Flash intensity falls off with distance, described by the Inverse Square Law. Guide Number (GN) 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.
Guide Number = Distance x fstop (values which actually give a proper exposure)
f/stop = Guide Number / Distance (aperture for other distances)
Distance = Guide Number / fstop (distances for other apertures)
The distance is the distance from flash to subject. The reference base is a known accurate Guide Number for one situation (with specified ISO and flash head zoom mm), from which other situations can be calculated. There are also other ways: 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. Even if you always only use TTL metering of flash, the calculator can be useful to show sample capabilities of your flash at the same settings.
Guide Number is a tool to determine exposure of direct flash with manual flash power levels.
The full details explaining use of guide numbers are below the calculator. I've never seen another GN calculator that offered any of these additional features.
Insufficient flash exposure is corrected with more flash power, smaller f/stop number (wider aperture), higher ISO number, or shorter distance.
Excessive flash exposure is corrected with less flash power, larger f/stop number (stopped down aperture), lower ISO number, or longer distance.
Shutter speed is not a factor of flash exposure (but shutter speed will affect ambient exposure and HSS exposure). Shutter speed used with speedlight must not exceed camera's Maximum Sync Speed (but again, regardless if shutter speed is 1/100 second or 15 seconds, it does not affect the flash exposure, which itself is relatively instantaneous).
Doubling Power level or ISO increases exposure 1 EV.
Doubling Guide Number increases flash exposure 2 EV.
Doubling f/stop number or distance reduces flash exposure 2 EV.
Calculator Limits on range of settings:
Power level: Full 1/1 0 EV to 1/128, -7 EV
F/stop: f/0.5 to f/90, -15 EV
ISO: 1.56 to 819200, -19 EV
You can change the settings manually for corrections, or if appropriate, use the calculated buttons to do it automatically. However, if a calculated button value is limited at its end value, then it can't go any further (so then change some numbers to a different combination, or use another button).
Note: Guide Number assumes Manual flash mode (where YOU set the flash power manually). If using TTL flash automation, the way to adjust metered flash exposure is by using Flash Compensation to adjust the metered goal (for example -1 EV Flash Compensation to reduce TTL flash exposure). Because, TTL automation adjusts flash power level to achieve its metered values, which overrides all other attempts of flash control. Note too, that Flash Compensation or Exposure Compensation do not affect Manual flash power level.
And Manual flash cannot tolerate Auto ISO, because Manual flash power level cannot react to ISO.
Distance Units: You can work in units of either feet or meters. All shown distances will be in those same units. Since there are 3.28 feet in one meter, the GN in feet is simply 3.28 times the GN in meters. Again, see the guide number chart in the flash manual for flashes that zoom (an example chart is below). The charts show GN in units of both feet and meters. Be sure you understand which zoom setting that GN applies to.
First enter the Full power Guide Number for the actual zoom mm you will be using (the GN chart in the flash manual shows these various GN for different zooms, and is probably for ISO 100). GN values are 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 simple basic GN calculator, offering the two choices (modes A and B) for computing either aperture or distance by simply dividing the GN, computed simultaneously, but independently, not related to each other. Use either/or, A or B. The Guide Number Chart in the flash manual is the source of the Guide Numbers for each speedlight zoom or power level.
However, mode C is a special option to enter Both aperture and distance as your goal of desired settings. Then setting Power or ISO to correct that exposure are the best choices to achieve that goal, but there are four possibilities (including changing goals of distance or aperture). You can always change any of the settings for a different result. Note that this method C is also how the GN chart is used manually without a calculator (see example of Use of GN Chart below). 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 required power and zoom settings to use. The GN charts offer full stops of exposure, but this calculator (and probably your gear) can work in third stops.
Calculating for Fill Flash Level in bright sun is possible, even if Guide Number computes a full flash exposure, and GN does Not consider ambient lighting. So it is not main stream procedure, but computing using the proper ambient exposure settings in Option C (using an aperture that results in shutter speed near Maximum Shutter Sync Speed, perhaps 1/200 second, and ISO 100 at perhaps f/11 or f/13 for bright sun) can compute the Fill flash level at that specified subject distance. This Fill Flash factor effectively increases the actual Guide Number so to speak (because it computes a perhaps -2 EV lower fill level.). So For fill level, a subject distance up to 15 feet may be possible with f/13 for ambient in speedlight mode (f/13 x 15 feet is GN 195, /2 for -2 EV fill falloff, is GN 98). Caution: Using a slower shutter speed or higher ISO will stop down f/stop more, which reduces effective speedlight flash power.
