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 flash exposure, then maybe you can get by for awhile without this part right now. It is no big deal if you skip to next page now, you can always come back later. It certainly does help understanding however, essential basics of flash we should know.
Guide Number is a prime fundamental, related to Inverse Square Law, and will always be important to know. Guide Number is the oldest system for flash exposure (for flash bulbs, before automation), but guide number only applies to direct flash. It is not very useful for bounce, because it requires knowing the distance in the path from flash to subject. 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.
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 (GN) as a guide to the flash power and its distance capability (again, it only applies to bare direct flash). Studio lights, not so much, these always use various modifiers (umbrellas, softboxes, etc), so they spec watt seconds of energy, but some may mention guide number as a reference (bare direct flash).
Specifically, Guide Number is a tool for direct flash with manual flash power levels to automatically deal with the Inverse Square Law. Guide Number is all about the Inverse Square Law (but GN makes its math be trivial).
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 in this case, 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 useful part is that this GN 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).
Guide Number = f/stop x Distance (those values which give a proper exposure)
f/stop = Guide Number / Distance (aperture for other distances)
Distance = Guide Number / f/stop (distances for other apertures)
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.
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 flash exposure. The main use is, if our subject is at 13 feet (from the flash), then we know GN 80 / 13 feet = f/6.1. This is a lot to know by simple division, and it really could not be any easier. There is a Guide Number chart in the flash manuals. The only complication is that we need to know the distance. However, there are two qualifications: This is speaking only of Direct flash, and this will not remain true if you zoom the flash head differently. Zooming the flash head changes the Guide Number (zooming in concentrates the flash power into a more narrow brighter beam, with a different Guide Number). See the sample Guide Number chart below.
You can work in 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. The Guide Number chart normally gives both values, the two values are usually shown as meters/feet. Use either unit, so long as you are consistent with all distances.
Guide Number is all we had in the old flash bulb days (and it still works), and since then, flash units always used to provide a little calculator on them to do this Guide Number division, but TTL use has made this less used today. The top few Nikon flashes have a GN Mode, which is that 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. 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).
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 offset and cancel in the math, so that the simple product (aperture x distance) is a CONSTANT for correct exposure (for this given flash situation, for any aperture or any distance). Don't worry about the math in the derivation of this Guide Number constant. 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.
The GN of multiple equal flashes used in combination acting as one, is (GN of one) times sqrt(number of flashes).
The Guide Number charts are typically always printed showing ISO 100 values, but we can just multiply ISO 100 GN values by 1.41 to get ISO 200 GN values, or multiply by 2 to get ISO 400 GN values, etc. Or we can divide if converting going the other way.
New Guide Number = (Old Guide Number) x square root of (New ISO speed / Old ISO speed)
Doubling ISO doubles distance range, or doubles f/stop number, which is two more stops.
So we know we need flash power of f8 x 12 feet = GN 96 (feet) at ISO 400. The charts are always for ISO 100, but we know the ISO 400 factor is x2. So to go the other way, we divide actual the ISO 400 value by 2 to get the ISO 100 value (to compare in the ISO 100 chart). Converting this to ISO 100 is GN 96/2 = GN 48 (feet, ISO 100). We search in the speedlight manual's guide number chart (always ISO 100), and maybe we find the value at 24mm zoom and 1/4 flash power to be say GN 49 feet. More than close enough to 48. 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 need.
The charts show GN as both meter and feet values (as meters/feet), so use either one, so long as you are consistent with units. Duplicate this situation by setting flash to 24mm zoom, 1/4 power, and set the camera at ISO 400 and f/8, and 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, GN varies numerically same as f/stop numbers, so another GN value of about 10% difference is roughly within 1/3 stop (and 41% is one stop). We don't necessarily have to match lens and flash zoom values, we can use any flash zoom if as wide as the lens zoom - which may waste a little light if the flash is wider than the lens, but 1/4 power is not much burden (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 roughly about 1/3 stop. This chart example is for the Nikon SB-800 flash.
If you don't have a GN chart, you can make your own. Maybe you have a factory specification of one GN at full power. If no flash head zoom, then it is a constant, otherwise it applies to the zoom specified (which may be the maximum zoom, which gives the highest number). Or if no specification, you can determine it yourself - if say f/4 at 11 feet is judged to give correct exposure, then the GN is f/4 x 11 feet = GN 44 (repeat this at each flash zoom value). Either way, you can make your own chart - each lower 1/2 power step divides that GN by 1.414 for that ISO, and then, each doubling of ISO multiples GN by 1.414. There are a couple of tricky points, judging the degree of "correct exposure", and also, slightly different distances give slightly different exposures.
Measured GN may vary a little from the factory value. Some theories we hear are that advertised values from off-brand manufacturers might exaggerate their GN specs. Or that GN can increase in a small room where reflections from the near walls combine, but GN is lower in wide open spaces with no reinforcement. My own notion is that it takes a lot to overcome the Inverse Square Law, but a very near wall can provide fill. Another possibility is that the capacitors in an old flash may have deteriorated somewhat, it may not still have full capacity. My 25 year old Nikon SB-24 still performs to spec however. I think the most likely reason is that our own perception of "correct exposure" is not always precise, so try multiple readings with a few different subjects.
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". The hot shoe 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 here, 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.
We do have to know 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. It does not work for bounce, and bounce is pretty good stuff (TTL is wonderful for bounce). GN mode is for camera mode A or M, and 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).
In the real world, we usually just guess at the distance, so the initial result might be off a little, but like TTL, it will be a close starting point. Camera Flash Compensation is not operative in GN mode (camera metering is not involved), but 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).
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.
One special case: If you attempt to verify your speedlight's Guide Number chart at maximum power level, be sure your shutter speed is a bit slower than maximum sync speed, maybe 1/125 second. The speedlight becomes slow at maximum power, so if at the fastest shutter speed, the shutter closes on it, and the sensor can't see it all. If metering it, and it is on the hot shoe, the fastest shutter speed can quench the flash off too, so an external meter can't see it all (and it can recycle faster too). This is only a slight effect and only occurs at maximum flash power and at fastest sync shutter speed.
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.
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), 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). This "rule" is more like three stops for a vertical flash angle.
Whereas, 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 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. 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).
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.