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Basics of Flash Photography

When Shopping - Comparing Speedlight Flash Power Rating using Guide Numbers

There is another section about using Guide Number, but for here, 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 x 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, which takes into account the Inverse Square Law. 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. 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 x 3.28 = 98 (feet) for American markets. Units should be specified, but not always are.

Equivalence of actual flash power and Guide Number cannot be computed - there is no equivalence of watt seconds of energy and Guide Number. There is an efficiency factor in converting electrical power to lumens of light (flash is higher efficiency, comparable to fluorescent, significantly higher than incandescent). Guide Number is about the meterable light, which largely depends on the 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.

The Guide Number (GN) is as much or more about the reflector coverage angle as about the power level.

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 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. 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 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. 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. We need to see that chart to know much.

Compare Flash Powers by Computing Difference in Guide Numbers

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.

To have any meaning, any two Guide Number ratings to be compared must be:

It used to be that flashes that zoom agreed to advertise guide numbers for a standard 35mm zoom, which was considered to be a typical useful working value, certainly conceivable. The power was comparable that way. It was about full frame views, but most Japanese flashes still do this.

But today, some marketing (especially Chinese flashes) advertise their maximum 105mm zoom guide number, simply because that is a larger number that looks better than others, regardless that we rarely use flash at 105mm zoom. I have seen one Chinese manual that simply advantageously had the wrong chart in it.

Today, to know very much about ratings, we need to look at the guide number chart in the user manual (sometimes online). The calculator to compare power can be useful when shopping for a flash.

If one GN is rated for ISO 200, then dividing the number by 1.414 will give the ISO 100 equivalent.

Guide Numbers can only be compared if both are at the same flash zoom settings (unless it cannot zoom).

Enter two Guide Number values to compare. Use either Feet or Meters, but be consistent.

Compare Power Levels of two flashes with GN

GN ---> GN

Same zoom, same ISO

  Difference is:

The simple rule is this:
A new GN 1.414x higher is one stop, either aperture, or 1.414x more distance range (for example, 2x ISO is 1.414x GN).
A new GN 2x higher, is two stops, either aperture, or 2x more distance range.
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.

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 3 meters is easy). 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 maybe consider two flashes, with guide numbers of 50 and 80. Assuming ten feet, and same zoom value, they compare as f/8 and f/5, which is about 1 1/3 stops exposure difference. Or if GN is meters, rough equivalents are GN 15 and 24 meters, divide by maybe 2, f/7.5 and f/12, about the same 1 1/3 stops.

Compare Flash Power of Nikon CLS iTTL Flash Models

The Nikon guide number charts in the user manuals show these specifications. Guide Numbers allow these simplifications to compare exposures (here, for GN / 10 feet, and GN / f8), at full power level, same zoom and same ISO. These are two arbitrary situations, which could be real, and then we better know how to compare power of two flashes as aperture or feet. These GN numbers are for feet (simply divide GN and distance values by 3.28 for meters. 10 feet is 3.05 meters.)

More words, but to spell it out specifically, the first chart shows at ISO 100 and 24 mm zoom, the DSLR internal flash at maximum power has to use f/4.2 to expose a subject at 10 feet fully and properly (direct flash). Whereas if with a SB-700, DX would stop down to f/9.2 to do the same exposure at 10 feet. This is the definition and usage of the published Guide Number. So f/9.2 is a bit more than double the f/4.2 number, so this is more than two stops, and more than 4x more power. Or, the chart also shows the internal flash distance for exposure at f/8 is 5.2 feet, but the distance is 11.5 feet for a SB-700 DX (both at maximum power, maximum range at f/8 for full exposure). This is a little more than twice as far, again meaning more than 2 stops and more than 4x power.

The Difference column compares flash models all relative to the camera internal flash. For example, the first 24 mm chart shows SB-700 DX GN is mathematically 2.26 stops more power, which is 4.8x more power and light, than the camera internal flash. The calculator above can compute these things. Typically, most external speedlights at 1/4 power easily match the internal flash, plus they can zoom to do much more, and tilt for bounce too, and can be used off camera, etc, etc.

