Flash intensity falls off with distance. Guide Number is a numerical method used to determine exposure of direct flash with manual flash power levels, to automatically deal with the Inverse Square Law, making the math be trivial. There are also other ways, today we might use a handheld flash meter, or metered TTL, or just trial and error works well with the digital LCD preview. But in the old days (including flash bulbs), guide number was all we had. The concept is still valid and useful, and is still a fundamental for understanding flash. Guide Number is about dealing with the Inverse Square Law.
The full details explaining guide numbers are below the Guide Number Calculator.
There are a few special features in the calculator here. One is that the marked camera settings are nominal numbers (the markings are rounded approximate numbers, not exactly the real numbers the camera actually uses). One example is f/11 ... we say f/11, but the math is actually f/11.314. This is a small difference, only about 0.08 stop, but it blurs calculations, prevents best precision. Same thing with ISO, we see ISO 1250, but another line in the Exif tells us it used 1270 (to be 1/3 stop from 1600). Shutter speeds too, 1/60 second is actually 1/64 second (see more). Any discrepancy is only in the rounded markings, and only in our mind, since the camera always knows to do it accurately. Computing GN manually still works well enough of course, because the camera only accepts third stop values, so 0.08 is not a big deal. We always have to choose the closest third stop, which could be up to 1/6 stop off (± 0.167 EV) anyway.
But this calculator also uses the precise values internally (same values as the camera uses). That's a small difference, but using the exact values allows more precision in the EV calculation (f/8 to f/11 is expected to compute as 1.00 stop difference, and now it actually does). I hope this is interesting rather than a confusion.
There are a couple of ways to use the calculator.
Insufficient flash exposure is corrected with higher guide number, wider aperture, or more flash power, or higher ISO, or shorter distance.
Excessive flash exposure is corrected with lower guide number, stopped down aperture, or less flash power, or lower ISO, or longer distance.
Guide Number (GN) is a prime fundamental, related to Inverse Square Law, and is about how light works, something to know, which will always be important to know. The light fall off means that direct flash exposure can be correct at only one specific distance from the flash. Anything closer is brighter, and anything farther is darker. But how much it changes works on sort of an exaggerated percentage basis (inverse square law), and a greater distance simply has more middle ground range. Bounce flash can seem to extend this range, but direct flash exposure falls off with the square of the distance. Flash will be two stops underexposed at twice the distance, or two stops overexposed at half the distance (inverse square law). So the general rule for flash is to keep all of your subject parts near the same distance plane from the lights (same idea as focus depth of field).
Photographing groups: Same distance plane is impossible for multiple rows, and multiple rows can be a deep zone for an even flash illumination, or for focus depth of field too. For long rows, curving the ends forward to equalize the distance helps. For large groups of a few rows deep, raising camera and flash height dramatically (with step ladder) to look down into the group can minimize difference of row distances (and won't hide faces with rows in front). Greater flash distance can extend the range of acceptable flash exposure. We normally think umbrellas ought to be "close as possible" for softness, but when back twenty feet, umbrellas don't add much (will cost power, and softness is not important then anyway). However, increasing the flash distance for greater flash exposure range, and stopping the lens down for greater depth of field, very significantly increases the flash power needed. Common notions for best group lighting for multiple rows is that multiple flashes ought to all be above the camera, pointing outward to cover the group evenly (lights at the camera see same as what the lens sees, without creating terrible shadows). Two flashes aimed different directions are individual units, NOT combining the same as multiple flashes ganged acting as one. But be careful about any central overlap, which is ganged. It would be good to meter the lights and the center, to verify all group areas are equal. For large groups, see Google - I'd suggest the Chuck Gardner link there.
This diversion is really about the distance range depth of the flash exposure. If you meter the flash, you can meter the range depth too (you certainly ought to plan and know the exposure difference at front and rear of a big group). Or use the calculator here. Or if computing guide number, simply computing distance at ± 1/3 stop apertures computes the range extents for that tolerance (Note ± 1/3 stop is 2/3 stop from front to rear). Exposure range is not about power or aperture, but is about the flash distance (inverse square law), so as to distance, the range distance is applicable to TTL too.
