GN Rating for Speedlights

[TCampbell]

The guide number (which is listed in either meters or feet) is not an indication of the "power" of the light (that's usually in watt/seconds)... rather it's the distance that the flash can adequately illuminate a subject with the assumption that the camera is using ISO 100 an aperture of f/1.0.

Realistically nobody makes an f/1.0 lens (they do... they're REALLY hard to find. Canon used to make a 50mm f/1.0 lens but hasn't made that thing in about 15 years I'm guessing.)

So why f/1.0? Because it makes the math easy.

You take the guide number and divide it by the f-stop you will use in the shot.

e.g. if you plan to shoot at ISO 100 and f/5.6 then divide the guide number by 5.6.

58 meters ÷ 5.6 = 10.4 meters (I've rounded that... the real value has a lot of digits after the decimal).

10.4 meters, when converted to feet, is 34 feet.

But at this point we've just converted the base f-stop (f/1.0) to our actual f-stop (in our example, f/5.6) with a simple division problem. But we haven't converted the ISO. Each time you double the ISO by 1 stop, you can increase the distance the flash will carry by a factor of about 1.4. E.g. if we switch to ISO 200 our 34 feet becomes 34 x 1.4 = 47'. If we go up 2 stops (ISO 400) then we can double our distance to arrive at 68' (when you take the square root of a number... and then you square it... you end up with your original number again. So the √2^2 = 2... hence 2 stops means we get to multiply by 2 instead of the 1.4 we use when it's just 1 stop.)

But remember that the guide number is not an indicator of how bright the flash is... instead it describes how far it will carry.

Imagine if you disassemble a flashlight and take out the light bulb and batteries but leave the reflector housing behind. When you turn on that light bulb, it won't be very bright. It's really the same amount of light leaving the bulb, but the light is scattering in every direction. When you put the bulb in the reflective housing, all the light that would have gone backwards or sideways ends up hitting the reflector and bouncing forward -- all the light is redirected so that it shines in the same direction (more or less) and the bulb "seems" very bright.

Photographic flashes work the same way in that we "modify" the light using various tools... reflectors, diffusers, etc. A high quality parabolic reflector can focus light so well as to send it to astonishing distances. Diffusers can "eat" a lot of light and really shorten the distance. When you "bounce" a flash off a painted white ceiling (which is typically a "flat" white paint... not a "glossy" paint) you lose a lot of light.

Guide numbers around 58 meters are typically of high-end flagship flashes from Canon & Nikon. There are more powerful (and not surprisingly more expensive) flashes. The speedlite type flashes are great because of their portability and while being fairly versatile, there's a reason they still make studio flashes and somewhat portable lighting that aren't really "speedlites".

The question is... what does the flash need to do for you? Speedlites are popular for "event" photography because of their mobility. High end photographers are still running with multiple lights (and assistances are holding these lights on mono-pods poles) so they are still "controlling" the lighting.

If you REALLY want to get correct flash exposures... you may want to invest in an incident light meter which also supports flash exposures. My first meter was a Sekonic L-308s. It's a little over $200, but it can meter "flash" as well as continuous light. When you put it in flash metering mode, it starts monitoring the light but as it knows it's looking for a flash, it looks for a bright spike in the amount of light and registers this as a flash (it is smart enough to know that when using TTL systems that there is likely to be a pre-flash. It's basically looking for that final spike in light and displaying the metered value during the spike.

This means you can put the meter at your subject position, use all the bounces, reflectors, diffusers, or any light modifiers you want, and the meter will tell you how much of your light ACTUALLY ARRIVED at your subject. You can also accurately tell what your modifiers are doing to your light (how much light do I lose because I put my flash into a soft-box instead of firing straight unfiltered light?) Knowing these (these are tests you do in your studio) tells you facts about your gear so that when you're out in the field, you know much light that diffuser eats (because you tested it in your studio (my old boss used to label the modifiers so we knew how much light that "ate" when we used them.))

 

[WayneF]

But remember that the guide number is not an indicator of how bright the flash is... instead it describes how far it will carry.

Partly true. It is defined that way. Power (actually energy) is in watt seconds, and GN is in feet or meters. There is no correlation between these. No way to know one from the other.

