FLASH SYNC
Sync speed is the fastest shutter speed you can use with flash. You cannot use a faster shutter speed than the sync speed with flash. If you try on a camera more than about 20 years old you'll get a partial blackout of the image, and modern cameras override you electronically when the flash is on.
The faster sync lets us shoot at larger apertures, which requires less flash power, which lets our flash recycle faster, which lets us shoot more frames per second.
Changing ISO does nothing to help since it's the ratio of ambient to flash light that's important.
Exposure equals the area under each curve.
The total area for the ambient light increases as you consider more time horizontally, due to a longer shutter opening.
The area (exposure) for the flash doesn't change so long as the exposure is at least 1/1,000 second to capture it all.
The exposure from ambient light increases as exposure time (flash sync speed) gets longer, but the contribution from the flash stays the same. One uses smaller apertures with longer

exposures, which keeps the ambient exposure constant but requires more power from the flash to keep up.
It is difficult for battery powered flashes to compete and balance with direct sunlight as flash sync speeds get slower. I often run out of range in daylight with slower syncing cameras, or my flash has to fire at full power and take a long time to recycle. a.) Flash is an instantaneous burst which lasts only about 1/1,000 of a second. You get the same contribution to exposure from the flash regardless of shutter speed. Only the distance, aperture and ISO affect it.
Flash contribution = f(Distance, Aperture, ISO) b.) The contribution to exposure from ambient light varies with shutter speed, but not distance.
Ambient light contribution = f(Shutter speed)
c.) Changing aperture or ISO does nothing to affect the ratio between flash and ambient light. The factors which do change the ratio are distance, shutter speed or flash power.
Ratio bw Flash and Ambient light = f(Distance, Shutter speed, Flash power)
d.) If you have the freedom to go to a higher sync (shutter) speed you can use a higher ISO or larger aperture to keep the same ambient exposure while increasing the sensitivity to the flash. Most battery powered flashes have to work very hard to compete with direct sunlight unless they are very close. Faster sync lets you get further away or use less flash power. Using less flash power increases battery life and reduces recycling time which increases frame rate!
See also Shooting Sequences with Flash .
e.) Here's a subtle distinction many people miss: simply using a lower ISO or bigger aperture setting doesn't help unless you also have access to faster shutter sync speeds to keep the ambient exposure unchanged.
Example one: You can't reduce the required flash output by increasing the ISO. You will need to use a smaller aperture to keep the ambient exposure constant because you can't increase the shutter speed above the sync speed. Because of the smaller aperture at the higher
ISO you still need the same flash output. A higher ISO can't get you more range or better battery life.
Example two: A lower ISO can let you use larger apertures for portraits, but it won't speed up recycle time (and thus frame rate) or increase range. If you set a lower ISO and use a larger f/stop you still need the same flash power output. A faster sync speed would give you more range or require less power from the flash when using the larger aperture.
There are no disadvantages to the availability of faster sync speeds. Faster rated sync speeds simply give you more options. You always can use slower speeds with flash.

At low light levels slow shutter speeds are needed to let in enough ambient light. Sync speed is not a limitation. We don't need to hold back the ambient light to keep it from overpowering the flash. In dim light we always wish we had more ambient light!
To sum up, sync speed is critical for using fill in daylight, and fill is critical to getting good images in direct sunlight.That's what makes sync speed critical, in addition to stopping motion.
Sync speed isn't an issue in dimmer light.
Trick Modes
Marketing departments like to push the "FP" trick flash modes that seem to let you sync at any speed, even 1/8,000. This doesn't count as far as I explain later here . Canon calls this
"High Speed Sync" and I forget what Sigma and others call it. It is not a substitute for genuine fast sync speeds.
At slower sync speeds like 1/180 you're far more likely to get subject or camera motion and blur than you are at 1/500. You knew this.
Depth of Field Flexibility
With faster sync speeds you can use larger apertures while retaining the correct ambient light exposure for fill flash. This is the key to all the other related benefits. Simply setting a slower
ISO does nothing to help all the below issues since you have to pump out more flash power at the slower ISO.
Maximum Flash Range
Larger apertures let in more light from the flash which allows you to get further away with any given flash. For a full stop increase in sync speed you get a 40% increase in range since you can open the aperture up a stop. For a two stop increase (going from 1/125 to 1/500 for example, which lets you open your aperture two stops) you double your maximum flash range.
Flash Battery Life
Larger apertures let in more light from the flash and thus less power needs to come from the flash for each shot at a given distance. Less battery power is used to recycle the flash for each shot, since modern flashes only use the power needed for each shot and conserve the rest for the next shot.
Flash Recycle Time
Since less power is used for each shot it takes less time for the flash to recycle since it only needs to replace the power that was used. If you haven't noticed this, time how long your flash takes to recycle when set to a manual power setting of 1/8 (almost immediate) and then see how long it takes when set to full (about 5 to 10 seconds).
Maximum Frame Rate

