How to balance ambient light with flash

Many camera and flashgun combinations do a great job of producing images with exposure that’s nicely balanced automatically in a wide range of situations, but if you want to take more creative images or be able to assume control when the automated systems fail, you need to understand how to do the work yourself.

This is something that many photographers are nervous about, but it’s actually pretty straightforward provided you’ve got a good grasp of shutter speed and aperture.

A common reason for balancing flash and ambient light is to illuminate a dark or dull subject in the foreground while having a nicely exposed background, but it’s also useful for adding a little contrast and giving your image a bit more impact. It’s a routine technique for portrait photography.

One of the fundamentals to remember when dealing with flash is that it usually has a very short duration and this means that as long as you are working within its shutter speed sync range, it is only affected by aperture.

Once you have determined the exposure settings that you want to use, the best way to control a flash’s impact is to adjusts its power or change the distance between it and the subject. Flash light falls off very quickly, so it won’t reach distant backgrounds.

Provided that you use a shutter speed that’s within your flash’s sync range you can control the exposure of the background using shutter speed and aperture as you would normally.

Setting the Ambient Exposure

The easiest way to set the exposure for the background/ambient light is to use your camera’s built-in lightmeter just as you would normally. Shoot in Manual exposure mode and use a fixed sensitivity value rather than let the camera set sensitivity itself in Auto ISO mode.

This puts you in complete control of the aperture (and depth of field) as well as the shutter speed. Ideally use a low sensitivity setting to keep noise levels down.

Take a shot without flash and adjust exposure as necessary until you are happy that the background looks right.

It depends upon the effect you want, but in many instances it’s a good idea to reduce the exposure of the background a little so it’s a slightly darker than you would have it in a ‘normal’ non-flash image.

When the flash is fired, your image will have more impact and your subject will standout. It also gives the scene a bit more contrast, but nowhere will be darker than in the non-flash image provided you keep the camera’s exposure settings the same.

Setting the flash power

Now to add a flash of light. With film it’s advisable to use a light meter to measure the brightness of the flash light and set appropriate exposure settings, but with a digital camera it’s quicker and easier to just take a couple of test shots.

Modern flashguns have clever automatic through-the-lens (TTL) exposure modes that can deliver just the right amount of light, but they can be hard to predict and prone to variation, so on the whole it’s best to take control and set it to Manual mode. Then set the flash power to a quarter as this makes a good starting point.

If the flash is too bright you have two options, move it further away from your subject or turn down the power. Alternatively, if it’s too dark, turn up its power or move it closer to your subject.

Next steps

That’s it, we’ve covered how you balance ambient and flash light in just a few easy steps. You can add more lights and flash modifiers to make the light softer or harder, but the basic principle is the same.

Bear in mind that any light modifiers cut some light so you may need to turn up the flash power or move it nearer to your subject to get the same exposure.

Also, light intensity is governed by the inverse square law which means that if you double the distance between the light source and the subject, the intensity of the light drops by a quarter.

Conversely if you halve the distance the intensity is increased by four times. SO small movements can have a dramatic imapct.

It’s also important to consider the direction of the flash light to produce natural looking images and as a rule you’ll get far better results if you shoot with the flash off the camera than when it’s mounted in the hotshoe.

There are many wireless triggers and flashguns that have wireless communication built-in, but if yours doesn’t, cables are widely available and inexpensive.

If your camera lacks a socket to connect a flashgun you can get a hotshoe adaptor very cheaply, so there’s no excuse for not using a flashgun remotely.