Guide number makes exposure computation very easy. Guide number is the oldest system to determine flash exposure (used for flash bulbs, before automation), but guide number only applies to direct flash. Guide number is not useful for bounce, because it requires knowing the distance in the total path from flash to subject, and also the reflection coefficient at the ceiling (very roughly, common situation bounce can need 2 or 3 stops more power than direct flash). But guide number still is fundamental today, and understanding guide numbers can increase understanding of flash and inverse square law, whether you actually use guide numbers or not. We should all spend a little time playing with this, to understand the concept. It is a genuine basic of flash photography, which simplifies the Inverse Square Law (which is a really huge factor for flash).
Shutter speed is not a factor of flash exposure (Part 2), but f/stop, ISO, flash power, and flash distance are the factors. Distance does not affect our sunlight, but it is pretty tricky for flash. Direct flash exposure falls off with the Inverse Square Law (with distance), a serious complication for determining exposure. If we don't actually meter the flash, then guide numbers can solve distance computation easily (for direct flash). Guide numbers have been calculated forever, at least since first commercial flash bulb about 1930. Guide number was the only system before light meters and electronic automation.
If you meter your flash, either via TTL flash automation, or by using a hand held flash meter, or if you just use the camera's rear LCD and histogram to tweak in your manual flash exposure, then maybe you can get by for awhile without it, but Guide Number certainly does help basic understanding, essential fundamentals of flash that we should know (how flash falls off with distance).
Guide Number works very well for unmodified direct flash (for example, on-camera direct flash). The one big issue is that guide number is not suitable for bounce or umbrellas, etc. So typically, direct bare flash is much less important for studio lights, because we normally heavily modify their light with umbrellas, softboxes, grids or snoots, whatever. This drastically changes their distribution coverage angles, and every change would create very different guide numbers. Guide numbers are typically more common of camera hot shoe speedlights (direct flash), and speedlights do provide specifications for Guide Number as a guide to the flash power and its distance capability (again, it only applies to bare direct flash). Studio lights, not so much about GN, since these normally use various modifiers (umbrellas, softboxes, etc), so they are likely metered.
So that's good to know to know, but guide number tells a lot more. If we know guide number is GN 40 (feet), then we know that 8 feet will need to use GN 40 / 8 feet = f/5 exposure. That is a lot to know (again, this is for unmodified direct flash).
Guide Number is a tool to determine exposure of direct flash with manual flash power levels, to automatically deal with the Inverse Square Law, making the math be trivial.
Guide Number = f/stop x Distance (those values which actually give a proper exposure)
f/stop = Guide Number / Distance (aperture for other distances)
Distance = Guide Number / f/stop (distances for other apertures)
For any given "correct flash exposure" situation, guide number is simply numerically equal to the aperture number (like the number 8 in f/8) multiplied by the subject distance (like 10 feet). Then for example, the guide number is f/8 x 10 feet = GN 80 (feet units). Specifically, that aperture and distance combination which gives the correct exposure, defines the guide number.
The Distance is from flash to subject. The flash might be on the camera, but the camera position is Not a factor. It is about the flash.
The useful part is that this guide number is a constant for that flash situation, good also for other distances or other apertures. If we know GN for the situation (flash power level and ISO), we can know correct direct flash exposure for any distance or any aperture. This constant GN is initially determined by some trial situation seen to give correct exposure. Or we can use the manufacturers chart of guide number (trial is what they did).
If for example, in any situation at all, if f/8 is seen to give the correct exposure at 10 feet (from the flash), then this defines that the guide number for this situation is determined to be 80 (feet, from f8x10 feet). Whatever situation gives a correct exposure, that determines the actual guide number, by definition.
The overwhelming advantage of knowing this guide number constant is that if we then move the light to be 5 feet from subject, then GN 80 tells us that GN 80 / 5 feet = f/16 will give us correct exposure there too. Or if we open the aperture to f/4, then the correct distance for this flash power will be GN 80 / f4 = 20 feet. This guide number 80 is a constant (in this same flash power situation), for any distance and any aperture, and its purpose is to make the inverse square law be trivial to compute.
Said again- From knowing this guide number constant (GN = aperture x distance) for one flash situation (power and spread angle), we can recompute any other aperture/distance combination for correct exposure, which automatically takes the inverse square law into account, involving only the simplest division. For example, if we know the guide number is 80 (feet), then we immediately know that all of these combinations give the same correct flash exposure:
If we know the correct exposure,
f/8 at 10 feet = GN 80
Or, if we know the guide number is 80,
GN 80 / 10 feet = f/8
You get the idea - any combination computing (f/number x distance) = GN 80 (in this example) also gives the same correct manual flash exposure. The main use is, if our subject is at 14 feet (from the flash), then we know GN 80 / 14 feet = f/5.7. This is a lot to know by simple division, and it really could not be any easier.
