Flash intensity falls off with distance. Guide Number is a numerical method used to determine exposure of direct flash for Manual flash power levels, to automatically deal with the Inverse Square Law, making the math be trivial. The reference base is a known accurate Guide Number for one situation, from which other situations can be calculated. There are also other ways, today we might use metered TTL, or a handheld flash meter, or just trial and error works well (for one flash) with the digital LCD preview. In the old days (including flash bulbs), guide number was all we had. The concept is still very valid and useful, and is still a fundamental for understanding flash. Guide Number is a solution to deal with the Inverse Square Law. Even if you always only use TTL, the calculator can be useful to show sample flash capabilities.
The full details explaining use of guide numbers are below the calculator.
Insufficient flash exposure is corrected with more flash power, higher ISO, wider aperture, or shorter distance.
Excessive flash exposure is corrected with less flash power, lower ISO, stopped down aperture, or longer distance.
Doubling ISO or power level increases exposure 1 EV.
Doubling Guide Number increases flash exposure 2 EV.
Doubling f/stop number or distance reduces flash exposure 2 EV.
First enter the Full power Guide Number for the zoom mm you will be using (probably GN for ISO 100, from the GN chart in the flash manual). For example, GN is usually stated like 30/98. The smaller number is GN for distance in meters, larger number is GN for distance in feet.
This can be a basic GN calculator, offering the two choices for computing either aperture or distance, done simultaneously, but independently, not related to each other. Choose either/or. Or see Example Use of GN Chart below, same idea using a GN chart.
Or, mode C is an option to enter Both aperture and distance as your goal of desired settings. Then changing Power or ISO to correct that exposure are the best choices to retain that goal, but there are the four possibilities. You can always change any of the settings for a different result.
Note that this method C is also how the Guide Number Chart in the flash manual would be used manually without a calculator. We compute the GN that the situation needs (fstop x distance), and then (for the ISO 100 chart) adjust it for ISO 100 so we can find that GN in the chart to determine the power and zoom settings to use.
Modern cameras are surely set to use third stops, which are clearly the most accurate exposure choice. But you might be using an old camera or lens or flash that is limited to full stops. Or you can select half stops here. If you select third stops, those results will find the nearest third. The idea of the nearest third is that results will be ± 1/6 EV.
Guide Number applies to Direct Manual Flash, at the specified zoom mm value and ISO. It all works well, but ...
Guide Number method can be very accurate, and is not affected by variations of reflected subject colors.
However Causes of Error are:
Repeating again: NOTE that if your flash specifies Guide Number for 105 mm zoom, but you are using it at 24 mm zoom, you absolutely need to know and use the 24 mm zoom Guide Number (see your flash manual for a standard Guide Number chart).
Ordinary guide number calculators just do a simple division, cases A and B above, independently. That's very important and useful, but there's a lot more here. For example, using the initial defaults of GN 98, ISO 100, full power, f/7.1 and 8 feet for mode C tells us full power of this GN 98 flash is +1.56 EV too high, but nearest third power setting of -1.67 EV will be correct exposure within 0.1 EV. Then the Change Power button will change that for verification.
Hopefully use will be fairly obvious. We can just try settings that seem right and sensible. The calculator will guide you about how much change is needed for a correct exposure of the goal. The A or B method is to simply use the distance or aperture as indicated at the A or B results, which will be correct exposure, but that changes the C goal, possibly drastically. Instead changing Power or ISO retains the goal. The calculator suggests a power level first, and then may offer an ISO Adjust (more reasonable numbers), but there are four choices that calculate change. Or you can of course always directly change any settings you want, and it will check the result. The suggestion for awkward cases showing tough numbers is, try changing the numbers towards what seems reasonable.
If you mix half and third stop settings, you'll get a warning that it conflicts with the step mode selected. Cameras set to do thirds will not do half stops. But it is possible to have a modern camera doing thirds and an old flash with only full or half stops. It still computes accurate numbers if mixed is how you really will work, but if any subsequent automation (that you select) changes it, it can honor the selected steps then. If a mix is intended, it could need an extra final tweak to reset the result.
The four Change buttons in C each only change their one property, which may or may not be enough range alone. The settings are limited, at least in useful range. This calculator will limit out at maximum or minimum flash power, or at extremes of ISO or aperture (in the list boxes), just meaning another change will also be needed. At limits, it still computes and advises about needed EV, but cannot adjust past these limits (without your changing something else).
