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Basics of Flash Photography

There is another section about using Guide Number, and there is a calculator below to compare the power of two Guide Numbers.

Guide Number is the specification of the flashes power capability for a given situation, like for each speedlight zoom setting. The purpose and advantage of Guide Number is to make the Inverse Square Law be trivial to compute exposure for unmodified direct flash. Guide Number simply is the multiplied product of (flash distance × f/stop) for a proper exposure situation (normally specified for ISO 100). For example, if a certain Guide Number were equal to 100 (feet), then it says a correct direct flash exposure is f/20 at 5 feet, or f/5 at 20 feet, or f/10 at 10 feet, etc. Any such combinations are a correct exposure. Guide Number is also how power rating for a speedlight is specified, however, we have to pay attention to the zoom situation too, to know what it means.

The Guide Number may be in feet or meters — we can use either so long as we are consistent (for Guide Number and Distance). There are 3.28 feet in one meter, so GN (feet) is always 3.28x GN (meters). Note that flashes for European or Asian markets probably routinely specify GN in meters, so if they say a low number like 30, it probably means 30 × 3.28 = 98 (feet) for American markets. Units should be specified, but not always are.

There is no equivalence of watt seconds of energy and Guide Number, so that comparison cannot be computed. There's an efficiency factor in converting electrical power to lumens of light (but flash is higher efficiency, comparable to fluorescent, significantly higher than incandescent). Guide Number is about the meterable light, which largely depends on the flash modifier distribution angle anyway (zoom, or reflector, etc, depends on how much area the light must fill?) But Guide Number is representative of relative flash power, and in some cases, we can "compare" flash power by using Guide Number.

If you are going to compare power of flashes by their Guide Number, 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 wider 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.

It used to be that flashes that zoom all advertised guide numbers at a standard 35 mm zoom, which was considered to be a typical useful working value. 35 mm zoom was common for 35 mm film format in a typical indoor setting, certainly conceivable. Most Japanese flashes still do this. The power was comparable that way, at the same zoom.

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 rarely use flash at 105 mm zoom. It will be up near 2x or 2 EV greater than the Guide Number for 24 mm. And I have seen one Chinese manual that simply advantageously printed the wrong Guide Number chart in it.

The Guide Number (GN) is as much about the flash head reflector coverage angle as about the power level. Zooming the flash head or adding an umbrealla or softbox drastically changes the guide number.

If the GN of one flash is specified as GN 180, but its narrow zoom coverage angle is concentrating all the power to only illuminate a small bright spot on the wall, and another flash specifies GN 90, but its wide reflector is illuminating the entire room, then we don't know much about the difference. We do know the first may meter brighter in its small area, but this second unit may have the highest power level, and may be brighter if its power were similarly concentrated into the small spot too. Wide angular coverage requires a lot of power (illumination per unit of area). GN is the illumination at one point, inside whatever area, but we can only compare "power" when the flashes are doing similar work, with similar coverages (using same zoom angle). Then compare the guide numbers.

Speedlights that zoom have a GN chart in their user manual, of many different Guide Numbers, a different GN for each combination of zoom setting (angle of coverage), and power level. A larger GN if the flash head is zoomed in tight, or a smaller GN if zoomed out wide. 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. Any such speedlight that only advertises a Guide Number for maximum zoom probably does not have much to brag about at lesser zooms. To know very much about the GN ratings, we need to look at the complete guide number chart in the user manual (sometimes manual is online). The chart provides GN for all possible zoom and power combinations. We need to see that chart to know much.

Compare Flash Powers by Difference in Guide Numbers

The simple rule is: Guide Number = distance × fstop Number (for any proper direct flash exposure).

Therefore, double GN is double distance or double fstop Number (which is 2 EV stops of exposure).

So GN compared as f/stops works too. If we assume these default values of GN 39 and GN 98 are both at ten feet (and at same zoom and same ISO), then these exposures are f/3.9 and f/9.8, which compare at the same 2.66 EV difference. Or in this case, the nearest thirds of f/4 and f/10 is also 2.67 EV difference.

Here is a calculator to compare two flash Guide Numbers (at same zoom coverage), to show the relative number of stops power difference these Guide Numbers represent.

Enter two Guide Number values to compare relative power of Direct flash. Use either Feet or Meters, but be consistent. And the Guide Numbers must be for same zoom and same ISO and same power level, typically the full power rating. Comparing power can be useful when shopping for a flash.

The GN of multiple equal flashes ganged in combination acting as one (see that calculator), is (GN of one) times the square root of (number of flashes).

Did I mention that GN comparisons are only meaningful at the same zoom, the same angle of light coverage?

Comparing Speedlight Power using the
"Same Zoom at Ten Feet" method

You can compare different brands/models of flashes with the calculator, or with the "Same Zoom at Ten Feet" method. This ten feet is an arbitrary divisor, and if GN is meters, you could use 2 or 3 or 10 meters. 10 feet is 3.05 meters, and the GN for feet is 3.28x greater than for meters (3.28 feet in one meter). The distance computes f/stop, which difference can be compared as flash power in EV.

If the Guide Numbers are for the SAME ISO and the SAME flash zoom, we can easily compare speedlight maximum power levels by assuming the same arbitrary flash distance to get an aperture from GN. Any distance, but dividing by ten feet is easy (or by 2 meters is also an easy divide). If GN is 95, then at 10 feet, f/stop would be f/f9.5. The purpose is to get an aperture number to be able to understand the power difference in stops, of the different flash models (at the same ISO and same Zoom coverage). Then comparing apertures, if the f/stop is one stop higher number, that is double power. Or, if the f/stop NUMBER itself is double, that is two stops, and 4x more power. The meaning of two stops more power is that you can use f/8 instead of f/4, or can use ISO 200 instead of ISO 800, or can have twice the distance range. One stop = 2x, two stops = 4x, three stops = 8x, four stops = 16x more power.

So for example, consider two flashes, with guide numbers of 40 and 80 (feet). Assuming ten feet, and both at the same ISO and zoom value, they compare as f/4 and f/8, which is 2 EV stops exposure difference.

Compare Flash Power of Nikon iTTL Flash Models

The idea is not so much about these specific flash units, but more about a way to compare any two camera speedlight flashes using Guide Number. Studio flash do not rely much on guide number, because they use all manner of lighting modifiers (umbrellas, softboxes, grids, snoots, etc). Even different reflectors change guide number.

The Nikon guide number charts in the user manuals show these GN specifications. Guide Numbers allow these simplifications to compare exposures (here, for GN / 10 feet, and GN / f4), at full power level, same zoom and same ISO. These are two arbitrary situations, and then we better know how to compare power of two flashes. These GN numbers below are for feet (simply divide GN and distance values by 3.28 for GN in meters.)

The Difference column compares all flash models relative to the camera internal flash. The DX modes zoom in more at same settings, brighter, but DX coverage won't still cover a full frame size. For example, the first 24 mm chart shows SB-700 DX GN is 2.47 stops more power, which is 5.55x more light, than the camera internal flash. The calculator above can also compute these things. Typically, many external speedlights at 1/4 power easily match the internal flash, plus they can zoom, and tilt for bounce, and can be used off camera, etc, etc, to do much more.

This seems a lot to know from simple division with Guide Number. This is the meaning of Guide Number. GN applies to direct flash, and it is as much about the reflector coverage angle as the power, but the published ratings are also representative of the flash power capability (if compared correctly, at the same zoom level).

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