Guide Number definition is (distance x f/stop), therefore doubling GN doubles distance range, or doubles actual f/stop Number, which is two more stops either way.
Where do we get this guide number? Whatever aperture and distance that gives an actually correct exposure can compute guide number. Or more commonly, there is also a guide number specification in the flash manuals (see below). Then we only need to know the distance between flash and subject. This guide number is speaking of manual Direct flash, and this guide number will change if you zoom the flash head differently.
Zoom: Zooming the flash head changes the guide number. Zooming in, to match the lens zoom (a more narrow coverage angle), also concentrates the flash power into a more narrow brighter beam appropriate for the lens zoom, with a higher guide number. There will be a guide number chart in the flash manual, with a different guide number for very zoom value. See the sample guide number chart below.
Flashes that do not zoom (like the DSLR camera's internal flash) will have one guide number value. It is printed perhaps as (the Nikon D3200 specification chart):
"Guide Number: Approx. 12/39, 13/43 with manual flash (m/ft, ISO 100, 20 °C/68 °F)"
For manual flash, this says GN 13 (meters) / GN 43 (feet). This implies at full manual power, but we can turn the flash power down as necessary, which lowers the guide number.
You can work in units of either feet or meters. Since there are 3.28 feet in one meter, the GN in feet is simply 3.28 times the GN in meters. Again, see the guide number chart in the flash manual for flashes that zoom (an example chart is below).
Guide number is all we had in the old flash bulb days (and it still works), and before flash units zoomed, they always had a little calculator on them to do this guide number division, but TTL flash mode has made guide numbers less used today. The top few Nikon flashes have a GN Mode, which is a GN calculator (sets flash power level to the aperture and distance). But we can often do the rough math in our heads (if distance is about 10 feet, then GN / 10 = aperture), which often gives a close starting point for proper flash exposure.
The published guide numbers (specs, charts, etc) are for unmodified direct flash and for the specified flash head zoom level. As the speedlight zooms in (longer mm to follow the lens zoom), the reflector concentrates the flash power into a smaller angle that becomes brighter, to cover the same appropriate view that the zoomed lens sees. There will be a different guide number for every zoom setting, and for every power level. Any other reflector situation - lighting modifier (diffusion dome, reflector, bounce, umbrella, whatever) - is a very different guide number. Any other path than direct flash is a different subject (involving longer path and bounce reflection losses, etc).
Guide number makes Inverse Square Law math be easy. The reason this product (of Distance x f/stop) works as a constant for exposure is due to the coincidence that each stop of f/stop numbers increase by the square root of two (1.414) to give half intensity, and the Inverse Square Law distance decreases by the square root of two to give double intensity, and these square factors of 2 offset and cancel in the math, so that the simple product (aperture x distance) is a CONSTANT for correct exposure for this given direct flash situation (ISO, zoom, power level), for any aperture or any distance. It is enough to know that the big deal is that the Guide Number automatically accounts for the Inverse Square Law, making its math be almost trivial for us. This is a big deal, but it is only applicable to bare direct flash.
Or even easier... Flashes compatible with the camera (communication) often know f/stop, ISO and zoom from the hot shoe. So in direct flash Manual mode, they can use their guide number to show the distance calculation (appropriate for the current power level) on their LCD. This can be a fine starting point (again, direct flash only). Can be very helpful.
ISO: The guide number conversion charts in the flash manuals are typically printed showing ISO 100 values, and then we know that GN increases by square root of 2, or by 1.414x for every doubled step of ISO. Or we divide GN by 1.414 if converting to half of ISO.
This first chart is a normal GN conversion chart, always aligning ISO 100 with GN 1x multiplier, because ISO 100 is usually of interest to us. Aligned that way, you have that standard chart.
But the rows are individually scrollable. This is something easy, but possibly made confusing here.There are no absolute relationships, we can slide these scales left or right as we wish. The meaning of the default values initially shown in this chart should be that IF you are using ISO 400, and align GN x1 there too (if ISO 400 is your base GN x1 value), then all other ISO values still have the GN multipliers shown. The doubling effect just mentioned always exists. For example (as shown initially), IF using ISO 400 and assuming it is the base at GN x1, then the next multiplier to the right gives the GN multiplier for the next ISO to the right (ISO 800 is 1.41x GN of current GN for ISO 400). And IF that combination also aligns an actual flash power level (like 1/4 power), then the GN multipliers also work for other power levels. Or, the GN calculator above will do all of this too.
