Flash Guide Number
The guide number (GN) of an electronic flash is a measure of the maximum amount of light it can produce with one discharge of its capacitor. The corresponding ISO standard uses the term “power of illumination”. The higher the guide number, the longer is the usable range of the light source and the farther away the subject can be for a well lit photo.
The effective range depends on the f-stop set on the camera body: At f4.0 you have a usable range of 8 meters with a GN-32 flash, and at f2.0 your range is 16 meters (32 divided by 4 = 8; 32 / 2 = 16). Built-in flashes in camera bodies have a guide number of 12 – 14; external flashes for hot-shoe use have GN between 20 and 40.
The guide number decreases with lower ISOs and wider flash lens positions and increases with higher ISOs and the longer zoom reflector positions. Therefore the guide number must be stated together with the angle of coverage of the flash reflector (expressed as a full frame focal length) and the sensor sensitivity (or film speed), otherwise the numbers are not comparable between flashes!
Guide number at Speedlights.net is always stated in meters, and for ISO 100 at 35mm (full frame). That’s why all test results here on the site comparable across makes and models. To get to the corresponding guide number in feet, simply multiply the metric GN with 3.28. GN 40 (meters) equals GN 131 (feet).
Flash Power Testing
The independent Speedlights.net guide number tests are always performed in a low reflection environment (as demanded by the ISO standard) with the respective speedlite flashing direct into a Sekonic L-358 with lumisphere lowered. ISO is set to 100, shutter speed to 1/200 sec. The ambient light is dimmed down to a minimum and a light meter reading of “100%” is confirmed to guarantee that exposure would come solely from the flash. There’s a 60-seconds waiting time observed between measurements to guarantee the flash capacitor has more than enough time to recycle completely, and freshly charged batteries are being used.
‘Calculated’ Guide Number
Adding exactly 1 f-stop to the flash meter result in the speedlights.net tests leads to the calculated guide number, so the f16 of an SB-400 becomes f22 = GN22, which is very close to what the Nikon specs say (GN21). Same for the 580EX II: the meter reads f22 + 6/10 for the Canon. Plus 1 f stop comes out at f32 + 6/10 which equals 39.4 as the guide number of the flash (a bit above the GN36 from the official Canon specs).
Further test data from Nikon, Metz, Nissin and Vivitar units show that this heuristic method provides very good results for a wider range of manufacturers and models. See the Speedlites Power Index for a comprehensive overview of guide number test versus specs.
Speedlites Power Index
The Speedlites Power Index exists in 2 versions: the TTL Index and an off-camera flash index for “strobist” flashes with a manual mode and radio-trigger compatibility. The light blue bar shows the official 35mm-GN, and the dark blue bar indicates our test results.
The latest evolution of the Speedlights.net Power Index shows the guide number from our tests as “RealGN” with metric values for the 35mm reflector position, and compares with the official manufacturer specs in the light blue bars. The Index is sorted by the test guide numbers in descending order so that the most powerful flash is found at the top of the list.
To get the feet-values for the flash guide number, simply multiply the metric guide number with 3.28 (example: GN 30 (meters) equals 30 * 3.28 = 98.4 (feet).
For comparison, the built-in pop-up flashes on DSLR camera bodies typically have a guide number of 12 – 14 at ISO 100.
Continuous Shooting Output
Guide number testing requires observing a 60-seconds waiting time between the test shots. This is because the flash capacitor has a charging curve and and it takes something like 10 seconds, or even longer after a full power pop, before it’s really 100% recharged.
Outside of the lab it certainly doesn’t make any sense to let the user wait that long between flashes – who cares if you have only 98% of the energy when you can fire every 3 seconds instead of every 10 seconds only? That’s why the “flash ready” light comes on sooner, but there’s this trade-off to make.
When shot with the maximum frequency in continuous fire, i.e. at the moment when “flash ready” lights up, a typical flash loses around 1/2 stops of power. To test this continuous shooting power a rapid series of full power flashes gets fired and the resulting guide number is then determined and displayed.
Flash Recycling Time Metering
Recycling times are tested according to ISO 2827. The definition of recycling time used here is the time span between the release of a flash and the moment when the ready-light comes back on; with this definition the reaction time is eliminated which typically adds between 0.2 and 0.5 seconds to the values displayed.
Flash recycling time tests are always performed in manual mode at the 1/1 output setting. For each battery type a series of 10 shots is fired and the procedure is captured on video and then analyzed later. Visit the Speedlights.net youtube channel to see different test videos.
Flash Duration Testing
Flash duration is the time between the beginning of the flash and the end of the light emission. Typical flash durations for compact speedlights are between 1/200 seconds at full power and 1/20,000 at the lowest output levels.
Photographers look at flash duration as it can have effects on (a) the ability to freeze action as well as (b) the usage of the fastest DSLR camera sync speeds such as 1/250 or even 1/320 seconds. If that is faster than the flash duration it means that not all of the light emitted from the strobe will be used for the photo.
There are 2 different flash duration time definitions in use, the so called t0.5 time and t.01 time.
t0.5 is the time between the moment where the flash intensity curve starts to exceed 50% of the maximum intensity point and the moment where the falling intensity curve goes underneath the 50% maximum level again. The t0.5 time is shorter by definition and therefore used by most manufacturers, e.g. Nikon.
The speedlights.net testing looks at the t0.1 duration i.e. the time between 10% of the maximum intensity is reached and back down to 10% towards the end of the flash discharge, so it is longer than t0.5 by definition. Some manufacturers, e.g. Metz, are using the t0.1 times in their specs. The t0.1 metering for speedlights.net is done with a Broncolor FCC meter.