For example, we could have instead first clicked the Change ISO button (from the same previous initial settings), to get ISO 32 at the initial Full power, and a correct exposure. Then an Adjust button appears that will shift us back towards ISO 100, at -1.67 EV power at ISO 100 again (more reasonable, in this default case). It may not always reach ISO 100 due to power limits.
An Adjust button (using power to shift ISO result higher or lower towards ISO 100) is offered at correct exposures that are not already ISO 100, but is only seen if power level is not already limited at Maximum or Minimum, to allow corresponding change. ISO can move much further than power can, but some cameras cannot go below 100, or some not much above it. ISO 100 is a known reasonable choice, and aperture and distance are our chosen goals. You may have reasons otherwise, and have already specified power level, so Adjust is optional, and it's just a click that can be ignored if not desired. You can of course do all of these actions yourself directly, the buttons just do the calculations.
A big part of the calculator purpose is about better understanding guide numbers.
Guide Number (GN) is a primary fundamental, related to Inverse Square Law, and is about how light works, 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: The 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, and when back 15 feet, umbrellas don't add much benefit (will cost power, and can't be soft from there anyway). 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.
This diversion is really about the distance range depth extents of the flash exposure. If you meter the flash, you can meter at the range extents too (you certainly ought to plan and know the exposure difference at front and rear of a big group). Or if direct flash, 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 exactly 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 (except for HSS flash, but see Part 2 for regular flash), but f/stop, ISO, flash power, and flash distance are the factors. Distance does not affect our sunlight (here on Earth), 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 direct flash falls off with distance).
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 (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 and zoom 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:
f/8 at 10 feet = GN 80
f/10 at 8 feet = GN 80
f/4 at 20 feet = GN 80
f/20 at 4 feet = GN 80
f/16 at 5 feet = GN 80
f/6.3 at 12.7 feet = GN 80
GN 80 / 10 feet = f/8
GN 80 / 8 feet = f/10
GN 80 / 20 feet = f/4
GN 80 / 4 feet = f/20
GN 80 / 5 feet = f/16
GN 80 / 12.7 feet = f/6.3
You get the idea - any combination computing (f/number x distance) = GN 80 (in this example) also gives the same correct manual flash exposure (due to Inverse Square Law). 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.
This works (and is conveniently used) because Guide Number definition is (distance x f/stop), therefore doubling GN doubles distance range (4x the light), OR doubles actual f/stop Number (1/4 the light), which is two stops either way. Actually for any number N, any GN gives same exposure at (N x distance) if using (fstop number / N). The N cancels for GN. This is true because of the coincidence that distance observes the inverse square law, and the area of f/stop number observes the square of the radius.
Where do we get this guide number? There is normally a guide number chart in the speedlight flash manuals (for each zoom setting, see next page). Or in the camera manual specifications for the internal flash. Then we only need to know the distance between flash and subject to know proper aperture for manual flash exposure. Or, we can also determine or verify it ourself, since whatever aperture and distance that gives an actually correct exposure can compute guide number. This guide number is for Manual Direct flash, and this guide number will change as you zoom the flash head.
Zoom: Zooming the flash head changes the guide number. Zooming in, to match the lens zoom (a more narrow coverage angle), 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 various zoom values. See the sample guide number chart next page.
If the speedlight does not zoom, then that's all it can do, so you can only use and 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 you'd think the calculator could calculate, but the actual reflectors can vary individually. 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 of course it depends on the individual reflector design. But guide number often maybe about doubles from 24 mm to 105 mm (4x, 1/16 area coverage), which typically achieves only about a two stop increase. There are variations, which prevent zoom calculation. The manufacturer creates a chart of guide numbers for nominal zoom values, and from it, the calculator uses the GN that you enter for the zoom you are using.
Flashes that do not zoom (like the camera's internal flash) will have one guide number value. It is printed perhaps as (the Nikon D3200 internal flash 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 / GN 43 (meters / 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 next page).
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.
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 24 mm 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 24 mm 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 24 mm 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 35 mm zoom... or that you could use 35 mm zoom. You can of course simply select any nearby zoom and power cell near 48 there (say 35 mm, 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, 35 mm 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 works too: Choose any zoom in your GN chart, and enter its full power GN (GN 125 here for this 35 mm 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). It is a full frame FX rated GN chart.
Or GN use can be even easier... Flashes compatible with the camera (communication) often know f/stop, ISO and zoom from the hot shoe. Nothing is metered here, but in direct flash Manual mode, they can use their guide number to show the distance calculation (appropriate for the current power level, ISO, aperture and zoom) 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.
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 on next page.
Use either feet or meters with the calculators, but you must be consistent with GN and Distance.