You can't shoot your next shot until the flash is ready. When the flash recycles faster you can shoot faster and at a higher frame rate. Most flashes are still ready to go when used at lower powers at the larger apertures you can use with higher sync speeds.
Buffer Depth
Most flashes can shoot continuous bursts at lower powers. The lower the power, the more flashes they can fire off at a time. If more flash power has to be used for each shot due to smaller apertures caused by slow sync speeds you cannot get as many continuous flashes without breaking cadence and having to wait a few seconds for the flash to recuperate. When you loose your flash oomph you can't continue shooting your sequence.
Allows Use of Smaller Flashes
For every stop of increased sync speed you only need half the flash power. Thus a smaller, less expensive flash may be all you need. Even better, the built-in flash might be all you need for fill!
Stopping Motion
Depth of Field Flexibility
Flash Range
Flash Battery Life
Flash Recycle Time
Maximum Frame Rate (FPS)
Buffer Depth
Allows use of smaller flashes
Faster Sync (1/500)
Better
More
Longer
More
Shorter
Faster
More frames
Yes
Slower Sync (1/125)
Worse
Fewer large apertures available
Shorter
Less
Longer
Slower
Fewer frames
No
Electronic flash is an instantaneous blast of light. At full power your flash may only last for a thousandth of a second. At typical power levels and with automatic settings it's probably be closer to 1/10,000 of a second.
The mechanical focal-plane shutter of film 35mm SLR cameras and Leicas are two curtains of metal or cloth that zip across the front of the film. At slow speeds like a full second they zip fast enough to appear to open and close immediately.

What's not obvious to the naked eye at fast shutter speeds is that the second curtain has to start zipping across the film right behind the first curtain. It has to do this because the curtain speed is not instantaneous. At fast shutter speeds the film is effectively exposed through a slit that zips across the film.
If you pop a flash at one of these faster speeds then only the part of the film behind the open part of the slit would be exposed to the flash.
The sync speed is the fastest speed at which the entire film or CCD can be open to light. This is determined by how fast the shutter curtains move.
At speeds faster than the sync speed the slit that travels across the film or CCD narrows. If you used flash at faster than the sync speed (you can't do this on modern cameras) you would only expose the part of the film behind the slit to the flash.
Nikon has done the world a great favor in the D1, D1X, D1H and D70 cameras by adding an electronic shutter to take care of the faster speeds. By doing this the sync speed becomes unlimited. The only reason the D1, D1X and D1H (and for all I know the D70) limit the speed to 1/500 is because of the loss of efficiency above that speed for shoe mounted flash.
Amateur digital SLR cameras omit the electronic shutter and are limited by the mechanical focal plane shutter.
Point-and-shoot digital cameras usually sync at high speeds like 1/500 because they also have electronic shutters.
Leaf shutters as used in professional cameras like the Hasselblad and large format cameras can sync at any speed. This is because their leaves open completely at all speeds, at which point the flash is fired. There is no slit or partial opening at the fastest speeds.
(also called High Speed/FP flash synch on Canon) back to top
Now that you know that the shutter needs to get all the way open to let the instantaneous pop of an electronic flash expose all of the film or sensor, what if instead we made the flash stay on long enough to let the curtains of a focal plane (FP) shutter complete their travel from one side to the other? We could let the flash expose the film or CCD at any shutter setting during the time it takes the curtains to travel from one side to the other.
You can do that as a trick mode with many high-end flashes and cameras. It's called the FP mode.
There are many disadvantages, which is why I don't ever use these modes.
1.) FP mode often reverts to totally manual exposure calculation. If so, it's only useful for shooting things that stay at the same distance so you can calculate it. I saw this feature illustrated in a Nikon brochure, and lo and behold, the two examples were 1.) a water skier shot from the boat (the rope stays the same length) and 2.) a shot of a race car made from a

