of distance is proportional to the square root of the ratio of intensity.

Thanks a lot for your input. The first part has always been clear, but why I made that second mistake is beyond me. All I know is that I know nothing

 

[astroNikon]

This is one reason I don't like to mix flash brands and prefer my OEM especially with multi-flash setups and multi-flashes per group controlled by one remote setting.

there was a thread a while back about this and how the cheaper 3rd party flashes really aren't accurate in their GN numbers.

 

[tirediron]

... 3rd party flashes really aren't accurate in their GN numbers.

Very common; some are off by as much as 50%