The guide number is multiplied or divided by 1.414x for each stop changed, which is each doubling of ISO, or for each doubling of flash power level. Two sequential doubles of ISO or power level doubles GN.
Then since GN = f/stop x distance, then we know doubling GN also doubles the computed f/stop number (which is two stops), or it doubles distance range (which is two stops).
The flash power level steps of Full, 1/2, 1/4, 1/8, 1/16, 1/32, 1/64, 1/128 are each half power of the previous step. The best fact to know about manual flash is that each half power step is one f/stop of exposure. One stop is a 2x factor, so said again, turning the flash from 1/4 to 1/8 power (which is half) reduces the exposure by one f/stop. This is extremely convenient to know.
Each half power step reduces GN by the square root of 2 (divide GN by 1.414). Two half power steps (1/4 power) is two stops of exposure, or 1/2 the GN value. Or use the calculator, or see the GN chart further below.
If the speedlight does not zoom, then that's all it can do, so you can only compare that. But if it zooms, increasing the flash zoom mm number concentrates the power into a smaller beam. Doubling the zoom mm theoretically covers a 1/4 smaller area with 4x brighter intensity, two stops. Which theoretically, the calculator could calculate the areas, but the actual reflectors vary to much to try. There is substantial area overlap (so frame edges are fully exposed), and usually double zoom mm might multiply guide number about 1.4x, or one stop (if that). Which is only a very rough approximation - because again, of course it depends on the individual reflector design. But guide number maybe about doubles from 24mm to 105mm (4x), which is near a two stop increase (half expected), but there are large variations. I'd love to be able to add zoom to the GN calculator, it is obviously an important factor, but this would have to embed GN charts from many different specific flashes, and there are too many flashes. You can do that easier, so instead see the Guide Number chart in your flash manual.
Use either feet or meters with these calculators, just be consistent.
Comparing power of two speedlight flashes with GN
Same zoom and ISO is required. Because zooming in concentrates the same flash power into a smaller area, which is then brighter (but only useful in that smaller area). It used to be that flashes that zoom agreed to advertise guide numbers for the same standard 35mm zoom, which was considered to be a typical useful working value, certainly conceivable (it was about full frame views then, but the major Japanese flashes still do this). The power was comparable that way, at the same zoom.
But today, some marketing (especially Chinese flashes) advertise their maximum 105mm zoom guide number, simply because that is a larger number that looks better than others, regardless that we rarely use flash at 105mm zoom.
Today, to know very much about ratings, we need to look at the guide number chart in the user manual (sometimes online). Comparing this calculation can be useful when shopping for a flash. However, I have seen one Chinese manual that simply advantageously had the wrong chart in it.
If one GN is rated for ISO 200, then dividing that number by 1.414 will give the ISO 100 equivalent. Guide Number can only be compared if both are at the same flash zoom and ISO settings.
Guide Number is used for speedlights, but is not very meaningful for studio flash. One reason is they are typically not used as direct bare flash, but also their GN rating situation is so unknown.They don't zoom, but comparison is difficult when we may not know what reflector was rated, or what its angular distribution spread is. Speedlight GN varies over probably a 2 to 1 range when they zoom... but we can only compare intensity at the same angular coverage, doing the same job.
Resulting GN of ganged multiple flashes
This context of ganged means flashes probably all mounted on the same stand, and aimed at the same point, specifically acting as one. A Main light and Fill light situation is acting as two, and is NOT two ganged acting as one.
The GN of multiple equal flashes ganged in combination acting as one, is GN of one times square root of (number of flashes). Each doubling of the number of equal flashes (from 1 to 2, or 4, or 8 flashes) results in one stop in brightness, each doubling increases GN by the square root of 2 (1.414). Two or four flashes may be reasonable, but thereafter, the law of diminishing returns will apply.
But ganging two unequal flashes acting as one, say of GN 58 and GN 80 (0.93 stop difference), will add as square root of (58² + 80²) = GN 99. This total is +1.54 EV compared to this smallest flash (more than double), and +0.61 EV compared to this largest flash (less than double).