camera bolted to the car and fired remotely. Some newer cameras like the D2H have automated this, finally.
2.) The flash always pops at full power on the flashes I've seen. Thus as above you lose battery life, have long recycle times, no high frame rates and all the other disadvantages above.
3.) Since only a fraction of the light at any time is exposing the film or CCD you lose a lot of light, again getting you back to the problems of limited flash range. The loss of light also depends on the shutter speed you use, thus the flash guide number used in manual calculations in 1.) above changes with shutter speed! You lose most of the light at the faster speeds and lose less at slower speeds.
At least FP mode gives you the flexibility to use any shutter speed and also because you lose a lot of light you can get to the larger apertures.
This can be a handy feature in limited applications, however there are so many limitations you see why I don't consider it anywhere as useful as a healthy true sync speed.
Some of the very latest cameras like the Nikon digital SLRs and Minolta Maxxum/Dynax 7 have FP modes that are TTL and do vary the power down from full. This way you have only the disadvantage of 3.) above. I have not researched these; believe it or not I'm too lazy.
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1/2,000 and above : Nikon D1, D1X and D1H cameras when used with an external PC cord.
The Nikon D70 when not used with a dedicated flash, for instance, an old 1980s nondedicated flash or probably also when used with the shoe-mount sync adapter for external flash. You can fool the D70 by undedicating the flash by covering all but the large central hot shoe contact.
These camera's electronic shutters are efficient enough to sync clear up to the full 1/16,000 or
1/8,000 speed, although of course with most strobes you'll lose a lot of light. You won't get a partially black frame as you will when using the wrong sync speed with a conventional focal plane shutter camera.
1/1,000: Rollei PQS series leaf shutter lenses.
1/500 : Most professional DSLR and film leaf shutter cameras including Hasselblad, 4x5" view camera lenses, the Plaubel 67 , and the Nikon D1, D1H, D1x and D70 when used with shoe-mounted flash. Also most point-and-shoot digitals, since they often incorporate electronic shutters.
1/250 : most good 35mm film SLRs from the 1990s through today. The Nikon D2H and D200 and Canon 20D and fancied DSLRs
1/125 - 1/250 : Amateur SLRs from the 1990s and amateur DSLRs like the Canon 10D ,
Digital Rebel , Nikon D100 and the Sigmas etc.

1/60 : Amateur SLRs from the 1970 and 1980s, like the Olympus OM-1, Minolta SRT series and X-700 .
1/50 : Leica rangefinder cameras, even today (pretty poor!)
1/30 : focal plane shutter SLRs before the 1970s, X-sync leaf shutters used with M type flashbulbs, Russian medium format SLRs like the Kneb/Kiev.
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Flashbulbs use magnesium foil that burns when triggered by an internal electrically fired squib. They usually burn for about 1/30 of a second and take a little while to reach full brightness. Thus shutters intended for flashbulbs have a position marked "M" which triggers the flash an instant before the shutter actually opens.
You can use all speeds on a leaf shutter with flashbulbs and M sync, although of course at the faster speeds you'll not capture all the light of the flashbulb. The guide number tables that come with your flashbulbs reflect this by having different values for different shutter speeds.
Flashbulbs, unlike electronic flash, are not very good for stopping action. That's why Nikon named their electronic flashes "speedlights" when they came out in the 1960s.
For focal plane shutters you need special "FP" (Focal Plane) flashbulbs if you want to use them at shutter speeds faster than 1/30. Just like the screwy FP modes of some fancy electronic flashes, use at higher speeds wastes a lot of light but it works. FP flashbulbs attempt to have a constant light output across about 1/30 of a second so as the focal plane's slit moves across the film you get even exposure. Regular flashbulbs have a peak of light and are best used with leaf shutters if you need to shoot at faster than 1/30.
Older cameras and shutters intended for use with both electronic flash and flashbulbs have either a two position switch or two separate sync terminals. "M" is for flashbulbs and "X" is for electronic flash. "M" fires the flash an instant before the shutter opens to allow the flashbulb to be at the best brightness. "X" sync triggers the flash the instant the shutter is completely open, since electronic flash fires instantly.
You can use most flashbulbs on modern cameras at 1/30 at either sync setting.
If you use an electronic flash at the M setting you'll see no flash on the film, since the flash would have been triggered an instant before the shutter opened. This used to mess a lot of people up in the 1970s since the flash would go off, but not show up on film!
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As you've read above, "M" sync on older shutters fires the flash before the shutter opens, usually ruining your photo with electronic flash.
On the other hand,what if you photographed something that glows in the dark? Yeah, the flash will pump it and your exposure will be made at the maximum glowing with no contribution from the flash. This is a cool trick for photographing phosphorescence.
Sync speeds

