7 things you should know about camera flash light

1. A FLASHGUN’S POWER IS EXPLAINED BY ITS GUIDE NUMBER 

If you’re in the market for a new flashgun, you’ll naturally want to know how powerful it is. The way manufacturers measure this is with a guide number (GN), which unfortunately is the least intuitive system ever conceived, and also the most exploited. Here’s the principle behind it. GN is calculated by multiplying the maximum flash-to-subject distance (in metres) at which a certain aperture will correctly expose the subject, assuming the ISO is 100. Confused? Let’s check out an example. Imagine you’re shooting a portrait at f/4 and ISO 100. If your flash has a guide number of 40, this means you can be up to 10m away (as 4x10=40), as any further will underexpose the subject. Some flashguns zoom in by narrowing the throw of light, which changes the GN, so bear this in mind when researching spec. You’ll also need to check a manufacturer isn’t measuring in feet rather than metres, or at ISO 200 rather than 100, as these, too, will distort GN in their favour. An entry-level external flashgun usually has a GN of 30-40, and a pro flashgun around 60. Your camera’s pop-up flash is around 10.

2. YOU CAN CHANGE FLASH BRIGHTNESS BY ADJUSTING DISTANCE

One quick and easy way to change the effective brightness of your flashgun is simply to move it closer to or further away from your subject. This is ideal for making microadjustments to your exposure, and saves you changing any settings on the camera or flash unit. If you do adopt this way of working, it’s worth knowing the basic idea behind the ‘inverse square law’, which explains what happens to light levels when you change the distance between an object and its light source. The formula for the inverse square law sounds complex, but it’s fairly simple. Here’s how it works. If you double the distance of your light, it’s two times further from the subject than it was. Square this number (2x2=4), then put a 1 over it, making it 1/4. Your light is now effectively 1/4 of the power. If you move the light three times the distance, it’s 3x3=9, so 1/9 the power. And four times the distance is 4x4=16, so 1/16 times the distance. Because the light drop-off is much greater than you’d expect, it’s important only to move your light in very subtle movements.

3. E-TTL METERING SETS YOUR FLASHGUN POWER AUTOMATICALLY

Go back a few years and all flashguns were fully manual – you had to set flash power yourself based on the camera settings you were using. Today, most modern flashguns, including built-in flashes, have electronic through the lens metering (E-TTL) functionality, where the camera sets flash output for you to ensure the subject is lit correctly. It does this by firing a pre-flash a fraction of a second before the shot is taken, then analysing the reflected light from the area surrounding the focus point. Depending on how well exposed it judges the subject to be, flash output is then tweaked accordingly. It can combine this with exposure data, metering information and subject distance, and if your flashgun has a zoom function it can even narrow the camera’s beam of light to match focal length. While E-TTL is hugely useful for day-to-day use, many creative flash applications require you to switch to manual mode.

4. WIRELESS TRIGGERS ALLOW YOU TO USE A FLASH OFF-CAMERA

One big advantage of an external flashgun, or ‘speedlite’, is that you have the option of firing it away from your camera, changing the angles at which you can light your subject. A small number of cameras have a wireless system built in, but for most you’ll need external triggers. These consist of a transmitter, which sits on your camera’s hotshoe, and a receiver (one for each flashgun), which attaches to the flashgun. Some wireless systems work on infrared with a range of a few metres, while others use a radio signal, so will work over larger distances (100m or more) and are much more reliable. A budget set of radio triggers is £20-30. We recommend the Yongnuo RF-603N II set, which costs £23.

5. SOME FLASHES ALLOW MANUAL ZOOM FOR SPOTLIGHT EFFECTS

Many advanced flashguns are able to narrow or widen the angle of their light output to match your lens’ focal length, and do this automatically as you zoom in and out. This ensures that all the light from the flash is focused within your image area, and not wasted on parts of the scene that the camera can’t ‘see’. There are times, though, when you might want to get creative and take control of the flashgun’s zoom settings yourself. One option would be to zoom out on your lens, but zoom in on your flashgun. If you ensure that any ambient light in the scene is underexposed, you’ll get a spotlight effect, as in the image above, that will add real drama to your shots. Most external flashguns now come with built-in zoom functionality, which typically ranges from 24-105mm. If your lens’ focal length stretches beyond this range, the flashgun will simply set itself to the nearest setting available.

6. FLASH BRIGHTNESS IS GENERALLY UNAFFECTED BY SHUTTER SPEED

A burst of flash is incredibly, unbelievably, remarkably fast. Depending on the power output of your flashgun, the pulse of light can be as fleeting as 1/50,000sec! This happens in the first split-second of the exposure, so whether you shoot with an exposure time of 1/200sec or 10min, the amount of flash light reaching the sensor will always be the same. This is useful to remember if you want to boost flash brightness in your shots – ISO and aperture will have an effect, but shutter speed won’t. The fleeting nature of flash makes it much quicker than the fastest shutter speeds available on most cameras, so some photographers use a burst of flash light to freeze their subject. This can be done in-studio for still life, as in project 4 on page 44, or out in the field when shooting fast-moving subjects.

7. A MODIFIER WILL HELP PRODUCE A SOFTER LIGHT FOR PORTRAITS

Using a bare flashgun is fine, but you might notice it creates relatively hard shadows. This is less than ideal for most types of shot, but is a particular issue when shooting portraits, for which soft, diffused lighting tends to be more flattering. For this reason it’s better to add a modifier to the end of your flashgun to soften the shadows or control the direction of the light. Modifiers come in lots of shapes and sizes, including softboxes (right), umbrellas (below), snoots, grids, beauty dishes and bounce cards, each of which produces a slightly different effect.

A good option to start with is an umbrella – it’s cheap, portable and will dramatically increase the size of your light source. Ideally you’ll also invest in a light stand and tiltable umbrella adapter bracket, for around £20 each. We recommend essential photo.co.uk for inexpensive modifiers, brackets and stands. But do bear in mind that modifiers will reduce effective flash brightness. It is possible to buy small modifiers for a camera’s in-built flash, or make your own by looping a strip of tracing paper around the flash.