The GN spec is in the flash manuals, and if it has zoom, it will have many guide numbers... and a GN chart in the manual (for zooms and levels). The charts usually show GN as both meter and feet values (specified as meters/feet). There are 3.28 ft in 1 m, so the GN conversion is: GN in feet = GN in meters x 3.28, or GN in meters = GN in feet / 3.28. Use either feet or meters, so long as you are consistent with units (same units everywhere).
The charts are always for ISO 100, but we know the ISO 400 factor is x2. We seek a ISO 400 GN in this example, so to go the other way, we divide the ISO 400 value by this 2x to get the ISO 100 value (to be able to compare in the ISO 100 chart). All of this was just shown above.
Suppose we plan to use direct flash at f/8 at 12 feet at ISO 400. So we obviously know this needs flash power of (f8 x 12 feet) = GN 96 (feet) at ISO 400.
Converting this example (f/8 at 12 feet, GN 96 at ISO 400) to ISO 100 is GN 96/2 = GN 48 (feet, ISO 100). Now we can search the guide number chart in the speedlight manual (ISO 100), and maybe we find this value at 24mm zoom and 1/4 flash power to be GN 49 feet. More than close enough to 48. The point of course is that same chart value also applies to GN 96 at ISO 400. This SB-800 GN chart below (for an example) says that 24mm flash head zoom and 1/4 flash power is GN 49, almost exactly the GN 48 that we seek. Measure the distance, set the settings (flash 24mm zoom and manual 1/4 power, camera ISO 400 and f/8), press the shutter button, and Presto, you've got it. You're very close on first try for a 12 foot flash distance (direct flash).
Or, the chart includes several other combinations of power level and zoom which are near GN 49. It need not be exact, another GN value of about 12% difference is roughly within 1/3 stop (and 41% is one stop). We don't necessarily have to match flash zoom to lens zoom values, we can use any wider flash zoom - which may waste a little light if the flash is wider than the lens, which is no problem if not at maximum power (some flashes do not zoom anyway). FX flash on DX cameras is already 1.5x wider anyway. To adjust results of this method, you can simply adjust the power level by 1/3 stop, or adjust the aperture by 1/3 stop. Or zooming the flash head makes steps sometimes roughly about 1/3 stop (no bets, that is a very rough approximation).
Say you really wanted to use 35mm zoom... or that you could use 35mm zoom. You can of course simply select any nearby zoom and power cell near 48 there (say 35mm, GN 44 at 1/8 power, that's close). Multiply it back to ISO 400 (44 * 2 = GN 88), and then compute a similar distance and aperture for that GN value (GN 88/12 feet = f/7.3) at ISO 400, 35mm and 1/8 power. That is comparable to 12 feet at f/8. All that is left is to measure off the 12 feet. There is some situation that will work, and it will work (if using your own chart of course).
Or the GN calculator above works: Choose any zoom in your GN chart, and enter its full power GN (GN 125 here for this 35mm case) at ISO 100. Enter the new ISO 400 you want to use, and for this example, enter f/8 and 12 feet. Click Compute, and it will advise setting -2 2/3 stop power (1/3 higher than 1/8 power). Do that and click Compute again, and it verifies it is within 0.09 stop. And results should be pretty close.
Or if you have it, use the GN mode (next below), set ISO 400 and f/8 on the camera, and 12 feet in the GN menu. It will set the power for 12 feet, and it will work too (feet or meters, whichever you have the flash set to use).
This GN chart is from the Nikon SB-800 flash manual for regular speedlight mode (page 42 PDF manual).
Both sun and HSS flash are continuous light, so fast shutter speed of course reduces their exposure, both the same way. It's natural, we expect that. For either or both, we can open the aperture wider to compensate, which creates the concept of equivalent exposure. If shutter speed is twice as fast, we simply open aperture one stop to recover. Any equivalent exposure is still the same exposure, true for both sun and HSS flash. This is extremely different than speedlight flash mode.
Probably what we all will do is to put the HSS flash in TTL mode, and go out and see what happens. But understanding the guide numbers will first give you a very good idea about what to expect, and add how to change things to get what you want it to do. The chart zooms and guide numbers probably don't exactly match your situation, but TTL should then take care of minor differences.
The Nikon flash manuals have a separate Guide Number chart for HSS (GN is for bare direct flash.) Typically, the later models (SB-900, SB-700, spec chart in rear of manual) give 1/500 second shutter for HSS GN. Earlier models (SB-800 page 106, SB-600 page 76) give it as 1/300 second. Since HSS is continuous light, this means we can use any Equivalent Exposure to shift HSS away from this referenced shutter speed number (and the HSS GN remains the same, if still an equivalent exposure).
