Universal measure for flashes?

[KmH]

I didn't see it mentioned that when wanting to compare the GN number of different hot shoe flash units we need to look closely at the numbers the flash unit maker uses to state the GN output of their device. As pointed out, we also have to consider the flash unit maker too if we are considering GN numbers for 3rd party units.

For instance Nikon's gives two GN numbers for their SB-900 flash unit. One for ISO 100 and the other for ISO 200 - 34/111.5 (ISO 100, m/ft), 48/157.5 (ISO 200, m/ft) - and the additional information of zoom focal length, frame size, and unit temperature - at 35 mm zoom position, in FX-format, standard illumination pattern, 20°C/68°F.
If you're outside on a summer day and the unit is at 90°F your flash in unit will not deliver 34/111.5 (ISO 100, m/ft), 48/157.5 (ISO 200, m/ft).

The bottom line is that as a hot shoe flash unit is zoomed, the power level is changed, the air temperature changes, or the camera ISO is changed, the GN number also changes.
Put another way, because of the nature of light there is no way a single unchanging number can be used to tell us in advance how much light a hot shoe flash unit will produce for any given set of settings.

Note that the good hand held electronic light meters report the intensity of light from a flash unit (hot shoe or studio monolight) in 1/10th stop increments.
Using a hand held flash meter is the best way to measure how much light from a hot shoe flash unit is falling on your subject.

 

[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.

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.

Sure, that is a little old school, I think. For me it is easier to make adjustments from an actual image. I can make an exposure, display it on the 25" monitor and work from there. I haven't used a guide number or flash meter for a decade. I sold my flash meter years ago. In the film days we couldn't do things this way because we couldn't display an image instantly on a monitor. Even a polaroid took a while to handle and cost money.

My avatar, by the way, was a polaroid. I liked the lighting so I used it to shoot the subject. It was the only image of myself I could find to put on the forum.

 

[Bebulamar]

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.

Doubling the light output doesn't double the distance but only increase the distance by the square root of 2 which is around 1.414. And that is the inverse square law. See the word square?

 

[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.

Doubling the light output doesn't double the distance but only increase the distance by the square root of 2 which is around 1.414. And that is the inverse square law. See the word square?

Thanks for your input, Alexr25 already pointed that out.
I found out why I made that mistake for such a long time. In german, there is the syno "das Licht nimmt mit dem Quadrat der Entfernung ab". That means light intensity is reduced with the square distance. That is what I was taught whan I learned photography. That sentence is misleading at best, because actually it is inversed. Not the distance is squared, but the light loss. Another example that it is best to always question what you get taught. Shame on me, that I did not