1/500 : Most professional DSLR and film leaf shutter cameras including Hasselblad, 4x5" view camera lenses, the Plaubel 67 , and the Nikon D1, D1H, D1x and D70 when used with shoe-mounted flash. Also most point-and-shoot digitals, since they often incorporate electronic shutters.
1/250 : most good 35mm film SLRs from the 1990s through today. The Nikon D2H and D200 and Canon 20D and fancied DSLRs
1/125 - 1/250 : Amateur SLRs from the 1990s and amateur DSLRs like the Canon 10D ,
Digital Rebel , Nikon D100 and the Sigmas etc.
1/60 : Amateur SLRs from the 1970 and 1980s, like the Olympus OM-1, Minolta SRT series and X-700 .
1/50 : Leica rangefinder cameras, even today (pretty poor!)
1/30 : focal plane shutter SLRs before the 1970s, X-sync leaf shutters used with M type flashbulbs, Russian medium format SLRs like the Kneb/Kiev.
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Flashbulbs use magnesium foil that burns when triggered by an internal electrically fired squib. They usually burn for about 1/30 of a second and take a little while to reach full brightness. Thus shutters intended for flashbulbs have a position marked "M" which triggers the flash an instant before the shutter actually opens.
You can use all speeds on a leaf shutter with flashbulbs and M sync, although of course at the faster speeds you'll not capture all the light of the flashbulb. The guide number tables that come with your flashbulbs reflect this by having different values for different shutter speeds.
Flashbulbs, unlike electronic flash, are not very good for stopping action. That's why Nikon named their electronic flashes "speedlights" when they came out in the 1960s.
For focal plane shutters you need special "FP" (Focal Plane) flashbulbs if you want to use them at shutter speeds faster than 1/30. Just like the screwy FP modes of some fancy electronic flashes, use at higher speeds wastes a lot of light but it works. FP flashbulbs attempt to have a constant light output across about 1/30 of a second so as the focal plane's slit moves across the film you get even exposure. Regular flashbulbs have a peak of light and are best used with leaf shutters if you need to shoot at faster than 1/30.
Older cameras and shutters intended for use with both electronic flash and flashbulbs have either a two position switch or two separate sync terminals. "M" is for flashbulbs and "X" is for electronic flash. "M" fires the flash an instant before the shutter opens to allow the flashbulb to be at the best brightness. "X" sync triggers the flash the instant the shutter is completely open, since electronic flash fires instantly.
You can use most flashbulbs on modern cameras at 1/30 at either sync setting.
If you use an electronic flash at the M setting you'll see no flash on the film, since the flash would have been triggered an instant before the shutter opened. This used to mess a lot of people up in the 1970s since the flash would go off, but not show up on film!