For example, if the HSS GN is 36 at 1/300 second (from the SB-800 HSS GN chart next below, 24mm ISO 100, feet), then HSS exposure for 10 feet is 1/300 at 36/10 = f/3.6. Then any equivalent exposure of 1/300 f/3.6 works the same at 10 feet for HSS, for example 1/1200 f/1.8 is the same GN, same distance, same exposure (you can get more GN and more flash range and higher equivalent shutter speeds by converting to higher ISO, ISO 400 is 2x GN). Both sunlight and HSS are continuous light, so both work this same way (equivalent exposures are the same exposure). The beauty is that changing equivalent exposures won't affect your flash or sun exposure, or your HSS fill ratio. The problem is that HSS guide numbers are quite low.
Equivalent Exposure concept of HSS, for both flash and sunlight. This is a big deal. It's a D800 and SB-800 at 5.5 feet, at ISO 400 and 24mm, hot shoe direct flash. Flash is at manual Full power level. The Guide Number is GN 72 at reference 1/300 second if at ISO 400, so f/16 1/320 worked for the ambient alone (Auto FP 1/250 mode). So at f/16, HSS GN distance is 72/16 = 4.5 feet. Using 5.5 distance instead of computed 4.5 was a flash reduction to become a lower fill level in daylight (computes 122% distance, roughly -0.6 stop). Both flash and sunlight have the same results at f/16 1/320 second and also same at f/4 1/5000 second with HSS flash (equivalent exposures). This is is a Big Deal, HSS works with the same Equivalent Exposures as Daylight (because both are continuous light). Specifically, this means the same one HSS GN works at any Equivalent Exposure (up and down the camera P mode range), which is very useful to know when using HSS flash.
This is the SB-800 HSS GN chart (page 106 PDF manual). Older models used this 1/300 second reference, newer models use 1/500 second (which is about 2/3 stop difference). The 1/300 is an awkward shutter number, so I call it 1/320 second. The idea is to compute HSS GN settings for that referenced shutter speed, and then any equivalent exposure works the same (for HSS or sunlight).
Another example of some numbers, the Nikon SB-910 flash guide number for FX 200 mm full HSS power is GN 64.3 (feet) at ISO 100 at reference 1/500 second shutter speed. ISO 400 would be 2x GN, and 2x flash range, so assuming ISO 400, this is GN 125. I do Not mean to imply HSS has much range, but a higher ISO gives more flash range, and of course, the only goal of HSS is for more shutter speed too. ISO can move you away from f/1.8 too, another plus. You definitely may need to consider higher ISO with HSS.
However, in sunlight, our exposure also has to match the suns ambient exposure. Bright sun at ISO 400 is near EV 17, which is 1/500 second f/16 (1/500 for the HSS GN, and yes, Sunny 16 is 1/3 stop brighter). At that f/16 1/500, this HSS ISO 400 GN 125 produces a 7.8 foot range. But one equivalent of that is 1/8000 at f/4, still same range.
But assuming bright sun (if we are using HSS), then fill flash level only needs to be a stop or two less than full illumination level, and if assuming -2 EV fill, that fill concept doubles the GN distance. That range becomes 15.6 feet for -2 EV fill, which is 20% flash. A -1 EV flash compensation is 1.4x GN distance, and 33% flash. This level at higher ISO can be usable for f/1.8 portraits at reasonable range, but sports action may be too distant.
(More about HSS here). If the ambient were dimmer, and if we can underexpose ambient a couple of stops, I would never consider HSS. Speedlight mode can run circles around it, power, range, and motion stopping, but it is limited to maximum shutter sync speed... which won't matter if the ambient is underexposed enough to not be a factor.
We should realize that neither TTL metering nor Guide Number calculations are always precisely correct. Things happen, but at least GN is independent of the subject colors which do fool camera meters. But BOTH methods are pretty close first attempts, tremendously better than having no clue at all. The standard used to be that flashes that zoom always advertised GN at 35mm, considered a probable useful working value for full frame cameras. Nowadays, the Chinese flashes advertise the GN at maximum zoom (the biggest number), an inflated representation, not a likely probable common actual use. You will want to see the GN Chart in the flash manual, instead of the advertised claims. Flash power levels can only be compared at the same zoom value for both.
