Flash photography, part 2: The suffering with the guide number
15.3.2019
Translation: machine translated
The luminosity of a flash unit is decisive. It is specified as a guide number. Unfortunately, this figure is deceptive and its calculation is much more difficult than it seems. Many manufacturers pimp their figures to suit themselves.
A separate flash unit has many advantages over the built-in camera flash. One of them is the higher luminosity. A clip-on flash has much more power. This allows you to correctly illuminate subjects that are further away.
There is a special unit of measurement for the luminosity of a flash: the guide number. It is calculated as follows:
Guide number = aperture value × maximum distance in metres
Example: If I use aperture F4 and can flash at a distance of up to 6 metres, then my flash has a guide number of 4×6=24.
Inversely, I can calculate how far I can illuminate with which aperture if I know the guide number of my flash unit (24): at aperture F4 this is 6 metres and at aperture F8 it is still 3 metres.
That sounds simple and exact. Simple maths. Unfortunately, a closer look reveals that this guide number is largely useless.
Numerous factors influence the guide number
Whether your flash is bright enough for a particular scene depends not only on aperture, luminosity and distance, but also on a number of other things.
Firstly: the ISO sensitivity. Logical - the flash reaches further at higher light sensitivity.
Secondly: The subject and its surroundings. Matte, dark subjects need a stronger flash than bright, reflective surfaces. The amount of light required also depends on whether there are reflective surfaces such as windows in the vicinity.
Thirdly: the zoom factor. Modern flash units know what focal length you are currently shooting at. This information is sent from the camera to the flash. Based on this information, the flash focusses the light more or less strongly depending on the focal length: at wide-angle it is scattered as widely as possible, at telephoto it is directed as narrow a beam as possible onto the small image section. What does this have to do with the guide number? It's like a garden hose: a narrowly focussed beam of light reaches further with the same amount of light than widely diffused light. This means that the guide number of a flash unit depends on the focal length currently being used.
Fourthly: Indirect flash. Of course, the maximum distance only applies if you point the flash directly at the subject. However, indirect flash via the ceiling or wall is one of the most important reasons why you want to use an external flash unit in the first place.
The following pictures of our office building are ghastly, but illustrate the factors well. All photos were taken with maximum flash power of the device, aperture F8 and 1/125 second.
The picture on the left is directly frontal, the flash is powerful enough. In the picture on the right, I used the flash over the ceiling. As the room is very high, the flash was far too weak.
On the left, the same image flashed across the ceiling. On the right the same with 400 instead of 100 ISO.
Frontal flash again. At a wide angle of 24 mm, the image can only be poorly illuminated. In the smaller section of 70 mm, the flash light is sufficient because it is focussed. You can also see that the plant as a subject needs much more flash light than the pot, for example.
Here the flash light comes indirectly from the window wall. The flash is unable to illuminate the right edge of the image. At the same time, you can see that the brightness decreases towards the top.
Where the problems lie
The ISO dependency is not a problem. The manufacturers agree on 100 ISO, problem solved. In fact, everyone adheres to this consensus. In everyday life, it is also no problem to convert to a different ISO number. According to Wikipedia, quadrupling the ISO number means doubling the range.
The dependence on the subject can also be standardised. Guide number specifications apply to a grey card with 18 per cent grey and no reflective environment. This standard makes it possible to compare the specifications of different flash units. However, it is of no use if you want to know how far you can actually flash in a particular situation.
The zoom factor is particularly problematic. Not all manufacturers use the same focal length for their guide number specifications. Nikon specifies the guide number for 35 mm. Canon and Sony, on the other hand, use the maximum focal length that is possible with the respective flash and thus achieve much higher values with similar flash units. Because not every flash unit covers the same focal lengths, the guide number specifications are not even reliably comparable within the same brand.
To make matters worse, the guide number cannot simply be converted from one focal length to another, as is the case with the ISO value. You have to download the manual of the respective flash unit and look for the corresponding table there. On page 101 of the manual for the Canon Speedlite 430EX III, I can see that this flash has a guide number of 28 at 35mm. This makes it as powerful as the Nikon Speedlight SB-700.
What to do with this non-information
Don't let yourself be dazzled: When choosing a flash unit, the guide number is useless without a focal length specification. Of course, you'd rather know what will help you with your purchase instead of what won't. My tip for this is very simple: don't buy the cheapest and weakest flash unit. With indirect flash, you'll be happy if your flash has reserves. The same applies if you want to brighten up with a flash in bright sunlight. The slightly more expensive flash units are not only more powerful, but usually also have a few more features. Weak flashes are really only useful in combination with several flashes or if you need something extremely handy. <p
David Lee
Senior Editor
David.Lee@digitecgalaxus.chMy interest in IT and writing landed me in tech journalism early on (2000). I want to know how we can use technology without being used. Outside of the office, I’m a keen musician who makes up for lacking talent with excessive enthusiasm.
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