Flash works by emitting a very short, very powerful burst of light. Energy is stored in a capacitor in the flash head and then released all at once in a fraction of a second.
Once you’ve fired the flash, the capacitor then needs to be recharged ready for the next flash. This typically takes a few seconds (or less if the flash was not fired at full power) and is called the ‘recycle time’.
One of the creative characteristics of flash is that the flash duration is very brief – as short as 1/500-1/1000sec, or even shorter. This means that you can use flash to ‘freeze’ fast moving subjects.
Different flashguns have different power ratings. The power of the flash falls away over distance and followed the ‘inverse square’ law. If you double the distance, the power of the light is reduced to a quarter.
Flash manufacturers quote a Guide Number (GN) which will give you an approximate guide to the power of a flashgun. It’s a simple calculation; you divide the guide number by the subject distance to get the lens aperture you should use. For example, if a flash has a guide number of 30 and your subject is 3m away, you should use an aperture of f/10.
In practice, it’s both simpler and more complicated than that. Most flashguns offer automatic TTL flash exposure so that manual guide number calculations aren’t necessary. It’s still possible to adjust the strength of the flash, however, using a flash compensation adjustment, which works a lot like exposure compensation in regular photography.
Guide numbers are not easy to compare, either, since not every maker uses the same focal length in the quoted value. Many flashguns have ‘zoom heads’ to narrow or widen the flash coverage to match your zoom setting. This makes the flash ‘go further’ at longer zoom settings, but encourages makers to quote guide numbers for longer focal lengths to make the flash look more powerful. A flash might give a guide number of 60 at a focal length of 105mm but it will be a lot less at a regular, shorter focal length.
Shutter speed, ISO and exposure
Guide numbers take no account of shutter speed. That’s because in flash photography, the shutter speed does not usually affect the exposure. The flash is usually the dominant light source, and the ‘exposure time’ is the duration of the flash, which is very short and effectively outside the photographer’s control.
The camera’s ISO setting does make a difference, though. You can make flash ‘go further’ by increasing the camera sensitivity. For example, if you increase the ISO from 100 to 400, you are making the camera sensor four times more sensitive. Hence, following the inverse square law, your flash will now have twice the range.
To return to the shutter speed, it is relevant in one important respect. Many cameras use ‘focal plane’ shutters, and at faster shutter speeds these do not expose the whole sensor at the same time. This is no good for flash. This means you can only use certain shutter speeds on these cameras, and the makers will always quote the maximum ‘flash sync speed’ for that camera. This is typically 1/180-1/250sec.
You can use slower shutter speeds, and this offers lots of creative potential for mixing flash with ambient light. The shutter speed won’t affect the flash, but a slower speed will increase the exposure for the ambient light.
Built in and external flash
Many cameras have a built-in pop-up flash, but this is best kept for emergencies. Built in flash is pretty weak and emits a harsh, short range light. You have no control over the direction of the light or its softness.
This is why many photographers invest in external flash units, sometimes called Speedlights or Speedlites. These are more powerful and more adjustable flashguns that clip on to the camera’s ‘hotshoe’ or ‘accessory shoe’.
An external flash will provide more power and hence longer range, and many come with heads that can be swivelled round and angled upwards to ‘bounce’ the flash off nearby surfaces to soften and vary the lighting effect.
You don’t have to fix and external flash to the camera. Using off-camera flash it’s possible to achieve a much greater variety of lighting effects. There still needs to be a way for the camera to trigger the flash, and the old-fashioned way is to use a ’sync cable’ to connect the flash to the camera’s flash sync socket, or ’sync terminal’.
Not every modern camera has a sync socket, however, and most photographers won’t want wires trailing everywhere. The modern solution is to use wireless flash, either via infra-red or radio-control.
This opens up the possibility of multi-flash lighting, where the camera or one of the flashguns is a ‘master’ or a ‘commander’ that’s used to trigger the others. Different camera makers have their own dedicated wireless flash control systems. Nikon calls its system, the ‘Creative Lighting System (CLS)‘, for example.
You don’t have to use the camera maker’s own flash systems. You can use third party flashguns and and plug-in wireless flash control systems consisting of transmitters and receivers.
This kind of flash control is quite advanced, the more the province of professional photographers. It’s also the point where many will consider swapping to a professional flash system with more power, speed and adaptability.