Measured GN sometimes may seem to vary a little from the manufacturer's chart value. I have wondered at times, but prefer to believe the chart numbers normally are carefully prepared. One factor is that our own procedures can vary. A near wall surface can provide reflected fill (affecting exposure), so GN can increase in a small room where reflections from the near walls/ceiling combine, whereas GN is lower in wide open spaces with no reinforcement. How much the ambient light is also contributing can be a factor. Flash capacitors do of course have a capacity tolerance (typically -10%, +20%. new), so individual flash units might vary slightly (but it's small in terms of stops of power). Another possibility is that the capacitors in an old flash may have deteriorated somewhat, not still full capacity. My 25 year old Nikon SB-24 still performs to GN spec however.
The GN charts generally seem to specify GN for meters always in whole numbers, and then GN for feet is simply m/3.28. Just my assumption, but if every zoom value on every brand and model has a rounded GN value in meters (no decimal points), it suggests the published GN precision is ± 0.5 (meters) or ± 1.6 (feet), which is possibly a noticeable variation if subject is close. But if at usual distances, GN can be surprisingly accurate, an excellent first try. Just try it, especially if your flash has the GN mode (next below). I don't have much trouble with guide numbers, but there certainly are variations in different situations..
But I really think (in my own case) the most likely reason for GN differences is that our own perception of "correct exposure" is not always precise, so check multiple tries in different situations. And actually do take the picture to check it. My own experience is that the Nikon guide number seems accurate. If I measure the distance and take the picture, I get fairly good exposures, at least a good starting point. One beauty of it is that GN is not affected by subject reflectance in reflected meters. But if I just meter the direct flash, it may not always match GN expectations precisely? That's a contradiction, and is probably my procedural error? Metering flash is a good thing to do, and if you can meter your lights, that's great. But if you have no other means to adjust manual direct flash, you ought to try guide numbers.
One special case: If you attempt to verify your speedlight's guide number at maximum power level, don't use your maximum sync speed (at full power). Unless you are metering it, then at full power, back off to maybe 1/160 second. The speedlight becomes slow at maximum power, so if at the fastest shutter speed, the shutter can close on it, and the sensor can't see it all. If metering it with a handheld meter, and flash is on the hot shoe, the fastest shutter speed can quench the flash off too, so an external meter can't see it all either. This is only a slight effect and only occurs at maximum flash power and at fastest sync shutter speed. Flashes vary, but I do see my SB-800 slightly reduce maximum flash exposure at 1/320 second, but it seems negligible at 1/250 second. Speedlight lower power levels are greatly faster of course. Most studio monolights are the opposite, slowest at lower power levels.
SB-700 manual page C-11
SB-800 manual page 44 (shown at right)
SB-900 manual page D-11
SB-910 manual page C-12
Nikon calls this GN mode "Distance Priority Manual Flash". When on the hot shoe, CLS flash already knows ISO, aperture, zoom, and the guide number chart, so these flash models have a GN Mode option where all you do is set the distance into the flash menu (ten feet shown here). The flash computes and sets the flash power level automatically, to be correct for the distance and camera settings (bare direct flash). Automatic computation in that sense, but it is a Manual flash mode - we enter the distance manually (The D-lens focus distance is not used by this mode, because it is not accurate or complete enough for this).
The details are that we know f/8 at 10 feet is 8x10 = GN 80 at this ISO 200. That's 80/1.4 = GN 57 at ISO 100. This is a SB-800, so looking at its ISO 100 GN chart above, we know 50mm at GN 57 must be set at a bit more than 1/8 power in this case. GN Mode simply knows how to do all that, and does it for you when you enter ten feet.
Camera Flash Compensation is not operative in GN mode (camera metering is not involved). If you want the flash a bit brighter, tell it the distance is a bit more (1/3 stop more is 12% more distance).
Guide Numbers are about direct flash, but this GN mode can surprise you if you will try it. We do have to know the distance, but one really wonderful GN advantage is that unlike TTL metering, GN Mode is independent of the subject colors which do affect TTL metering according to how well they reflect light. GN mode is independent of the subjects reflectance (like incident metering is also independent of seeing the subject). It just sets the right light level for any subject (at that one distance), and black things will come out black, and white things will come out white. So this mode would be fabulous, except that it is direct flash only, and we have to know distance. GN mode is for camera mode A or M (aperture is set and does not keep changing), and for direct flash only with the flash head straight ahead (the Nikon GN mode simply disappears from the menu if the flash head is tilted or rotated).