Rapid Fire
These are simply firing a camera rapidly. This is the most common. When used without flash you can just keep firing away, at typically a few frames per second
(FPS) with most cameras. FPS is usually specified as a feature.
Flash recycle times are usually a few seconds, however that's when they are used at full power in manual or at their maximum distance. For the past 20 years automatic flashes that don't shoot their full load of power can make their next shot immediately without having to recycle fully. If the flash uses only a small amount of power for each shot you also can shoot with the flash at several frames per second. For instance, if the flash only needs 10% of it's full power for each shot you can make ten shots as fast as you want before draining all the power from the flash's capacitor.
You keep the flash power low by moving closer, using a higher ISO, or using a larger aperture. This is why fast flash sync is important, since the faster sync allows larger apertures for sequence shooting. (more here about fast sync.) Of course you also can dial down the power in manual mode, but that's the hard way.
More powerful flash units can get off more shots before they are exhausted.
After you've made all the shots of which the flash is capable in one burst you have to wait the full several seconds for it to recharge or recycle before you can do it again.
Of course as you're shooting it's doing it's best to recycle, too. Don't bother calculating all this. Just shoot away and the flash will deal with this as best it can. If you can't get enough shots in a sequence just get closer, crank up the ISO or open your aperture.
Do this a lot for a long time and you may be able to melt a flash unit. Just feel the front of it now and then and back off if it's getting too hot. Nikon specifies in their user manuals how many full power dumps you're allowed and how long you have to wait for cooling without melting the flash. I find I can do much more than that and my flash still works. I don't worry about this, but I'm not shooting pro sports all day long either.
Just be careful.
Stroboscopic
These shots are rare and usually only seen in sales flyers for flash units with too many features. These show a moving object exposed several times on the same frame with a dark background. These sales brochures usually show something lame like a bouncing ping pong ball or someone swinging a tennis racquet in the dark.
Wow.
These strobe modes are where the flash fires many times per second as long as you hold down the flash button, just like repeating strobes on an airplane at night. I find the best use of this feature is if you are transported back to the 1970's and find yourself in a disco. To use this for photography you set the camera on Bulb and hold open the shutter in the dark to get a sequence multiply exposed in one frame.
Because the long exposure has to be as long as the sequence I've only seen these made in the dark. Wow.

You can tell I'm not a fan of this mode, which is a popular feature put on more expensive flash units to make you think you need them. Personally I prefer flash units without this feature, since this feature gets in the way when I'm selecting among the modes I really use.
Composites
These are seen in action sports magazines like "Snowboarder" where you have sequential multiple images of an athlete pulling an aerial maneuver in front of the same background. These great images are often used to illustrate the steps in a complex move.
These take a long time to composite together in Photoshop. You start with a conventional sequence of rapid fire shots, with or without flash. You then choose one shot as the background image, and of course save a little time by choosing one with the athlete in it. You then select out the athlete from the other images and using layers composite them on top of the background image. It involves a lot of selecting, so these are easiest to do if framed against a sky.

http://www.scantips.com/lights/flashbasics1c.html
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, then we know GN:
Or, if we know the Guide Number is 80, then we know 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/11 at 7.3 feet = GN 80 f/2.8 at 28.6 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 / 7.3 feet = f/11
GN 80 / 28.6 feet = f/2.8
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 at the different distances, because 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.
ISO 25 50 100 200 400 800 1600 3200 6400
Factor x0.5 x0.71 x1 x1.41 x2 x2.83 x4 x5.6 x8
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.
Suppose we plan to use direct flash at f/8 at 12 feet at ISO 400.
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.
Guide Number is really easy with flashes that have a GN mode.
These Nikon CLS flash models do have GN mode in the menu:
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, it 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).
Downsides of Guide Number: - So to repeat: Problems are, the guide number method needs to know a fairly precise distance from flash to subject, which leaves out bounce flash. Or, we can always guess roughly, 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 learn 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 and the sensor can't see it all. If 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 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 selfmetered 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.
Summary
Guide Number and Exposure are not quite the same thing. Guide Number = Aperture number times Distance (for combinations of correct exposure), and these two factors cancel each other's √ 2 factor to easily account for the inverse square law. This major point is a big deal, that the purpose of Guide Number is the easy way to bypass the math of inverse square law.
Equivalence of flash power and Guide Number cannot be computed - there is no equivalence of watt seconds of power and Guide Number. Guide Number largely depends on the modifier

