mixing flash and ambient light

Mixing Flash and Ambient Light

mixing flash and ambient lightAfter completing a family portrait shoot recently, I had the opportunity to take some fun photos of my client’s girls bouncing on their trampoline. A bright but rich evening sky provided the perfect backdrop, but there are some technical challenges to overcome to get a shot like this, so I thought I would share the issues and solutions with you here.

My first tip would be to consider the sky first, using manual settings to slightly underexpose it, hence achieving that beautiful deep blue colour. But therein lies a problem: the sky is bright and requires either a very fast shutter speed or a small aperture to get it dark enough. Fine you say, but then a small aperture will let little flash light through, and a fast shutter speed won’t work at all with flash!

Let’s focus briefly on flash photography. The pulse of light from your flash which lights your subject is extremely brief – perhaps lasting only 1/40000th of a second. This means that shutter speed has no effect on exposure when using flash light alone. Think about it – it doesn’t matter whether that very brief pulse of light occurs during a 1/125s exposure or a 1/25s exposure – if the flash supplies the only light available, then the exposure will turn out the same.

So – what’s the problem? Well – all cameras have what is known as a ‘flash synch speed’. This is the fastest shutter speed that the camera is able to accurately synchronise the flash pulse with. Any faster than the synch speed and the flash pulse occurs after the shutter has started closing again, meaning that the flash light fails to get fully to your subject. Typically the maximum synch speed of an SLR is about 1/250s.

It would appear then, that the only option would be to shoot at 1/250s and set one’s aperture small enough to underexpose the sky. For this image, I would have needed to set my aperture to F11.

Back to flash photography. When using flash light alone, it is only aperture that affects exposure; the smaller the hole through which your flash light passes, the less light gets through.

So – having set one’s shutter speed to 1/250s and aperture to F11 to get that perfect sky, how does one then ensure one’s subjects are also perfectly exposed? Simple – switch your flash to manual. Manual flash you say?! Isn’t that all ‘light meters and horribly complex calculations’? Actually – no – it doesn’t have to be. Once in manual, simply try full power output, review your image, and if the subject is too bright, reduce flash power accordingly. It will only take a few attempts before you nail the flash power setting and everything looks great – and besides, the girls will be having great fun on the trampoline, so are quite happy to keep bouncing!

Of course, as Hamlet said – there’s always a ‘rub’; some confounding problem. In this case, it may simply be that, for your chosen aperture setting, full flash power is insufficient to fully light your subjects. Happily, there are always solutions.

Most simply – and never forget this – just get closer to them! Light obeys the inverse square law, meaning that, if you halve the distance to your subject, four times the original light will fall upon it!

Another solution – if you can’t get any closer – is to increase your aperture to get more flash light through, accepting that your sky will then be a bit too bright. If you have the processing knowhow, you can always darken the blues on your computer to bring something of that richness back. And if not? Well it’s generally better to have your subject correctly exposed than the background – at least you have the memory.

So – what were my final settings? What solution did I opt for? In fact, I shot at 1/1000s, F5.6. But wait – didn’t you say that cameras can’t synchronise flash above 1/250s?

Yes, I did, but it’s worth knowing that some flash guns have a high speed ‘focal plane’ (FP) option. Rather than emit a single pulse of light, in this mode the flash outputs a series of pulses, adding up to the requisite amount of light. This permits the flash to be used with very high shutter speeds, as the series of pulses enables even light across your subject, even whilst the shutter is closing.

FP mode does, however, reduce the maximum amount of light which a flash can output, so why did I use it? Simply because I was photographing a moving subject and I wanted to freeze the action. The more astute among you may now be thinking ‘but flash always freezes action irrespective of shutter speed’. That’s true if flash is your only light source, but here during the day there was plenty of ambient light as well, which would blur the subject edges at slower shutter speeds.

Finally – why use flash at all? If there was plenty of ambient light, why not make things simple and just take a snap without the flash? As I have discussed in previous articles, our eyes are far more sensitive to light than a camera sensor, and what we see is also based on our brains’ interpretation of a scene. In fact, in a shot like this, the dynamic range (dark to light tones) between the sky and subjects would be so great that unless you burnt the sky out to almost complete white, your subjects would be dark almost to the point of silhouette – which of course might look great but is a completely different photo 😉