If the flash can zoom, then it is required to compare Only at the Same zoom values. Because zooming in concentrates the same flash power into a smaller area, which is then brighter (but is only useful in that smaller area). If comparing GN of a speedlight to a studio flash, then to have any meaning, only the same reflector angular coverage (that was used for the GN rating) must be compared (softboxes and umbrellas drastically change GN and coverage). If it does not zoom, then of course it only has that one setting to do one thing (with that same reflector). It used to be that speedlights that zoom agreed to advertise guide numbers at the same standard 35 mm zoom, which was considered to be a typical useful working value, certainly conceivable (it was about full frame views then, and the major Japanese flashes still do this). The power was comparable that way, at the same 35 mm zoom. But today, FX and DX sensor coverage can change GN at the same zoom. So, just saying, for power comparisons to be meaningful, all things must be equal.
But today, some marketing (especially Chinese flashes) advertise their maximum 105 mm zoom guide number, simply because that is a larger number that looks better than others, regardless that we may rarely use flash at 105 mm zoom.
Today, to know very much about ratings, we need to look at the guide number chart in the user manual (sometimes it is online). Typically more meaningful than simple advertising. Comparing this calculation can be useful when shopping for a flash, but know what it specifies, what zoom and ISO it's for.
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 power and intensity when lighting the same angular coverage, when doing the same job.
Studio lights: Guide Number works very well for unmodified direct flash. One big issue is that guide number cannot be specified for bounce or umbrellas, etc. (because, it depends on them). 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 different reflector coverage would create very different guide numbers. Here is a great clarification of that from Alienbees.
Otherwise, the guide number that may be specified for a studio flash probably applies only to the included standard bare reflector it ships with (we have no clue what it means if conditions are not stated). If the applicable reflector (and its angular coverage) used to specify GN is not specified, then we don't know how GN applies for our usage. Any wider reflector providing wider area coverage will always have a lower guide number, and a more narrow reflector concentrating the light into a smaller area will have a higher guide number (within that smaller area). To be able to compare guide numbers, we can only compare at the same area coverage. Specifically saying, comparing GN of a speedlight zoomed to light a spot on the wall to GN of a studio light with wide reflector illuminating the entire wall is just being dumb (but we see that done).
So guide numbers are typically more common of camera hot shoe speedlights (direct flash), and speedlights do provide specifications for Guide Number at each zoom as a guide to the flash power and its distance capability (again, it only applies to bare direct Manual flash). For studio lights, GN is commonly specified with the one default bare reflector, which probably does not apply to your usage, since these normally use various modifiers (umbrellas, softboxes, etc). So studio lights are likely metered.
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, there are diminishing returns.
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). Two lights added are always (at least a little) brighter than the brightest one light. Two equal lights are 2x brighter than one (+1 EV).
Guide Number is just the Inverse Square Law, and should be very accurate. GN is also independent of the subject colors which do fool camera meters. But we should realize that neither TTL metering nor Guide Number calculations are always precisely correct. Things do happen, but BOTH methods are pretty close first attempts, tremendously better than having no clue at all. The GN standard used to be that flashes that zoom always advertised GN at 35 mm zoom, considered a conceivable working value for flash on full frame cameras, real world useful. Nowadays, the Chinese flashes advertise the GN at maximum zoom (the biggest number, which advertises better, at least to buyers that don't understand GN), 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, and I have seen one Chinese lower powered speedlight manual that included the wrong GN chart for a more powerful unit. But I believe the chart numbers from major manufacturers are very carefully prepared and very carefully verified.
A bigger 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 rated -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 (turn them on awhile every six months or so, to keep them formed). My nearly 30 year old Nikon SB-24 speedlight 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 divided by 3.28 (3.28 feet in one meter). 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 very 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 color reflectance in reflected meters. But if I just meter the direct flash, it sometimes 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 50 mm 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).
Bounce is often the very good stuff, but Guide Numbers do not work so well for bounce, because the total distance up and down is greater than the direct path, and there is also reflection loss at the ceiling. TTL is wonderful to actually meter bounce (reflected metering), but bounce will need two or three stops more flash power then the direct distance 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. One advantage of digital cameras today is that we immediately see our result, and we can simply correct it and retry until we are happy. The camera body has a flash compensation menu, and the flash body has a flash compensation menu, and the camera will add these to a new total. Changing TTL compensation works, but changing the other values like aperture or ISO just recalculates the same TTL exposure goal.
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 105 mm 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.
Continued - HSS Guide Numbers on next page.