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.
The full details explaining use of guide numbers are below the calculator.
Please report ( Here ) any problems with the calculator, or with any aspect of this or any page. It will be appreciated, thank you.
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.
Modern cameras are surely set to use third stops, and it's 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 is that results will be ± 1/2 step for any step selection.
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.
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 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 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 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 online). Comparing this calculation can be useful when shopping for a flash. However, I have seen one Chinese manual that simply advantageously had the wrong chart in it.
If one GN is rated for ISO 200, then dividing that number by 1.414 will give the ISO 100 equivalent. Guide Number can only be compared if both are at the same flash zoom and ISO settings.
Guide Number is used for speedlights, but is not very meaningful for studio flash. One reason is they are typically not used as direct bare flash, but also their GN rating situation is so unknown.They don't zoom, but comparison is difficult when we may not know what reflector was rated, or what its angular distribution spread is. Speedlight GN varies over probably a 2 to 1 range when they zoom... but we can only compare 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 has many variables, and 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).
Continued - More Guide Numbers next page.