distribution angle anyway (zoom, or reflector, etc). But in some cases, we can "compare" flash power by using special cases of GN.
If you are going to compare power of flashes by their Guide Numbers, then you have to know this tricky part (many terrible errors are made by not understanding this part). We cannot just compare any two Guide Numbers . This comparison is only valid for the same flash situation, specifically, for the same reflector and same angle of coverage (same zoom for speedlights). If a wider reflector or zoom distributes the same "power" over larger area, then the metered "intensity" goes down at any point inside that large area. Or concentrate the same power into a more narrow beam, and the metered intensity goes up, inside that narrow beam.
That increase is not more power, it is the same flash and the same power, but just not doing the same job. The "power per unit area" times the coverage "area" is the same flash power level. Speedlights have many Guide Numbers, one for each zoom angle. We should know which one are we comparing. We can only compare like things, but not apples and oranges.
The guide number is as much or more about the reflector coverage angle as about the power level . If the guide number of one flash is specified as GN 180, but its zoom coverage angle is concentrating all the power to only illuminate a small spot on the wall, and another flash specifies GN 90, but its wide reflector is illuminating the entire wall, then we don't know much (I'd bet this second unit was the strongest though). Wide angular coverage requires a lot of power (illumination per unit of area). It does not help to zoom tight to concentrate the power into a small bright spot, if we are trying to photograph the large wide group or area.
GN is the illumination at one spot, inside that area, but we can only compare "power" for similar coverages. Make the coverages equal in size (same zoom angle), and then compare the guide numbers. It is part of something else, but this point is discussed more Here .
Speedlight manuals generally have a chart of many different Guide Numbers , a different
GN for each zoom setting (angle of coverage), and for each power level. A larger GN if the flash head is zoomed in tight, or a smaller GN if zoomed out wide. There is a Guide Number chart in the flash manual. This published GN is always speaking of unmodified direct flash, and usually for ISO 100 by convention. Any different ISO, or different power level setting, or zoom angle, or bounce or umbrella or any other modifier will change it to be a new situation, when any old guide number is no longer applicable. GN may be in feet or meters - we can use either so long as we are consistent. There are 3.28 feet in one meter, so GN (feet) is always
3.28x GN (meters).
Guide Number is not affected by subject reflectivity (like camera light meters are). But in contrast to speedlights, studio lights use a large variety of possible light modifiers (umbrellas, softboxes, grids, snoots, etc), and so do not bother with Guide Number, since every case would be very different. Studio photographers simply use handheld flash meters to measure what is actually happening. The studio light manufacturers instead publish an input power level in watt seconds (or may be called joules in Europe, 1 joule is equal to 1 watt second), which is a maximum capability. Actual output depends on efficiency, and also how the modifier concentrates and distributes that light.
To be told that the Guide Number of some flash is GN 120, means nothing in itself. We hope the rating is not exaggerated, some manufacturers do. We can assume ISO 100 if not stated otherwise - ISO 100 is sort of the standard norm. But GN is as much about the reflector angle as the power, and the situation must be stated to be meaningful. If not stated (then who knows?) but it possibly could mean:

If it is a speedlight, speedlights have many guide numbers. The GN rating advertised may be when concentrated at maximum zoom (highest number). That is the best marketing number.
Older flashes used to specify the 35mm guide number, sort of an "average" value. Neither may be how you will use the flash. The manual likely has a Guide Number rating at each zoom value.
If it is a studio light, any GN rating probably implies when using only the standard supplied reflector, but what coverage is it? 40 degrees? 80 degrees? It makes a big difference. Is it how you will use it? The GN value no longer applies when you put an umbrella on it.
We can only compare like things, doing the same job - specifically, when the angle of coverage is the same, doing the same job. If one light is zoomed to make a small spot on the wall, and the other is illuminating the entire wall to the same intensity (same exposure) - that is NOT a meaningful comparison.
Examples, comparing power of speedlights without having them to meter. I call this the
"Same Zoom at Ten Feet" method.
If the Guide Numbers are for the SAME ISO and the SAME flash zoom, we can compare power levels by assuming the same arbitrary flash distance to get an aperture from GN. For this, we can simply assume ten feet, to simply divide GN by 10, to determine apertures at ten feet (from the published GN chart). The purpose is to show an aperture number to be able to understand the power difference in stops, of the different flash models (again, any comparison is only meaningful at the same ISO and same Zoom coverage). If the aperture is one stop higher, that is double power. Or, if the f/stop NUMBER itself is double, that is two stops, and
4x more power.
The standard for GN charts always use ISO 100 values. We could multiply each value by 1.4x to create ISO 200 values, but which does not matter just to compare levels, like we are comparing here, from the flash and camera manuals (online in most cases). We only need to be consistent.
The Nikon guide number charts (for feet) in the manuals show these Guide Number specifications for full power level, which allow these simplifications to compute correct exposure (here, for GN / 10 feet, and GN / f8). This assumed ten feet of course allows us to compare the power potential of the flash models, in stops of aperture, or f/8 shows a distance range. These numbers are feet (divide GN and distance values by 3.28 for meters).
Doubling ISO doubles distance range, or it doubles f/stop number, which is two more stops.
At 24mm zoom and ISO 100, and maximum power level (and direct flash of course)

24mmNikon Flash Model
SB-800 (FX)
SB-900 DX
SB-900 FX
SB-600 (FX)
SB-700 DX
SB-700 FX
SB-400
DSLR internal flash
SB-R200
GN 91.9
GN 75.5
GN 69
GN 42
GN 33
Guide Number
Feet, ISO 100
GN 98
GN 111
GN 88.6
GN 85.3 f/9.2 f/7.5 f/6.9 f/4.2 f/3.3
Aperture at 10 feet f/9.8 f/11.1 f/8.7 f/8.5
11.5 feet
9.4 feet
8.6 feet
5.2 feet
4.1 feet
Distance at f/8
12.2 feet
13.9 feet
11.0 feet
10.7 feet
If at 105 mm zoom and ISO 100, then: (GN is definitely also about the reflector and zoom)
105mmNikon Flash Model
Guide Number
Feet, ISO 100
Aperture at 10 feet
Distance at f/8
SB-800 (FX)
SB-900 DX
SB-900 FX
SB-600 (FX)
SB-700 DX
GN 184
GN 172
GN 162
GN 131
GN 125 f/18.4 f/17.2 f/16.2 f/13.1 f/12.5
23 feet
21.5 feet
20.2 feet
16.4 feet
15.6 feet
Notes
85mm max zoom
85mm max zoom
Notes
SB-910 is similar no zoom no zoom no zoom

SB-700 FX
SB-400
DSLR internal flash
SB-R200
GN 121
GN 69
GN 42
GN 33 f/12.1 f/6.9 f/4.2 f/3.3
15.1 feet
8.6 feet
5.2 feet
4.1 feet
This seems a lot to know from simple division. This is the meaning of Guide Number. GN of course applies to direct flash, but the published ratings are also representative of the flash power capability (if compared correctly). Again, the advantage of GN is to make the inverse square law be trivial to compute for unmodified direct flash.
There is a f/stop table in tenth stops , to determine the difference in say, f/8.5 and f/4.2 (two stops). We almost know this one in our head, because 2x f/4.2 is f/8.4, and double the f/stop number is two stops change. So, if maybe we don't want to use the f/8 at maximum power, it still means that we can use 1/4 power at f/4.
Examples:
One stop is double power, two stops is 4x power. One stop more power may allow stopping lens down to f/5.6 instead of f/4 (better for bounce). Or one stop difference means the higher power flash might use ISO 400 to bounce, where the lower power flash needs ISO 800 to do the same.
Two stops difference means higher power flash probably could bounce at the same aperture the lower power flash was using for direct flash (crudest rule of thumb, not precise, distances and heights are not mentioned).
As always, the assumption is that the guide number specification is adequately accurate
(never a great bet).
Higher power is always good stuff for bounce. This "same zoom at ten feet" method is trying to compare to show how good (but it is only meaningful and comparable at the same angle of coverage). Possibly there is sometimes "more" good at longer zooms, if one flash cannot zoom the same degree. We always want more power for bounce, all we can get. no zoom no zoom no zoom

http://neilvn.com/tangents/using-higher-iso-settings-with-flash/
December 27, 2010
The advice for optimal camera settings for best image quality are usually:
- use the lowest possible ISO:
- at an aperture about 3 stops down from maximum (the widest) aperture;
- at a shutter speed fast enough to avoid camera shake and unintentional subject movement.