We don't know illumination from energy, things can vary.

But of course, GN is exactly about how bright the flash is (what direct flash will actually meter), in a specified situation (distance or aperture, zoom, ISO, power level). Increase GN and the flash gets brighter. If we do it by zooming in, the field gets smaller (concentrated beam, of same energy, but a brighter concentrated beam). When we add modifiers (umbrellas, softboxes, grids, domes, whatever), then we no longer know GN.

But we certainly can compare relative power of two flashes from the guide numbers - but only if at the same ISO and zoom factor. Zoom of course meaning what field of view is covered by the flash. We can only compare equal coverages. If one flash is lighting a small spot on the wall, and one is lighting the entire wall, and we cannot compare those that way. A wall requires much more energy than a spot on the wall. But same coverages, we can compare.

If one flash doubles Guide Number (at same ISO and zoom), that is 2.0 stops more power.
If it multiplies GN by 1.414, that is 1.0 stops more power.
(I am saying power, which is actually exposure, illumination per unit of area, from a concentrated beam of energy which I will call power. ).

This is why Guide Number is published, to give expectations what the flash will do.
We just need to know what coverage situation their GN applies to.

Handy calculator for odd values at bottom of page at Compare Power Rating of Flashes with Guide Numbers

Since it is about the square root of two, the same calculator works for inverse square law (distance differences)
or lens f/stop differences.

 

[Derrel]

Here is a flatbed scanner image made off of the side of a really small Speedotron Brown Line D202 model, showing the pack's power distribution chart, along with Guide Number information for three different power output levels when used with the standard reflectors.

As one can see, at full power, the 200 Watt-second power output setting when used with one flash head with its standard reflector yields a Guide Number of 190, which means that at 10 feet, the correct exposure for ISO 100 would be 190 divided by 10, or f/19; at 100 Watt-seconds, the proper f/stop for a 10-foot distance would be 135 divided by 10, or f/13.5; at 50 Watt-seconds, the 10-foot exposure would be 95 divided by 10, or f/9.5.

 

[WayneF]

As one can see, at full power, the 200 Watt-second power output setting when used with one flash head with its standard reflector yields a Guide Number of 190, which means that at 10 feet, the correct exposure for ISO 100 would be 190 divided by 10, or f/19; at 100 Watt-seconds, the proper f/stop for a 10-foot distance would be 135 divided by 10, or f/13.5; at 50 Watt-seconds, the 10-foot exposure would be 95 divided by 10, or f/9.5.

Not sure of your point, but that is not a universal conversion of WS to GN. It is Not generally applicable to other units.

We don't know conversion efficiency, electrical to lumens. Power packs are a little lower efficiency than monolights, due to the resistance loss of the long thin wires to the light heads. Not that it matters, we probably don't know other efficiencies either. Final GN is what matters, what it can actually do. Except studio lights are normally used with other modifiers anyway.

And we don't know (from this) about the reflector, and its beam angle, so we cannot compare its GN to anything else with equivalent coverage. We don't know coverage. We don't know what its GN means.

 

[Derrel]

I just posted the power pack and its information simply as a point of reference, as an illustration of the way Guide Number information is presented by one flash manufacturer, as a way for users of that equipment to calculate their exposures, using a specific set of three different power levels with three different, real-world Guide Numbers.

I never said ANYTHING about "universal conversion of Watt-seconds to GN"--that's something you brought up, for some oddball reason. I just provided some numbers; and as you stated, you are obviously not sure of much behind the data I posted.

I posted some simple examples of Watt-seconds, at 200, 100, and 50 Watt-seconds, and the corresponding Guide Numbers for each power level. Pretty simple really. I presented some simple data, and then described how to do the rudimentary math to arrive at the needed f/stop for each of the three different power levels/Guide number values. Almost self-explanatory...

The standard reflectors in use when that pack was shipped were the so-called 11.5-inch 50-degree grid reflector.

 

[Derrel]

Here's a scan I made off of an old flashbulb reflector that I scrapped. I made this so people can print this out, then cut the center circle out and then glue that to a rounded piece of paper, and then paste the larger-diameter outer area onto a second piece of paper, and then put a brad through the center of the two "dials", to make a simple rotating calculator.

