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Infobank

Flash: Flash synchronisation

The duration of the flash from Canon Speedlites is faster than 1/1,000 second, and often as short as 1/10,000 second. This suggests that you should be able to shoot with the camera shutter set to 1/1,000 second, or even 1/2,000 second.

This might be possible if your camera used a leaf shutter. This is a shutter made of interlocking metal blades that block all the light while closed, but open to let the light through to the film. A leaf shutter is normally located within the lens, between the glass elements.

Focal plane shutter

EOS cameras, however, use a focal-plane shutter. This consists of two metal curtains positioned just in front of the digital sensor or film (image frame). At the start of the exposure, the first curtain opens to expose the image frame. At the end of the exposure, the second curtain moves to cover the area up again.

Each curtain takes time to travel the width of the frame. If you wait for the first curtain to reach the end of its travel before the second curtain is released, the fastest shutter speed possible is somewhere between 1/60 and 1/500 second (depending on the camera).

The speed at which the curtains move is constant. They don’t speed up or slow down at different shutter speed settings. Slow shutter speeds are obtained by increasing the time between the opening of the first curtain and the closing of the second.

Fast speeds are obtained by releasing the second curtain before the first has reached the end of its travel. This exposes the image frame through a moving slit formed by the two curtains.

However, the duration of an electronic flash is so short that if it fires while the curtains are forming a slit, only the area of image frame uncovered by the shutter blades at that precise moment will be exposed.

If you want the entire frame area to be exposed at the same time, you need to set a shutter speed that allows the first curtain to reach the end of its travel before the second curtain starts to move. The fastest shutter speed that allows this to happen is called the flash synchronization speed. On EOS cameras it is never slower than 1/60 second, and can be as fast as 1/500 second, depending on the model.

However, the flash synchronisation speed is not the only shutter speed you can use for flash photography. All speeds slower than the flash synchronization speed will also synchronize with flash − a fact that offers plenty of creative opportunities.

The shutter speed setting has no effect on the exposure of the digital sensor or film to the flash illumination. The flash duration is shorter than the exposure time. You cannot increase or decrease the amount of flash illumination reaching the film by altering the shutter speed setting. The flash synchronization speed is simply a speed that allows all of the film to be exposed to the light at the same time.

High-speed synchronisation (FP flash)

With EX-series Speedlites, it is possible to use flash at higher shutter speeds than the flash synchronisation speed. This is called high-speed synchronisation, or FP (focal plane) flash, and is set on the control panel of the Speedlite.

This is done by increasing the duration of the flash so that it gives a constant output while the moving slit formed by the focal plane shutter moves across the sensor or film. This is called ‘long-burn’ flash. Although different areas of the sensor or film are exposed at slightly different times, each area receives the same amount of flash illumination.

The increased duration of the flash comes at the expense of the flash power, and the faster the shutter speed used, the lower the power. At a shutter speed of 1/500 second, the guide number of the Speedlite is less than half its normal rating, and this drops to less than a quarter at 1/2000 second.

High-speed synchronisation is useful for fill-in flash with fast-moving subjects − athletes and cyclists, for example. Here, the faster shutter speed helps to ‘freeze’ the movement of the subject, while the flash adds detail to shadow areas.

High-speed synchronisation can also be useful when shooting portraits outdoors with fill-in flash in bright sunlight. If you are limited to a shutter speed of, say, 1/200 second, you might have to use a small aperture to get correct exposure. A small aperture might give more depth-of-field than you want. By shooting at 1/500 second, or even 1/1000 second, you can use a wider aperture, reducing the depth-of-field.

Slow speed synchronisation

Slow speed synchronisation simply means shooting with flash at a slow shutter speed. With a moving subject, this often captures two different images − a sharp image of the subject taken with the short duration of the electronic flash, and a blurred image of the subject taken by ambient light at the longer duration of the slow shutter speed. Results are often unpredictable, and can be varied by altering the shutter speed.

Combining flash with a slow camera shutter speed merges two images − one sharp, the other blurred. The result can give a strong impression of movement.