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This is the way to compare power of two speedlights being considered for purchase, but GN comparison is Not very suitable for studio flash since the angular coverage of the various reflectors and modifiers used is rarely known. Even if direct flash, it is generally not known which reflector that any GN spec applies to.

If the flash can zoom, then it is necessary to compare with everything the same, including 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 (both direct flash with bare reflector), 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 the flash does not zoom, then it only has that one setting to do one thing.

Today, some flash marketing advertises their maximum mm zoom guide number, maybe at 105 or 200 mm zoom, simply because that is the largest number that looks better than others, regardless that we may rarely use flash at extreme zoom. Much flash use is indoors, and back in the day, manufacturers generally always specified GN at 35 mm as a reasonable number (for full frame cameras then). Some still do, but today, some of them specify the largest possible number. But for GN power comparisons to be meaningful, all things must be equal. So most speedlights offer a Guide Number chart in their user manual for the various zoom values, and that chart will be very necessary for using Guide Number. A speedlight that zooms has many guide numbers.

Example of a Guide Number chart (for a Nikon SB-800 speedlight): (values are meters/feet)

If using Manual Full power, ISO 100, and 35 mm zoom, then it says the measured Guide Number is 125 (feet). If the subject is at 10 feet, that appropriate f/number is GN 125 / 10 ft = f/12.5 (use f/13).

So today, to know very much about ratings, we need to look at the guide number chart in the user manual (sometimes the manual is online). It is very much more meaningful than the simple advertising. This correct Guide Number is be useful when shopping for a flash, but know what it specifies, what situation that the GN is specified for. Guide Number can only be compared if both are at the same flash zoom and ISO and power settings.

There is a very easy short cut needing no calculator to compare power levels. Just assume an easy hypothetical distance, like 10 feet (or depending on the GN units, maybe 2 or 3 meters). Going with 10 feet here, easy numbers done in your head. Perhaps one flash is GN 80 and so 10 feet gives f/8. The other is GN 56, and the same 10 feet gives f/5.6. These numbers easily indicate the first has one full stop greater power (because f/8 can be stopped down one stop more than f/5.6).

Or maybe the numbers are not as neat, maybe GN 90 / 10 feet is f/9 and GN 70 / 10 feet is f/7. And f/9 and f/7.1 happen to be third stops, 2/3 stop apart, so f/7 is a slightly greater difference (than f/7.1). Or this 90/70 is 1.29x, which is the distance ratio and f/stop Number ratios. As a rough guide, in both ratios, √2 is 1.414x which is one stop more power, and 2x is two stops more. This 1.29 GN ratio is 1.29² = 1.66x power ratio . And then the powers of 2 for EV is log₂(1.66), for which you'll need maybe the Windows or Google calculator, which is also log₁₀(1.66) / 0.3 = 0.73 EV difference (again slightly more than the 2/3 EV of f/7.1). Or, the little calculator here will do it.

The GN squaring for power or intensity is because the two factors of GN are the Inverse Square Law of distance and the numbering system of full f/stop steps, both using √2 steps to indicate 1/2x intensity values. You'll see that in the next calculator too.

This context of ganged means flashes probably all mounted on the same stand, and aimed at the same point, specifically acting as one (for a greater power level). But the common Main light and Fill light situation is acting as individual lights on the subject (see this about that). However, the background or hair lights are other separate lights.

Option 1: The GN of multiple equal flashes ganged in combination acting as one (at same distance from subject), 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.

Option 2: But if ganging two unequal flashes acting as one (at same distance from subject), say of GN 58 and GN 80 (0.93 stop difference), the 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). It's Not the average GN. 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).

Option 3 is camera Exposure of adding the f/stops of flashes individually metered at the subject which illuminate the subject frontally, which is typically the Main and Fill lights. Main and Fill are NOT likely at same distance but this case individually meters them at the subject. These should NOT include background light or hair light, since they don't illuminate the subjects face. We would set our main light, and then set the fill a lower ratio for fill level, and then we need the overall exposure for the camera. Normally we would setup the power of each light metered individually, one turned on one at a time, and then simply meter Main and Fill together (without background or hair lights on) for the camera aperture setting.

Metering a low number like f/2.8 measures a dim light, meaning that the camera aperture must be opened wide to match the dim light. Metering a high number like f/16 is a bright light needing the lens to be stopped well down, and f/16 is five stops brighter than f/2.8. This sum is only f/16.25, which is still nominal f/16 (only 0.044 EV different), but two lights added together are always some brighter than the brightest one.

When there are only two f/stops entered here, their difference in EV is shown, which is a measure of the lighting ratio. However, there are two definitions of lighting ratio. One is a power or f/stop ratio as shown here (which is very popular because we can easily meter it), and is popular in internet talk. But also see the real lighting ratio (ratio of the actual light on areas of the face) which might need a bit more thinking. When reading a lighting book that mentions a 3:1 lighting ratio, it may not be clear which way they mean it unless they mention the lights. Bottom line, we adjust the lights to be the look we desire, but a 1 EV difference in Main and Fill is a good general starting point for color portraits. Maybe 1.6 EV for B&W portraits, but 2 EV is becoming dramatic.