As one can see, when I made the scan, I set the calculator dial to a Guide Number of 110, in Feet; that is evident if one looks at the Distance in Feet scale, and notes that f/11 is directly opposite the 10 foot distance.

I suppose there are many younger people who have never seen such a simple calculator as this; this one dates to the late 1950's.

 

[photoguy99]

But that would be copyright infringement!

 

[TCampbell]

But remember that the guide number is not an indicator of how bright the flash is... instead it describes how far it will carry.

Partly true. It is defined that way. Power (actually energy) is in watt seconds, and GN is in feet or meters. There is no correlation between these. No way to know one from the other.

We don't know illumination from energy, things can vary.

But of course, GN is exactly about how bright the flash is (what direct flash will actually meter), in a specified situation (distance or aperture, zoom, ISO, power level). Increase GN and the flash gets brighter. If we do it by zooming in, the field gets smaller (concentrated beam, of same energy, but a brighter concentrated beam). When we add modifiers (umbrellas, softboxes, grids, domes, whatever), then we no longer know GN.

The source of light could be brighter but isn't necessarily brighter ... if the beam is focused then the same light carries much farther as compared to a beam which is allowed to spread.

A true "parabolic reflector" has a focus. If the light is positioned at that "focus" point then all points on the reflector will bounce the light forward in a parallel path. For the most part... the beam doesn't "spread out" as it travels forward. You can change the reflector and thus change the angle of spread and massively change the guide number of the flash... without changing the power output of the light bulb at all.

When you do this (change the reflector to focus the spread of the beam) you are actually changing the guide number.

The guide number is the net-effect of the light, reflectors, and anything intended to diffuse or scatter the light. When a manufacturer of a speedite states a guide-number, they are referring to the maximum distance of the flash with the reflector focusing the beam as narrowly as possible, no other modifiers in the path (straight beam), and flash at max power.

If you apply modifiers to the light, then you change the guide number. This effect is directly measurable and can be expressed mathematically. It is not subjective.

 

[WayneF]

I just posted the power pack and its information simply as a point of reference, as an illustration of the way Guide Number information is presented by one flash manufacturer, as a way for users of that equipment to calculate their exposures, using a specific set of three different power levels with three different, real-world Guide Numbers.

I never said ANYTHING about "universal conversion of Watt-seconds to GN"--that's something you brought up, for some oddball reason. I just provided some numbers; and as you stated, you are obviously not sure of much behind the data I posted.

I posted some simple examples of Watt-seconds, at 200, 100, and 50 Watt-seconds, and the corresponding Guide Numbers for each power level. Pretty simple really. I presented some simple data, and then described how to do the rudimentary math to arrive at the needed f/stop for each of the three different power levels/Guide number values. Almost self-explanatory...

The standard reflectors in use when that pack was shipped were the so-called 11.5-inch 50-degree grid reflector.

OK, sorry, I did not know your purpose. I had just said there was no conversion between watt seconds and guide number, so my paranoia thought you were saying "oh yeah? Here is one." So it was my problem then, I didn't understand it was just an example of guide numbers.

Yeah, 50 degree reflectors on studio lights are rather strong. Alienbees offers 50 degrees optionally, but their standard AB reflector is 80 degrees (will fill an reflected umbrella). With the same WS energy, their 50 degrees is concentrated narrow to be two stops stronger (4x) than 80 degrees, because 80 degrees lights up so much more scene width.

We have seen a lot of laughable mistakes about comparing guide numbers. We can compare, and it is useful to compare, but we can only compare GN at the same field of coverage, meaning for speedights, same zoom. Not everyone seems to know this, which must be why speedlight manufacturers seem to specify their highest number. There is a site online saying with good intentions that a SB-800 is more powerful than a Alienbees B400 (due to confusing GN & reflectors), but which happens to meter a good stop stronger than a SB-800 (in the same umbrella). Watt seconds is 1/2 CV², and SB-800 computes 75 watt seconds, and B400 is rated 160 watt seconds. That is a stop stronger.

Guide Numbers have been around at least since flash bulbs. I don't know who gets credit, but the purpose of guide numbers is to simplify the inverse square law, determining exposure for any distance using just simple division. Makes it real simple, and GN used to be printed on every flash bulb package. Still very useful (for direct flash), but strangely, GN sure seem to have a lot of mystery about them.