Shutter speed would be a factor for HSS flash (or for ambient light), but does not matter to speedlight flash, except for a speedlight, it cannot exceed the cameras Maximum sync speed. The speedlight flash is very fast to freeze motion, so (just to mention some speed) a shutter speed of maybe 1/160 second should be fine (the speedlight duration will be faster). You might use a faster shutter speed just to keep out any ambient light. There can be a small risk that fastest Maximum Sync speed (some cameras offer 1/250 second or faster) could very slightly truncate highest Full power level illumination, but any lower speedlight power level would avoid that.
Guide Number applies to Direct Manual Flash, with camera Manual mode (both ISO and aperture are set and fixed) at the specified zoom mm value. It is NOT point&shoot, but it all works rather well, except ...
Some obvious Procedural Errors are:
Using a handheld flash meter (incident, metered at the subject) will be much more practical and desirable for umbrellas or softboxes, certainly in a studio setting involving multiple flashes. These lighting modifiers affect GN value greatly, and even then, the Inverse Square Law (light path distance) for umbrella or softbox distance would have to be measured from the actual flash tube. Much easier to simply meter the light at the subject. The flash meter makes great studio setup be very fast, easy and repeatable, especially so for adjusting each of multiple lights for lighting ratio.
The Guide Number method can be very accurate for direct flash, and one big plus is that it is not affected by variations of reflected subject colors that fool reflected light meters.
Repeating again: If your flash specifies Guide Number for 24 mm zoom, but you are using it at 105 mm zoom, you absolutely need to know and use the 105 mm zoom Guide Number (see your flash manual for its complete Guide Number chart for various zoom settings).
Ordinary guide number calculators just do a simple division, modes 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 A & B goal. The calculator suggests a power level first, and then may subsequently offer an ISO Adjust (to more reasonable numbers), but there are four choices that calculate change. Or you can 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.
Modern cameras are surely set to use third stops, which clearly allow the most precise exposure choice. But you might be using an old camera or lens or flash that is limited to full or half stops, those can also be selected here too. If you select third stops, those results will find the nearest third. Minor, but using third stop aperture could still deviate up to ± 1/6 EV (0.167 EV) from calculated precise value (which is shown). And if you do mix half and third stop settings, it will compute, but you'll get a warning that it conflicts with the step mode selected (because 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 will still compute accurate numbers if mixed is how you really will work.
If visible, the Change buttons in C are suggested possible exposure solutions, but each button only changes its one property, which may or may not have enough range alone. The settings have wide maximum range, but do have limits. 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 second change may also be needed. At actual limits, it still computes and advises about needed EV, but cannot adjust past these limits (you then need to change 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 optional Adjust button appears that will shift us back towards ISO 100, at -1.67 EV power at ISO 100 again (if desired, often more reasonable). It may not always reach ISO 100 due to other limits, like power. You can do all of these actions yourself directly, the buttons just do the calculations.
A big part of the calculator purpose is about better understanding exposure and 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. Even if you only use TTL flash, you need to know that the light fall off means a 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).
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. 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).
Cropped sensors: If using a full frame flash on a cropped camera body, that's fine, The sensor may be more narrow, but the fash field required depends on the lens and what it sees. On the cropped sensor camera, if flash power is an issue, maybe for still adequate cropped area coverage with a little more concentrated power, then you might zoom the flash head a bit more than the lens focal length, up to the Equivalent Focal Length (which is crop factor times actual focal length of the lens). TTL or a flash meter works well then, or you could use the GN Chart for that new zoom value. If your frame corners go dark, that's too much flash zoom. But today, a few newer dedicated speedlights (same brand as camera, at least for Nikon) have both full frame and cropped modes, which can handle this automatically. These flash user manuals should have both GN Charts, for the two sensor types (cropped mode will zoom the flash head a bit more than the camera lens zoom, with still adequate coverage for the smaller sensor).