Option 4 is the same as 3, but metering in tenth stops, typically the Main & Fill lights. Tenth stops are very convenient for metering portraits, to easily compute your lighting ratio in your head. You will of course already have the flash meter that will meter the total too (for camera exposure settings, but turn the background and hair lights OFF to meter the camera exposure of the combined Main and Fill).

Studio lights: Metering each flash is the way to know what multiple lights are doing to set lighting ratio. Here’s a description of that metering procedure. Guide Number is commonly used for speedlights, but GN is not very meaningful for studio flash. One reason is they are typically not used as direct bare flash, but even if otherwise, their GN rating situation is too unknown to actually have meaning (which reflector that the GN applies to is rarely disclosed). Metering is far best, otherwise you should verify the GN of studio units yourself. They don't zoom, but comparison is difficult when we may not know which reflector was rated, if any, or what its angular distribution spread is. When speedlights zoom, their GN varies, over often a 2 to 1 range. But we only can compare GN power and intensity when lighting the same angular coverage, when doing the same job. Instead studio flash is commonly used with a light meter (specifically to meter each light to control lighting ratio).

Still, Guide Number does work very well for any unmodified direct light (even your table lamp or yard light), if you determine the correct GN for the reflector in use. If you verify that f/4 at 10 feet is a correct proper exposure, then the GN is 4x10 = 40 (feet), for the ISO being used (until you change something about the light). One big issue for studio flash is that guide number cannot be specified for bounce or umbrellas, etc. (because, intensity depends on them). So typically, direct bare flash is much less frequent 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.

Otherwise, the guide number that may be specified for a studio flash possibly applies only to the included standard bare reflector it ships with (but we have no clue what it means if conditions are not precisely stated, and it's normally not). 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).

Zoom mm	Vertical Degrees	Horizontal Degrees
24	60	78
28	53	70
35	45	60
50	34	46
70	26	36
85	23	31
105	20	27
120	18	25

Here's a typical speedlight chart of flash angular degrees coverage (Nikon SB-700, page H-20, for Full frame). The flash will necessarily be several degrees wider than lens coverage at same zoom. The absolute meaning is hard for users, because the intensity falls off so gradually at the edges, it’s difficult to decide when it disappears. Is it gone at 10%, or 1% or 0.1%? So engineers typically rate such fields measured at the half power points (because that can be determined more precisely, but there is a lot still left). So the point, if comparing to the standard Alienbees 80 degree reflector, maybe at least compare speedlight GN at 24 mm? A fully powered speedlight will compare as about 75 watt seconds, and the SB-700 as maybe 60 watt seconds. Watt seconds cannot be converted to guide number, because watt seconds is electrical Input power, and GN relates to relative output illumination per unit area. Meaning, guide number is also affected by electrical efficiency, and by the concentration of the light by the reflectors coverage area.

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, and that is very convenient to do in the studio.

Converting Guide Number for New ISO or a New Power Level

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.

Guide Number is always (f/stop x distance) giving correct exposure.
If determining this GN when using ISO 800, then it is the GN for ISO 800.

Relative to an Old ISO of 100, then:

Using this second chart, to convert GN for new ISO is:

  New GN = Old GN × GN Factor

or the general case:

  New GN = Old GN × √New ISO / Old ISO

The Guide Number Calculator (previous page) will do any of this.

Guide Number Math

The guide number is multiplied or divided by 1.414x for each stop changed (same as f/stop numbers), which occurs at each doubling of ISO, or at each doubling of flash power level. 4x ISO or 4x 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). One significance of that is that the ratio of two f/stops or the ratio of two distances must be squared to reflect the ratio of the intensity or power. This is NOT true of ratios of shutter speed or ISO, but only for f/stops and distances.

Generally, the effect of flash power level on maximum Guide Number is GN Reduction = √PwrLvl, or example, 1/4 power is √0.25 = 50% of maximum GN.   Each half power step reduces GN by √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 example GN chart above.

But the absolutely most useful thing to realize is that 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, so the best thing to know about manual flash is that each step of half power is one f/stop change of exposure. One stop is a 2x factor, so said again, reducing the flash power from Full to half, or from 1/4 to 1/8, or from 1/16 to 1/32 (each of which is half power) reduces the exposure by one f/stop. This is extremely convenient in use.

GN Accuracy

Guide Number is accurate if it is accurately known. GN is just the Inverse Square Law, it is all just math and physics, and GN exposures should be quite accurate if the initial GN was correct. We can determine and verify that for ourself. Another advantage is that GN is also independent of the reflected subject colors which do fool camera meters. We should realize that neither TTL metering nor Guide Number calculations are always precisely correct, but the bet is on Guide Number if the initial Guide Number was accurate. 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 flashes.