Guide Numbers do not work so well for bounce, and bounce is often the very good stuff. TTL is wonderful for bounce, but bounce will need two or three stops more flash power then the direct distance GN would indicate. In the real world, we usually just guess at the direct distance, so the initial result might be off a little, but like TTL, it will be a close starting point. The Nikon flashes can use the compensation in the flash body - press the Center SEL button, see the manual. Or you can reenter a different adjusted distance (changing the other values like aperture or ISO just recalculates).
Guide Numbers are used for direct bare flash, but it becomes tough and unknown for bounce and umbrellas, etc. Path distance has to be measured from the light source (the flash tube), via the reflection surface (NOT just from the fabric panel). In the old days (before TTL electronics), we used to approximate for ceiling bounce with the rule of thumb "open two stops for bounce" (from the direct values). Three stops was usually closer, which was sometimes adequately ballpark for negative film (much more latitude than digital), but of course, very crude and vague, because every situation was different (ceiling height and texture and reflectance, and flash head angle, etc). FWIW, the three stops might be reasoned this way: a 45 degree path was 1.4x longer which is one stop, the reflection at the ceiling was one stop loss, and half the light went elsewhere, another stop. The film had much latitude, and it usually worked OK. Digital is more critical, but TTL makes this easy today.
So TTL excels for bounce, it simply meters the actual light arriving via that path, whatever it is. However, regardless if bounce or direct, TTL accuracy is still always affected by the reflectivity of the various subject's colors (clothing, walls, etc) - which does not affect guide number, which does not even take the subject into account (only the distance and GN are considered). For direct flash, the beauty is, if we know GN and distance, we KNOW the exposure, independent of the subject colors. There is a good case for that, but frankly, measuring distance and doing division is more awkward than automatic TTL metering. We likely have to adjust it slightly either way (due to TTL reflectance, or GN distance).
So to repeat: Problems are, the guide number method needs to know a fairly precise distance from flash to subject, and this leaves out bounce flash. Or, we can always guess roughly at the distance, and get a rough trial answer, and then tweak that result better by trail and error. It is a good starting point, but we also need to know the guide number fairly precisely, which implies direct flash only. The guide number chart in the flash manual can differ a bit from our results, which could be due to added reflections from walls in a tiny room, could be flash capacitor aging, could be marketing exaggeration of specs. Or, often it is rather accurate. But it will be a constant after you know it.
But if we do know one precise exposure result, we can change distance and still know the right exposure. We have to do some division, but sometimes we can approximate this in our heads, or many flashes have (or used to have) guide number calculators, where we enter distance, and it tells us f/stop, or vice versa.
While guide number is a fundamental basic we ought to know (it handles the Inverse Square Law), and which is still dead on today, frankly, the method may seem old fashioned now. It is what we used back in the 1940s to 1960s - it was all there was for the flash bulb era. But by about 1970, we had electronic flashes with the photo sensors for the Auto modes that self-metered the reflection back from subject. We certainly liked that, and it worked for bounce too. The 1980s introduced TTL, metered and controlled by the camera computer. We liked that too, it was great to actually meter the flash. Reflective metering certainly can have issues, it often needs some correction (called Flash Compensation, which we add manually by trial and error and experience). But TTL is metered, and is generally always a pretty close starting point.
Many users use TTL flash today, but also many prefer manual flash mode, for the control it offers. Both modes must be watched and adjusted, frankly, both are just the first starting point for determining actual proper exposure. TTL may start closer, but frankly, there is much less difference than we may imagine, in that we adjust both for a final result. Manual flash users just quickly "know" (remember) that this familiar situation will need about 1/4 power, same as last time. Honest, neither method is difficult except first day.
But either way, guide number is really about the least we can know about flash. For example, you're at home wondering about the graduation picture tomorrow. You think you can sit with 50 feet of the stage. You have a suitable lens, say 105mm for DX, but you're wondering about the flash. The zoom on your SB-600 maxes out at 85 mm, and the GN there is 131 (ISO 100), and x2 for ISO 400 is GN 262. At 60 feet (safety factor), GN 262 / 60 feet = f/4.4. Piece of cake, ISO 800 should not be necessary. Even if planning to use TTL, this is good to know before you get there. Take a couple shots of the empty stage before things start, to get setup right.
See a page about comparing power ratings of flash units using guide numbers (NOT the same page as Next page below).