Shutter speed is Not a factor of flash exposure (except for HSS flash, but see flash basics Part 2 for regular speedlight flash mode). Flash power, f/stop, ISO and flash distance are the factors of flash exposure. Distance does not affect the direct sunlight (here on Earth, the Sun is always at the same distance from Earth), but distance 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.
So that's good to know to know, but guide number tells a lot more. If we know guide number is GN 80 (feet), then we know that 8 feet will need to use GN 80 / 8 feet = f/10 exposure. Or 10 feet will use f/8. That is certainly a lot to know (again, this is for unmodified direct flash).
If you meter your flash, either via TTL flash automation, or by using a handheld 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 knowing about Guide Number, but Guide Number certainly does help basic understanding, essential fundamentals of flash that we should know (how direct flash falls off with distance).
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 = Distance x fstop (values which actually give a proper exposure)
f/stop = Guide Number / Distance (aperture for other distances)
Distance = Guide Number / fstop (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.
We know that multiplying distance by √2 reduces light by 1 EV due to Inverse Square Law.
We know that multiplying a f/stop number by √2 reduces exposure by 1 EV.
And that doubling either distance or f/stop number reduces flash exposure by 2 EV.
Both Distance and f/stop numbering involve full stop steps of √2.
The product of Guide Number = Distance x fstop is a situation which if known to be correct exposure, then from the same Guide Number, any other distance or f/stop combinations can be calculated for correct exposure too. The great beauty is that this Guide Number system automatically and conveniently takes the Inverse Square Law into account for computing exposure of direct light from either flash or continuous light.
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 distance.
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 (same flash power level and ISO), we can know correct direct flash exposure for any distance or any aperture (those that multiply to same GN). 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). 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.
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 or lighting modifier (diffusion dome, reflector, bounce, umbrella, whatever) is a different guide number. Any other path than direct flash is a different subject (like bounce 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.
Zooming the flash head changes the guide number. Zooming in, to follow the lens zoom (a more narrow coverage angle), concentrates the flash power into a more narrow brighter beam with a higher guide number. There will be a guide number chart in the flash manual, specifying a different guide number for various zoom values. See a sample guide number chart just below.
If the speedlight does not zoom, then that's all it can do, so you can only use and compare that. Flashes that do not zoom (like the camera's internal flash) will have one guide number value. The Nikon D3200 internal flash specification shows it this way:
Guide Number: Approx. 12/39, 13/43 with manual flash (m/ft, ISO 100, 20 °C/68 °F)
For manual flash, this says GN is 13 / 43 (meters / feet). This is at full manual power, but a menu turns the flash power down as necessary, which lowers the guide number. The addition of temperature puzzles me, perhaps that's for the battery.
The GN chart is in the flash manuals, because if it has zoom, it will have many guide numbers (one for each zoom and power level combination). 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, or any other units, so long as you are consistent with units (compute equivalent GN, and use same units everywhere for both guide number and for distances).
For example, 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. Your flashes manual should have the GN chart for it. The chart entries are meters/feet, as specified. Feet GN is 3.28 x meters GN, so there is some rounding.
Example: Suppose we plan to use direct flash at f/8 at 12 feet at ISO 400 (this procedure is NOT difficult).
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). Next page has more detail about converting GN for ISO.
Or, if 24 mm is not best in your situation, of course don't choose that one. The chart includes a few other combinations of power level and zoom which are near GN 48. In some cases, you may have to modify your ISO goal, distance, or aperture. The GN need not be precisely exact, It's not linear but a 12% difference in GN is about 1/3 EV (and 41.4% difference is one EV). We don't necessarily have to match flash zoom to lens zoom values, we can use any wider (shorter) 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). Full frame flash on 1.5x cropped camera is already 1.5x wider anyway, so a cropped sensor allows a full frame flash to zoom in a bit tighter than the lens.
Say you really wanted to use 35 mm zoom instead of 24 mm. You can simply select any nearby zoom and power cell near 48 there (say 35 mm, GN 44 at 1/8 power, feet, 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).
Or the GN calculator above works too: Choose any zoom in your GN chart, and enter its full power GN (GN 125 here, feet, 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 your flash has it, use the GN Mode on next page, and 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).
So 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 (as shown typically next). This can be a fine starting point (again, direct flash only). Can be very helpful.
Guide Number continued on next page, including:
HSS Guide Number calculator is on third page.