Our 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 where the importer included the wrong GN chart from 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 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 a short while every six months or so, to keep them formed (don't store with batteries in them though). But my 30 year old Nikon SB-24 speedlight still performs to GN spec. Nikon advises replacing the flash tube every couple of years (white balance can change a little), but my notion imagines that is about heavier professional use.

I feel sure the honest GN charts (at least from major brands) carefully meter exposure for one ISO in f/stops, and then GN is computed and usually rounded to two significant digits in meters. Then I assume the original measured distance is divided by 3.28 for feet (3.28 feet in one meter). Honest measurements are easy to do, and then 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 can be variations in different situations.

But it's easy to verify Guide Numbers yourself. Use f/2, f/4, f/8, or f/16 for that (those nominals are actually precise values, but like f/11 nominal is actually f/11.314, or about -0.08 EV different than numerical 11). I do think (at least in my own case) the most likely reasons for GN differences in use is 1) guessing distance, and not actually accurately measuring the actual distance, and 2) scenes vary, and our own perception of "correct exposure" is not always precise. So check multiple tries at GN in different measurement situations and verify in different usage situations. Do actually take pictures to validate it carefully (in more than one situation). My own experience is that the Nikon guide numbers always seem very accurate. If I measure the distance and take the picture, I get fairly good exposures, at least certainly 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 (not checking the picture), 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 guide number at maximum power level, don't use your maximum shutter sync speed (if at full power). 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. Even if metering it with a handheld meter, and flash is on the hot shoe, the fastest shutter speed closing can also 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 possibly only at fastest sync shutter speed. No problem at any lower power level or slower shutter speed. Flashes vary, but I do see my SB-800 at slightly reduced maximum flash exposure at 1/320 second, but it seems negligible at 1/250 second. Lower power levels of speedlights are greatly faster. Most studio monolights are the opposite, slowest at their lowest power levels. This is because speedlights implement lower power by truncating the flash shorter, but most studio lights just turn the voltage down lower.

GN Flash Mode

Guide Number is really easy with flashes that have a GN mode. These Nikon CLS flash models do have GN mode in the menu:

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 manual flash mode with GN (camera metering is not involved). If you want the flash a bit brighter, open the aperture a bit, or tell the calculator 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).

Macro

Macro situations may be troublesome for Guide Number, focusing at inches at instead of feet. You may prefer to bounce the flash, which does not use GN then. Or the flash can be more distant, or you can work GN in fractions of a foot, or the Guide Number can be converted to inches by multiplying GN in feet by 12 (GN 10 feet / 1 foot = f/10, or GN 120 inches / 12 inches = f/10. The GN calculator works with any units (inches, cubits, yards, etc) if you are consistent this way (with accurate converted GN), and measure the flash distance accurately and in same units.

But the bigger problem is that when macro lens (or if extension tubes too) is focused at macro distance, then you must understand that changes its effective f/stop number, for example, if at 1:1 magnification, the f/stop number actually used is doubled (and doubled is -2 EV). Meaning for example, if the lens is marked f/16, and it is set to f/16, it actually becomes f/32 if at 1:1, and GN needs to use that f/32 number (See Google). And this increase varies with magnification:
This macro Exposure Increase Factor (EIF) = (magnification + 1)², which is log₁₀(EIF) / 0.3 EV
1:1 is magnification 1, which is EIF 4x which is -2 EV due to increase in f/stop number.
1:2 is magnification 0.5, which is EIF 2.25x which is -1.17 EV.
1:4 is magnification 0.25, which is EIF 1.56x which is -0.64 EV.

Some macro lenses (Nikon) automatically report this new modified f/stop number to the camera, so you would actually see and set f/32 (if desired). Also internal focusing macro lenses today can shorten macro focal length a bit to extend less to change reported f/stop number to perhaps 1/3 or 1/2 stop less than calculated.

However, TTL meters the light through the lens, and exposure is automatically metered as modified, and TTL works with bounced flash too (and there's lots to be said for both TTL and bounce). Worst case with TTL just means you have to use a bit of flash compensation. If metering with a handheld meter, you do have to adjust the camera f/stop number for this macro light loss. And guide number must also use the correct modified effective f/stop number. TTL becomes very a convenient choice for macro.

Bounce flash is an Exception

Bounce is often the very good stuff, but Guide Numbers do not work for bounce, because the total distance up and down is greater than the direct path, and there is also considerable reflection loss at the ceiling. TTL is wonderful to actually meter bounce (reflected metering), but bounce will typically 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 this is quite crude and vague, since every situation was different (ceiling height and texture and reflectance, and flash head angle, etc). But 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).

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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. Guide number 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 seeing exactly what the lens view sees. We liked that too, it was great to be able 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 good, but can be 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 50 feet, GN 262 / 50 feet = f/5.2. Piece of cake, ISO 800 should not be necessary. 50 feet is at least a 10x7 foot field with 105mm on an APS cropped sensor at 50 feet. Even if planning to use TTL, this is all 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 Number calculator on next page.