 

[WayneF]

The source of light could be brighter but isn't necessarily brighter ... if the beam is focused then the same light carries much farther as compared to a beam which is allowed to spread.

A true "parabolic reflector" has a focus. If the light is positioned at that "focus" point then all points on the reflector will bounce the light forward in a parallel path. For the most part... the beam doesn't "spread out" as it travels forward. You can change the reflector and thus change the angle of spread and massively change the guide number of the flash... without changing the power output of the light bulb at all.

This is going to be one of those?

I am still back on this:

But remember that the guide number is not an indicator of how bright the flash is... instead it describes how far it will carry.

GN 80 means stand at ten feet, and expect to meter f/8. That is exactly how bright it is, and how to use that brightness.

Either metering or using Guide Number is our only means of knowing exactly how bright it is.

We can make it brighter by increasing the power, or by concentrating the same power into a more narrow beam, and the GN goes higher, but that is then still exactly how bright it is.

It does not have specific meaning about how far it will go.

GN 80, stand at 1000 feet, and meter f/0.08.

 

[TCampbell]

The source of light could be brighter but isn't necessarily brighter ... if the beam is focused then the same light carries much farther as compared to a beam which is allowed to spread.

A true "parabolic reflector" has a focus. If the light is positioned at that "focus" point then all points on the reflector will bounce the light forward in a parallel path. For the most part... the beam doesn't "spread out" as it travels forward. You can change the reflector and thus change the angle of spread and massively change the guide number of the flash... without changing the power output of the light bulb at all.

This is going to be one of those?

I am still back on this:

But remember that the guide number is not an indicator of how bright the flash is... instead it describes how far it will carry.

GN 80 means stand at ten feet, and expect to meter f/8. That is exactly how bright it is, and how to use that brightness.

Either metering or using Guide Number is our only means of knowing exactly how bright it is.

We can make it brighter by increasing the power, or by concentrating the same power into a more narrow beam, and the GN goes higher, but that is then still exactly how bright it is.

It does not have specific meaning about how far it will go.

GN 80, stand at 1000 feet, and meter f/0.08.

Your flash (and everybody else's for that matter) follows the Inverse-Square law. See: Inverse-square law - Wikipedia, the free encyclopedia
This is a law of physics. At no point will it ever be wrong. This is a very important law to know when shooting flash photography. If you don't know how it works, then you're "guessing" whenever you do flash photography.

I bring this up because the reason the inverse-square law works is BECAUSE of how light spreads as the distance increases. But notice that the inverse-square law doesn't apply to absolutely distances... but rather it applies to relative distances. If I have some amount of light at 10' and I move the light to 14' then I will get half as much light per unit area on my subject. Likewise if I was at 100' and I move to 140' (really 141 because I'm rounding) then I'd ALSO get half as much light.

When a speedite manufacturer claims a specific guide number, they are grounding the concept of the inverse-square law to a specific model light with a specific reflector and it's all tuned for the maximum distance they can milk out of it. They're also assuming you won't use any other modifiers. If you change any of these conditions then you changed the guide number.

The guide number is the distance that a flash can adequate illuminate a subject assuming ISO 100 and f/1.0.

The fact that you can then DERIVE a flash's guide number by standing at 10' is a nice way to test a flash or determine it's guide number when you don't already know what the guide number is. But it's not practical for regular shooting (not conducting a test) because it's unlikely that you'll always use the flash at 10' distances.

But lets visit your 10' f/8 example. If you MULTIPLY your 10' by "8" (because your example used f/8) then you'd get... 80'. And "80" is your guide number. See how that works?

So I stand by my original point... the reason for the guide number baseline is because it makes the math easy to apply in any situation. I can take the guide number and divide it by the f-stop I'd like to shoot and the result is the new distance at which you'll get proper illumination for the shot.

Example:
If my flash has a guide number of 69 meters (197') and I'd like to shoot a subject 25' away, I can divide 197 by 25 and arrive at 7.88. That means that I can now dial my aperture to f/7.9 (or the nearest fraction of a stop that your camera will allow) and get a correct exposure (in this case I'd use f/8).

