The Guidenumber – GN

First place a Speedlight in the studio, then click on the + symbol as shown in the graphic. Now the menu “Add Speedlight” opens.

After you have entered any name in the name field, you must select the range of your flash. The most common control range is 1/1 to 1/128. At the bottom of the list you will find the appropriate value for many devices. If your device is not listed, you will find the control range in the manufacturer’s manual.

Your flash can usually control certain zoom levels that you can adjust. In order to have the correct zoom levels available in the simulation, you have to select the corresponding range. The manufacturers often only differ in one value in the whole series. Therefore pay special attention to the gradations (e.g. does your flash have a zoom level of 80 mm or 85 mm? Often the manufacturers differ only in this one value). The lens hood is not included in the specification.

If the rule range and the zoom range are selected, set.a.light 3D has already preselected the guide numbers automatically and shows you only the values that are possible for the selected constellation. The guide number always includes a certain zoom level in mm. In the following article (The guide number in detail and definition) the guide number is explained exactly and what it stands for.

You can find the appropriate value in the table below. You will also find this value in the instruction manual of your flash. Detailed information about the guide number and its definition can be found in the following section. The operating instructions of the flash manufacturer often don’t tell you so much about this.

The guide number thus indicates the maximum distance to the object at which it is still correctly exposed. A guide number of 60 therefore means that under certain conditions an object is still correctly exposed at a distance of 60 meters.

The exact flash distance is calculated as follows:

Example: L 60 / B 5.6 = 10.71 meters

So if the flash has a guide number of 60 and I use an aperture of 5.6 (at ISO 100), the range of my flash is 10.71 meters.

However, the guide number always refers to a certain zoom position (zoom reflector) of the flash.

In the past, the guide number of the flashes was almost always given at 35mm (35mm format) and ISO 100 in meters at aperture F1 (1:1). These 35mm were not standardized, but still the standard. The advantage was that you could easily compare the devices and didn’t have to calculate back and forth. Unfortunately, the manufacturers nowadays almost always deviate from it.

The larger the zoom level (e.g. 200mm), the more focused (bundled) and thus brighter the emitted light energy and the higher the corresponding guide number (range in meters). In order to be able to indicate a nice high guide number as a manufacturer, this is then determined at the highest zoom level. The user is at a disadvantage because he can hardly compare the performance of the individual products.

The reason for this “trick” is quite simple: Marketing – because a device with a higher guide number simply sells better!

At Canon, for example, the guide number of the Speedlite 580EX II is 58. In this case, however, this guide number was determined at 105 mm. In set.a.light 3D L:58/GN:191 @105mm should be selected. If you are using a Canon Speedlite 600EX you would have to choose L:60/GN:197 @200mm (GN is the abbreviation for Leitzahl) and is given in feet). The guide number of the Speedlite 600EX is given on the basis of 200 mm. In reality, however, both devices have the same performance – at 35mm the guide number is 36!

If one now takes the Canon Speedlite 600EX, then this is indicated by the manufacturer with the guide number 60. If one now uses the Nikon SB-900 as a comparison, one sees that it is only indicated with a guide number of 34. Now one could think that the Canon device is much stronger.
BUT: Canon indicates the guide number at the highest zoom level (here 200mm), Nikon on the other hand at the zoom level of 35mm.

In order to be able to compare the two devices, one must first find out which guide number the Canon device has at 35 mm. With the Speedlite 600EX the “real” guide number is 36 at 35mm- that means: in reality the Canon flash is only slightly stronger! At a zoom level of 200 mm, the Nikon flash has a guide number of 56.

The zoom position of the reflector on the flash refers to the illuminated area (image section) which is required to obtain a correctly illuminated image with the same lens (e.g. 105mm). For a correctly illuminated image, the appropriate aperture for the guide number (in combination with the zoom) must be determined, the distance to the object must be correct and the lens and the zoom reflector of the flash must be set to the same value (e.g. 105mm) for the selected image section. Now one can only be lucky that all these things are normally taken over by the electronic control of the flash (E-TTL) and set correctly.

In order to be able to control the lighting design of unleashed flashes according to your wishes, you have to set the flashes manually and should have understood the above-mentioned connections.

If the user really wants to compare the manufacturer’s devices in terms of performance, it is usually necessary to laboriously find out which guide number the flash has at 35 mm.

In our table below, we have given the guide numbers for the common devices at 35mm and at maximum zoom level, so that the flashes of the manufacturers can be compared very easily and the appropriate value for the input in set.a.light 3D can be determined.

In set.a.light 3D, the corresponding base (e.g. 105mm) must always be specified in addition to the guide number. The possible combinations of guide number and zoom have been combined to simplify the input.

In the following table you can find the appropriate values for your device. If your device is not included, you will find the values in the manufacturer’s operating instructions.

Please let us know if your device is not on the list. We will be happy to add it to the list.

Footnote:

On Wikipedia you can find additional information and formulas: https://en.wikipedia.org/wiki/Guide_number.

GN is the abbreviation for “Guide Number” – the English term for guide number – and is expressed in feet.

If you find a failure in the list of devices, or a device is missing, write us an e-mail.