Fill-in flash

It might sound strange to use flash in daylight, but it is one of the most useful techniques available to photographers and it can dramatically improve portrait shots. Daylight flash is often called fill-in flash, because it adds light to shadow areas and brings out detail in your subject.

If your camera has a built-in flash, it's incredibly easy to add fill-in flash. Just set the Fully Automatic (Scene Intelligent Auto) shooting mode ("green square") and raise the flash head. Then, when you take the shot, the camera will fire the flash with just the right amount of light to give a perfect image.

It works like this. Your camera divides the scene in the viewfinder into a number of zones and takes a separate meter reading from each. If the reading from the centre of the frame is the strongest, or there is little difference between the different readings, the flash remains inactive. But if the centre meter reading is weaker than the surrounding readings, the camera assumes that the subject is backlit and adds flash to light it.

Of course, the system is not perfect. There can be situations where the central subject area is dark but fill-in flash is not appropriate. In this case you can switch to another mode to disable the automatic fill-in flash function.

However, it is usually better to work the other way round. Rather than activate Fully Automatic mode but then switch off fill-in flash when you don't need it, instead simply switch it on when you do. It is easy to recognise situations where fill-in flash will help − and making the flash fire when you want it to is very straightforward.

The aim with fill-in flash is to give the sensor the correct exposure for the daylight, plus enough flash exposure to reveal detail to the subject's shadows. Too little flash leaves the shadows dark, while too much makes the image look artificial. In the past, creating good fill-in flash images required measurements and calculations. Today, however, your EOS camera does all the work for you. It's just a case of setting a suitable shooting mode (more on this in a moment) and the camera will come up with the correct shutter speed or aperture setting for a well-balanced daylight flash image.

Although they are very useful, built-in flash units are not very powerful. At most, the guide number will be 12 (ISO 100, metres). If you divide the guide number by the aperture used for the camera exposure, you get the maximum subject distance or the "reach" of the flash. So if you are shooting at an aperture of f/5.6, for example, the subject you want to be affected by fill-in flash should be no more than about 2 metres away when you use a setting of ISO 100.

Provided the subject is not too far away, the built-in flash found on many EOS cameras is perfect for fill-in flash photography. Being close to the camera lens, it does not throw any shadows across the subject, which means there's no conflict with the shadows thrown by the sunlight.

Here is a three-step guide to using a built-in unit for fill-in flash:

1. Raise the built-in flash head. Many EOS cameras have a small button marked with a lightning-bolt arrow near to the built-in flash unit. Press this button and the flash head will pop up. If there isn't a button, you just need to lift the flash with your fingers.
2. Set the shooting mode to Aperture Priority (Av). Set the aperture to f/5.6 or f/8. Check that the shutter speed shown in the viewfinder is not blinking (if it is, adjust the aperture setting until the speed value display is steady).
3. Frame your subject and press the shutter button. In most situations, the camera will automatically capture a well-exposed image that balances the daylight with the flash. When you have finished taking fill-in flash pictures, press down on the flash head to close it away.

A Speedlite is a great alternative to a built-in unit for fill-in flash, and if your camera has a built-in flash, attaching a Speedlite activates a switch (inside the hot-shoe) that stops the built-in unit popping up.

Speedlites are more powerful than built-in flash units. The Speedlite 600EX II-RT, for example, has a maximum guide number of 60 (ISO 100, metres), which indicates that it can deal with subjects around 10 metres from the camera. Outdoors, the reach may be a little less, but still a lot further than a built-in flash.

When a Speedlite is attached to an EOS camera and switched on, it will always fire, even if the situation is one where the camera would not automatically activate the built-in flash in Full Auto, Portrait or Close-up modes.

The most convenient camera shooting mode for fill-in flash with a Speedlite is Program (P). In this mode, the flash output will automatically adjust to give a good balance between the flash illumination and the ambient light.

Outdoors, the output of the fill-in flash depends on the shooting conditions. With lower light levels (below about EV 10), you get a flash output just as if you were shooting a subject at the same distance indoors. Above EV 10, the flash output is gradually reduced, to a maximum of -1.5 stops (-2 stops with E-TTL and E-TTL II autoflash) at EV 13 and above. This auto flash output reduction helps to create a better balance between the daylight and the flash illumination in bright sunlight.

Most of the shooting modes on an EOS camera can be used with fill-in flash. Here's a guide to which you should use, and why.

• Fully Automatic (Scene Intelligent Auto): As mentioned, Full Auto mode activates the built-in flash when the camera decides that flash is needed. It does this in low light, as well as backlit conditions. This is fine if you are in a hurry and need a quick and easy way to shoot with fill-in flash, but it offers no creative control.

• Program mode (P): This is a good mode for simple fill-in flash. As in Full Auto mode, the camera sets the shutter speed and aperture automatically, but leaves you to choose whether or not to use the flash. If you raise the built-in flash unit, it will fire. Otherwise it won't. Or you can attach a Speedlite and switch it on.

In both Full Auto and Program (P) modes, the shutter speed with flash is limited to between 1/60 second and the camera's flash synchronisation speed (between 1/90 and 1/250 second, depending on the camera). This is a fail-safe mechanism built into newer cameras to avoid images being ruined by the effects of camera shake at slower speeds. Most of the time you will see the lower speed being set. The higher speeds come into use only in really bright daylight. However, 1/60 second is a perfectly good speed for fill-in flash.

• Shutter Priority mode (Tv): In Shutter Priority mode (Tv), you select the shutter speed and the camera automatically sets the aperture for correct daylight exposure. The camera won't let you select a shutter speed higher than the camera's flash synchronisation speed – if you try, it will set it to the synchronisation speed.

• Aperture Priority mode (Av): This is a good shooting mode for creative fill-in flash. With Aperture Priority (Av), you select the aperture and leave the camera to set the shutter speed. On the EOS R and all EOS cameras released since, the same fail-safe mechanism applies in Av mode as explained above in Full Auto and Program modes, limiting the shutter speed. Older cameras can go down to 30 seconds. Aperture is one of the main determinants of depth-of-field (how much is sharp in front of the point of focus and behind it).

However, you don't have total control over the aperture. If you set a very wide aperture on a bright day, the camera might not be able to select a shutter speed fast enough for correct daylight exposure. The result will be an overexposed image. You can avoid this by checking the shutter speed value displayed in the viewfinder before shooting. If the value is flashing, it means you are out of range and should set a smaller aperture.

If you set a very small aperture, you might find that the camera sets a slow shutter speed, especially if the sun is not shining. You should get correct exposure, but the slow speed may mean you need to use the camera on a tripod to avoid the effects of camera shake.

• Manual mode (M): This allows you to set both the shutter speed and aperture independently of the camera while retaining full autoflash exposure. This is very useful for some flash techniques − but not fill-in flash. Most of the shutter speed/aperture combinations will not balance the flash with daylight. Use Aperture Priority (Av) mode instead.