At 24 mm zoom and ISO 100
Nikon
Flash Model
Guide Number
(Feet)
ISO 100
Aperture
at 10 ft
Distance
at f/8
Difference from
Internal flash
24 mm zoom

Notes

Stops Intensity
SB-800 (FX) GN 98f/9.812.2 feet2.445.4x
SB-910 DX GN 111.5f/11.113.9 feet2.827.0x
SB-910 FX GN 88.6f/8.911.0 feet2.154.5x
SB-900 DX GN 111f/11.113.9 feet2.807.0x
SB-900 FX GN 88.6f/8.911.0 feet2.154.5x
SB-5000 DX GN 111.5f/11.113.9 feet2.827.0x
SB-5000 FX GN 88.6f/8.911.0 feet2.154.5x
SB-600 (FX) GN 85.3f/8.510.7 feet2.044.1x
SB-700 DX GN 91.9f/9.211.5 feet2.264.8x
SB-700 FX GN 75.5f/7.59.4 feet1.693.2x
SB-500 GN 78.7f/7.9 9.8 feet1.813.5xno zoom
SB-400 GN 69f/6.9 8.6 feet1.432.7xno zoom
SB-300 GN 59f/5.9 7.4 feet0.982.0xno zoom
  DSLR internalGN 42f/4.25.2 feet0-no zoom
SB-R200 GN 33f/3.34.1 feet-0.70.6xno zoom

At 50 mm zoom and ISO 100
(same flash power concentrated into a smaller area, brighter intensity)
Nikon
Flash Model
Guide Number
(Feet)
ISO 100
Aperture
at 10 ft
Distance
at f/8
Difference from
Internal flash
50 mm zoom

Notes

Stops Intensity
SB-800 (FX) GN 144f/14.418 feet3.5611.8x
SB-910 DX GN 147.6f/14.818.4 feet3.6312.4
SB-910 FX GN 131.2f/13.116.4 feet3.299.8x
SB-900 DX GN 150.9f/15.118.9 feet3.6912.9x
SB-900 FX GN 131.2f/13.116.4 feet3.299.8x
SB-5000 DX GN 148f/14.818.5 feet3.6312.4x
SB-5000 FX GN 131f/13.116.4 feet3.289.7x
SB-600 (FX) GN 118f/11.814.7 feet2.987.9x
SB-700 DX GN 113f/11.314.1 feet2.867.2x
SB-700 FX GN 102f/10.212.7 feet2.565.9x
SB-500 GN 78.7f/7.9 9.8 feet1.813.5xno zoom
SB-400 GN 69f/6.98.6 feet1.432.7xno zoom
SB-300 GN 59f/5.97.4 feet0.982.0xno zoom
  DSLR internalGN 42f/4.25.2 feet0-no zoom
SB-R200 GN 33f/3.34.1 feet-0.70.6xno zoom

At 105 mm zoom and ISO 100
(GN is definitely also about the reflector and zoom)
Nikon
Flash Model
Guide Number
(Feet)
ISO 100
Aperture
at 10 ft
Distance
at f/8
Difference from
Internal flash
105 mm zoom

Notes

Stops Intensity
SB-800 (FX)GN 184f/18.423 feet4.2619.2x105mm max zoom
SB-910 DX GN 169f/16.921.1 feet4.0216.2xGN 177.2 at 200mm
SB-910 FX GN 160.8f/16.120.1 feet3.8714.7xGN 173.9 at 200mm
SB-900 DX GN 172f/17.221.5 feet4.0716.8xGN 187 at 200mm
SB-900 FX GN 162f/16.220.2 feet3.9014.9xGN 183.7 at 200mm
SB-5000 DX GN 169f/16.921.1 feet4.0216.2xGN 177 at 200mm
SB-5000 FX GN 161f/16.120.1 feet3.8314.7xGN 174 at 200mm
SB-600 (FX) GN 131f/13.116.4 feet3.289.7x85mm max zoom
SB-700 DX GN 124.7f/12.515.6 feet3.158.8x85mm max zoom
SB-700 FX GN 121f/12.115.1 feet3.058.3xGN 124.7 at 120mm
SB-500 GN 78.7f/7.9 9.8 feet1.813.5xno zoom
SB-400 GN 69f/6.9 8.6 feet1.432.7xno zoom
SB-300 GN 59f/5.9 7.4 feet0.982.0xno zoom
  DSLR internalGN 42f/4.25.2 feet0-no zoom
SB-R200 GN 33f/3.34.1 feet-0.70.6xno zoom


The Glory of Zoom! :) This seems a lot to know from simple division. 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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