You don't need to use a 10' test for anything. This is because the old 10' guideline is a great way to find a guide number if you don't already know what the guide number is. Also... if you use a light modifier and want to determine how much light it's changing the light... this would be a good way to test how much it's changing the light. When I buy a light modifier, I do a test with and without the modifier to determine how much light it eats (that white fabric in a shoot-through umbrella or on the front panel of a soft-box is eating a bit of light and you'd want to know how much light you lose (usually measured in stops) by using it.)

My flashes (guide number 60 meters) have reflectors that zoom to 200mm and that's good for just about 200' at f/1 (197' if you want to be fussy about it). But that also constrains the spread of the light to throw it that far such that I'll get even illumination when shooting with a 200mm lens. A 200mm lens creates a circular image just 12.3º across on a full-frame DSLR. Now suppose I use a 50mm lens to shoot a subject at 10'. I can't use the flash with the reflector set to the same position and get even illumination because NOW my angle of view has increased to about 47º... just shy of 4 times wider. I would end up with a "hot spot" in the middle of my image. I need to back off the reflector to allow for a wider spread of light. As soon as I do that... I just changed the guide number of the flash. That change is measurable (objectively measurable... not subjective.)

But if you already know the guide number of the flash and you aren't using modifiers, then you can use the original rule... which is that the guide number is the distance at which the light can adequately illuminate a subject for an exposure assuming ISO 100 and f/1.0 -- then you can calculate the f-stop for any other distance (or if you want to change the f-stop you can calculate the new subject distance... of course the math works both ways.)

 

[WayneF]

The guide number is the distance that a flash can adequate illuminate a subject assuming ISO 100 and f/1.0.

But if you already know the guide number of the flash and you aren't using modifiers, then you can use the original rule... which is that the guide number is the distance at which the light can adequately illuminate a subject for an exposure assuming ISO 100 and f/1.0 -- then you can calculate the f-stop for any other distance (or if you want to change the f-stop you can calculate the new subject distance... of course the math works both ways.)

What is this f/1.0 business?

f/1.0 means nothing to Guide Number definition, it is just another aperture.

If f/8 at 10 feet, or f/4 at 20 feet, or f/16 at 5 feet, is the proper exposure, then the GN is 80. No f/1.0 about it.

But yes, f/1.0 at 80 feet would also be GN 80. I suppose you are making up your own definition from this numerical concept involving the number 1, but f/1.0 seems mighty impractical, and a confusion. No one else does that. Absolutely no need of it.

guide number - Google Search

No link on that first search page mentions f/1.0 once.

 

[photoguy99]

I thought the use of f/1.0 was quite slick. It takes away all the confusing rubbish from the definition and make it an easy consequence of the now very simple and easy to grasp definition.

That's the hallmark of an excellent definition.

 

[Yzerwing]

Given the debate going on here I guess it wasn't as dumb a question as I originally thought. I have to admit some of this went over my head, but all in all I learned that if I use TTL mode most of it won't matter. When I want to manually control the flash I will have to revisit this thread.

Thanks much and keep up the debate its way more interesting than politics!

 

[beachrat]

The guide number is the distance that a flash can adequate illuminate a subject assuming ISO 100 and f/1.0.

But if you already know the guide number of the flash and you aren't using modifiers, then you can use the original rule... which is that the guide number is the distance at which the light can adequately illuminate a subject for an exposure assuming ISO 100 and f/1.0 -- then you can calculate the f-stop for any other distance (or if you want to change the f-stop you can calculate the new subject distance... of course the math works both ways.)

What is this f/1.0 business?

f/1.0 means nothing to Guide Number definition, it is just another aperture.

If f/8 at 10 feet, or f/4 at 20 feet, or f/16 at 5 feet, is the proper exposure, then the GN is 80. No f/1.0 about it.

But yes, f/1.0 at 80 feet would also be GN 80. I suppose you are making up your own definition from this numerical concept involving the number 1, but f/1.0 seems mighty impractical, and a confusion. No one else does that. Absolutely no need of it.

guide number - Google Search

No link on that first search page mentions f/1.0 once.

It might not mention it on that page,but a flash guide number was explained to me over 30 years ago exactly the way TCampbell described it.
f/1 and all, which made it very easy for me to understand.