Taking this general advice at face value, means using the camera at its base ISO, which would either be 100 ISO or 200 ISO. However, while this advice is sound in theory, in practice this doesn’t have direct consequence on my decision about my camera settings.
In terms of exposure settings, we obviously want correct exposure, even if ‘correct exposure’ is open to interpretation. Now if we are using only available light, then we have what we have for that specific scenario. If the ambient light is low, we would need a higher ISO / wider aperture / slower shutter speed. There’s no wriggle room here.
But if we’re using flash, why not use the flash to give us correct exposure at these optimal settings? Why would we even go to a higher ISO?
The reason: when using flash on location, I am mostly concerned about balancing my flash with the ambient light. Or somehow taking my ambient light into account to give some context. It just looks better!
Let’s get back to the photograph at the top:
Looking at the photograph of this couple, Jessica and Michael, my camera settings were:
1/125 @ f3.2 @ 800 ISO
Two observations about the settings:
I bounced flash behind & above me into the area, so that I could have nice clean open light on them. My settings were chosen specifically so that the background detail is there. Not too dark. At the ‘optimal settings’ the background would’ve been pitch black for a hand-holdable shutter speed. Alternately, if I had somehow chosen the ‘optimal settings’ of f5.6 @ 200 ISO, then I would’ve been forced to use a tripod at slow shutter speeds. Tripods are an absolute nono in a train station. The surest way of being escorted out by the cops. Besides, working with a tripod here would’ve killed the spontaneity of the photo session.
Also, in bouncing my flash behind me, I am losing a lot of power from my flash. It really isn’t the most efficient way of using the flash. But the light that returns is sweet. Therefore, in bouncing my flash like this, I wouldn’t have been able to get proper exposure on them at a proposed f5.6 @ 200 ISO.
So why was my ISO setting so high? Because that was the only practical way to get enough of my ambient light in, and enough of my flash to make a difference.
To recap: you would go to higher ISO for a variety reasons:
 Balancing flash with the ambient light.
Having the ambient light register to some extent, whether as:
- just some color and texture in the out of focus background, with flash dominating, or
- as the main source of light, with flash just being fill light; my consideration is still the available light.

So while I could get correct exposure for nearly any situation at 100 ISO or 200 ISO by using direct flash, and then still use a medium or small aperture .. the results just wouldn’t look good. The lighting would be harsh, and would be obviously flash.
But most importantly, I would lose the available light entirely at low ISO settings, when I am working in less bright environments.
 Bumping up the ISO also means my flash works less hard, and can recycle faster.
 A higher ISO can mean a faster shutter speed, which would help with freezing subject movement or eliminate camera shake.
 Do keep in mind that not every photo will be used at full resolution. So you might not even need your image to be at the theoretical ‘optimal’ settings. Consider the final use of the image.
 I might need the extra ‘reach’ that the higher ISO gives to my flash’s output.
As an example of why I need that high ISO for that extra bit of ‘reach’ with my flash:
I recently met up again with a previous wedding couple of mine. They have featured on the
Tangents blog before . For this simple impromptu portrait of them at the wedding reception, I bounced my flash behind me to get nice clean open light on them.

1/60 @ f3.5 @ 1600 ISO
Here is the reception venue. It will give you an idea of the size of the place, and also that the entire room was a glass and white-painted metal structure. Not an obvious place to use bounce flash … but it works. It only works at higher ISO and wider aperture settings though.

Finally, about the composition of this image:
I know I will get comments about the angle. And while tilted compositions as a reflex is something I dislike, and have to actively concentrate on to make sure it doesn’t creep in due to sloppy camera work …
I do think the tilted composition here works , with Jessica as the centered element in the frame. But as always, these decisions are up to individual taste and interpretation.