GN Rating for Speedlights

[Yzerwing]

Hi fellow Lens Heads,

I have been unable to find a good explanation of the GN rating of a Speedlight and how to calculate it. While I have found some definitions of it I still don't totally understand it.

Could someone please enlighten me on this seemingly difficult subject?

Thankyou all for your cooperation and your patience.

 

[tirediron]

As good as any explanation of which I am aware... As for calculating it, do you mean how to determine exposure based on guide number, or how to actually calculate a strobe's GN? Calculating exposure is very simple: GN = distance × f-number. As for how to determine the actual GN of a speedlight? Don't know, and to be honest, don't much care.

 

[photoguy99]

One doesn't really calculate it. It's a manufacturer specification.

First things first. If the light is farther from the subject, it does a less good job lighting the model, right? So you'd need to open up your aperture to compensate. Conversely if you move the light closer, it pushes more light onto the subject, so you can stop down.

So the farther the distance, the smaller the f number and vice versa.

It turns out that these two numbers change in mathematical-step. So the product of them is constant. F number times distance is always the same, for a given flash power.

Now, you got to pick a unit for distance. Some people use feet some meters. Also you got to pick an ISO. Everyone uses 100.

The number you get by multiplying any valid f number with the distance that goes with it, for a specific flash power, is the guide number. Any distance and f number that produce the same number will work and give you proper exposure.

Bigger numbers are more power.

For automatic flash modes you can consider the rated guide number to be a max.

If the guide number is 50 in meters, then you may not stop down below f/10 when the light is 5 meters away from the subject, for example.

 

[Yzerwing]

So is this important to know and understand with TTL capable flashes? I have been chasing the whole speedlight thing around for a few weeks and am confusing myself on the subject.

 

[photoguy99]

No. It will only give you a sense of maximum per available. In pretty useful terms, but you can probably get there in ten minutes of hands on filling around as well.

Bigger is more power.

Twice as big means you can have the light twice as far away from the subject.

 

[Derrel]

To calculate the GN of your speedlight, use this decades-old, proven method: shoot test photos at precisely 10 feet of a subject with small cards showing the exact f/stop in use written on the card in pen, so there is no doubt later as to what exact f/stop was used for each test shot. Use ISO 100 on the camera. If the "best" exposure happens to be f/11 at 10 feet,then multiply the f/numer, 11, by ten, and get 110, or....the GN, in Feet of 110, at ISO 100.

If the best exposure was obtained at an aperture setting of f/16 at 10 feet, you'd have a powerful flash, like a handle-mount Metz, with a GN in Feet of 160. If f/8 produces the best exposure at 10 feet with ISO 100 in use, then the GN is 8 x10, or 80.

The GN is stated in the USA in Feet, and at 100 ISO. In much of the world the GN appears lower simply because it is specified in Meters.

A GN is for average subjects in an average room, with average reflectance, such as for example, inside a normal home or business. Outdoors, there is a huge amount of lost light that is not reflected by ceilings and walls, so outdoors GN's can be lower.

A GN is also quite dependent upon the beam spread of the flash; at wide-angle beam spreads, GN is lower; with a more-narrow beam spread, GN will go up.

 

[480sparky41 40]

............ If the light is farther from the subject, it does a less good job lighting the model, right?...........

Not necessarily. Many times lights are moved in order to adjust the amount of light striking the subject...... it doesn't mean the job is not as good.

 

[Yzerwing]

So..... if I needed a farther reach out of my speedlight at f 5.8 than I would increase my ISO to 200?

Everything stated so far is all based on ISO 100 what effect does doubling it have?

 

[480sparky41 40]

Doubling your ISO would gain you 1.4x more distance.

Or, in terms of your GN, it would increase by a factor of 1.4x.

 

[wfooshee]

Same as any exposure. Doubling the ISO adds one stop of exposure, the same as opening the aperture one stop, just like it does in non-flash photography. When you have a guide number at ISO 100, you get a higher guide number at ISO 200. The guide number doesn't double, though, it's multiplied by 1.4 (the square root of 2.) Light from a flash falls off as the square of the distance increase, so doubling the exposure sensitivity (as in doubling the ISO or going from f:5.6 to f:4) means you can move the light back by the-square-root-of-two times your distance to use the same power.

With TTL, doubling your ISO doesn't mean you'll get a brighter flash picture, it means your flash will use less power to reach the metered exposure. It does mean that you get more reach with your flash at a given aperture, though, as you asked. You get 1.4 times the reach for every doubling of exposure, either by ISO or aperture.

TTL-metered auto-flash knows about everything going on in the camera. If your aperture is at a certain setting and your ISO is at a certain setting, the flash is metered with those values in mind. Any adjustment that reduces exposure (lower ISO or smaller aperture) causes the flash to fire with more power. Going the other way causes the flash to fire with less power.

When using TTL flash, there's really no need to worry about guide number other than knowing that higher comes from a more powerful strobe. The guide number is useful in manual flash exposure when you want to control everything. Using what you want for aperture, distance, and ISO, the guide number lets you calculate the flash power needed. That's really more useful in multi-light setups, where you want to balance lights against each other.

 

[WayneF]

So..... if I needed a farther reach out of my speedlight at f 5.8 than I would increase my ISO to 200?

Farther reach, is either greater ISO or a wider aperture, or a bigger flash.

Re Guide Numbers: Four Flash Photography Basics we must know - Understanding Guide Numbers

 

[Yzerwing]

What is considered to be a decent GN? I am looking at a flash with a GN 58 is that good or bad? I don't have anything to compare it to. Price is an object as I am on a budget.

 

[WayneF]

GN 58 is probably (commonly) in units for meters, and probably at maximum zoom like 105mm zoom. GN at say 24mm zoom is probably about half of that. 58 probably implies a full power flash, respectable performance. Normally for ISO 100. For GN in feet, multiply GN (meters) by 3.28.

But GN has to be described better for us to know what it means. Marketing likes to tell us the one highest maximum number, but 1) we need to know for which zoom value it is defined at 58, and 2) units meters or feet, and 3) what does it do at 24mm zoom? We probably rarely use it at 105mm.

The flash manual surely has a GN chart in it that will tell us what we need to know (for all zoom settings, and at all power levels). It is the only way to know. Often the manual can be downloaded.

I would suggest Four Flash Photography Basics we must know - Understanding Guide Numbers

and Compare Power Rating of Flashes with Guide Numbers

and maybe Beginners Guide to Select a Hot Shoe Flash

 

[photoguy99]

58 in meters is a pretty beefy hotshoe flash. 58 in feet is on the weak side.

 

[Yzerwing]

Thank you all I feel a little more informed about what I am getting in to.