Flash presents opportunities to deal creatively with many situations in photography. It can be used outdoors to soften shadows on a bright day or highlight the subject, indoors to supplement poor lighting, or in a studio to create a fully-controlled lighting environment. Flash works by emitting a very short pulse of bright light, lasting about 1/1,000 to as short as 1/30,000 second. This short duration has a number of implications.
The shutter of a DSLR camera usually consists of 2 metal curtains, which when at rest, block the light from the lens getting to the sensor. When taking a picture, the first curtain starts to open, allowing light onto the sensor – and then the second curtain closes, blocking the light again. At low shutter speeds, below about 1/200 second, there is a period when the first curtain is fully open and the second curtain has not yet started to close – i.e. the shutter is fully open. To achieve higher speeds, say 1/1,000 second, the second curtain starts moving (closing) even before the first curtain is fully open. The shutter is never fully open but the gap between the 2 curtains moves across the opening, allowing a band of light to sweep across the sensor. An animation of shutter operation is provided: Shutter animation
This is fine when taking a picture in daylight, when the subject illumination remains constant while the band of light sweeps across the sensor. But when using flash, we must ensure that at the moment the flash fires, the shutter is fully open. Firing the flash must be synchronised with the shutter fully open and this only occurs at lower shutter speeds, up to about 1/200 second depending on the camera.
So flash is generally only usable in the low to mid shutter speed range. To get over this limitation, some flash units allow “high speed synchronisation”, where the flash fires rapidly and repeatedly during the exposure, allowing any shutter speed to be used.
The second implication of the flash’s very short duration is that the exposure setting for flash is independent of the shutter speed. If the flash lasts just 1/1,000 second, it doesn’t matter whether the shutter is open for 1/30 or 1/125 second, as long as it is fully open when the flash fires.
Classically, setting the exposure for flash photography involves using a flash meter placed at the subject, pointing back at the flashgun. We do a test firing of the flash and the meter will provide the f/number to set on the camera for any given ISO setting. Most flash units allow the power of the flash to be varied: full, 1/2, 1/4 power and so on. In this case we can choose an f/number (to control depth of field) and then use the flash meter to help us adjust the power of the flash to provide sufficient illumination for that f/number.
The brightness of the flash’s illumination of a subject depends on the power of the flashgun and the distance between the flash and the subject. As a point source of light, the intensity falls off with the square of the distance to the subject: i.e. if the distance is doubled, the intensity will drop to 1/4. So we can control the illumination of the subject either by adjusting the power of the flash or by moving the flash relative to the subject: further away from the subject to reduce illumination – or closer to increase illumination. In passing, one implication of this is that when photographing a group of people it is important to ensure that everyone is roughly at the same distance from the flash unit.
For correct exposure, we adjust f/number and ISO on the camera – and power and distance to subject on the flash unit. This is more straightforward than it sounds. Each flash unit has a guide number which is an exposure constant for the unit. For example if a unit has guide number of 40 metres at ISO 100, it means that a subject 10 metres away will be correctly exposed at f/4 (10 * 4 = 40) and so on.
To set up a picture with this flash unit, we might set the camera to ISO 100 and f/8 (based on consideration of depth of field) and then place the flash unit 5 metres from the subject (5 * 8 = 40). If we wish to keep things a little closer, we could move the flash unit in to half that distance, 2.5 metres, and set the flash unit to 1/4 power.
Foreground v background
Flash units are only effective at short range, as the illumination falls off rapidly with distance to the subject. If we have a person standing in front of a city backdrop, we can illuminate the person with a flash unit but not the cityscape behind.
So in a sense, when using flash we are creating a double exposure. The background scene is still exposed in the normal way, according to shutter speed, f/number and ISO. But in addition the flash exposes the subject, according to f/number, flash power and distance between the flash and the subject.
Many modern SLR cameras allow the flash to be metered and controlled by the camera using similar through-the-lens (TTL) metering as is used to meter any other type of subject. In such cameras we have the ability to set exposure compensation separately for both the normal exposure and the flash exposure. We can thus control the exposure of the subject and background independently.
Here are two classic examples of using such exposure compensation. First if we are taking a portrait on a sunny day, there may be harsh shadows on the subject’s face. If we set the camera exposure in the normal way but use a flash with its exposure compensation set to –2 stops, the flash will just add a little fill-in light to the shadows, giving a more balanced overall illumination.
On the other hand, if taking a portrait in front of a city scene, we may wish to make the subject stand out from the background. This could be achieved by setting the camera exposure compensation to –1 or –2 stops while setting the flash unit to deliver a normal exposure. The flash will illuminate the subject but will not have the reach to illuminate the background, which will be slightly underexposed as set in the camera. The subject should thus pop out of the background.
The same principles apply if setting the exposures of the camera and flash unit manually. The settings on the camera will affect the level of overall background exposure, while the flash settings will determine the subject exposure.
Note that these 2 exposures, background illumination and flash, are additive. If we set the camera to expose the subject normally and also set the flash to illuminate the subject normally, the effect will be to over-expose the subject. Thinking in terms of a double exposure helps keep this in mind.
On or off camera
Many cameras come equipped with a pop-up flash or a hot shoe to mount a flash unit on camera. While convenient, this has the disadvantage that illumination is very flat, i.e. all surfaces of the subject facing the camera receive the same amount of illumination. It is usually far better to have the flash off to one side, so that the light creates some modelling of the subject. This of course requires a cable to connect the flash unit and camera – or better still, an infra-red or radio link.
The figure below shows a standard studio flash setup. The camera and main flash (key light) are set up as follows:
- ♣ Camera – fastest sync speed, typically 1/200 sec
- ♣ Camera – standard ISO, 100 – 200
- ♣ Camera – set reference f/number, f/5.6 – f/8 (f/8 in the figure)
- ♣ Flash meter – facing the flash to be adjusted (key light first)
- ♣ Key light – take a flash reading and adjust flash output until reference f/number achieved
- ♣ Other lights – adjust output to achieve desired lighting effect
For high-key lighting, set the fill-in light to –1 EV. For high contrast / low key lighting, set the fill-in light to –3 EV. The edge light might be set +0.5 to +1 EV.
Note that where 2 (or more) lights are illuminating the same area, perhaps the backdrop, the results will be additive. So to achieve f/8 with 2 lights, adjust each individually to give f/5.6.