Canon EOS mag

Technique Flash factors

lee beel

Flash synchronisation and shutter speeds

EOS cameras (and most other SLR makes) use a focal plane shutter. This is situated just in front of the sensor at the back of the camera. The focal plane is the area where all the light from the lens is focused – that is, the sensor. Hence the name of the shutter. The way in which the shutter works limits the range of shutter speeds which can be used when shooting with electronic flash (see panel at left). The fastest shutter speed that can be used with flash is called the ‘synchronisation speed’ and varies with the camera (see list at right). Most of the time EOS cameras will not let you set a shutter speed faster than the synchronisation speed when a Speedlite is attached and switched on or the built-in flash activated. Flash duration The shutter speed does not control the amount of flash illumination which reaches the sensor. This is because a typical flash burst from a Speedlite lasts for between about 1/1000 second and 1/10,000 second, depending mostly on subject distance. If flash is the only light source, you could shoot at 1/25 second or 1/125 second and still get the same exposure (assuming no change to the aperture or ISO). High-speed sync (FP flash) If you are shooting indoors, the synchronisation speed of the flash is probably not a problem – you will need a slow shutter speed to capture ambient light, or you will be shooting just with the flash illumination. Outdoors, using flash as a fill-in light, it is a different story. Shooting portraits in bright daylight you might find that the ideal ambient light exposure is 1/500 second at f4. The wide aperture limits depth-of-field and throws the background out-of-focus. Setting a smaller aperture to bring the shutter speed down will give a distracting in-focus background. The answer is ‘High-speed sync’ (HSS), sometimes called FP (focal plane) flash. In this mode the flash does not give a single burst – it emits a lot of short bursts very quickly. The Speedlite, in effect, becomes a continuous light source, albeit for a very short period of time. This means that although the shutter blinds never fully uncover the sensor at fast shutter speeds (see left), the sensor is evenly illuminated during the exposure – just as it is in daylight. The downside of HSS is that the power of the flash output is reduced, but it is usually more than adequate for portraits. Simply set HSS and shoot – the flash exposure will adjust to suit the ambient light exposure, giving good fill-in light. EX-series Speedlites and EOS digital cameras can be used for HSS – see right. Right Despite the fast shutter speed, a Speedlite has been used to provide fill-in flash for this outdoor portrait. EOS 5D Mark II, EF 70-200mm f2.8L IS USM lens at 200mm, 1/800 second at f5.6, ISO 400.

The focal plane shutter has two main advantages. It is built into the camera body (the alternative leaf shutter is usually fitted into the lens, so each lens needs a shutter unit). Focal plane shutters are also capable of very fast shutter speeds.

Sync speeds (sec) EOS 1D 1/500 EOS 1D Mark II 1/250 EOS 1D Mark II N 1/250 EOS 1D Mark III 1/300 EOS 1D Mark IV 1/300 EOS 1D X 1/250 EOS 1Ds 1/250 EOS 1Ds Mark II 1/250 EOS 1Ds Mark III 1/250 EOS 5D 1/200 EOS 5D Mark II 1/200 EOS 5D Mark III 1/200 EOS 6D 1/180 EOS 7D 1/250 EOS 10D 1/200 EOS 20D 1/250 EOS 30D 1/250 EOS 40D 1/250 EOS 50D 1/250 EOS 60D 1/250 EOS 300D 1/200 EOS 350D 1/200 EOS 400D 1/200 EOS 450D 1/200 EOS 500D 1/200 EOS 550D 1/200 EOS 600D 1/200 EOS 650D 1/200 EOS 1000D 1/200 EOS 1100D 1/200 EOS D30 1/200 EOS D60 1/200 EOS M 1/200

This is the shutter unit from an EOS camera. The shutter blinds in the rectangle are formed from multiple metal blades so that each blind can retract into a relatively small space above and below the frame.

The ‘ready’ position has the top blind retracted above the frame while the bottom blind is extended to cover the frame. When the shutter button is pressed, the bottom blind retracts, exposing the sensor as it goes (above).

The bottom blind continues to retract

If you shoot at or below the flash synchronisation

until the sensor is fully uncovered (above). At this point the Speedlite is triggered. The sensor remains fully uncovered for the brief duration of the flash.

speed, the full frame should be correctly exposed (above), providing that the coverage of the flash

matches (or exceeds) the field-of-view of the lens.

After the flash has fired the top curtain starts to move, covering the sensor. You can set a shutter speed slower than the flash sync speed. This delays the travel of the top curtain, capturing more ambient light.

The top blind reaches the end of its travel, terminating the exposure. Both curtains then move back to their start positions, but with one overlapping the other so that no further light reaches the sensor.

Above Set HSS on the Speedlite 430EX II by pressing the third button from the left (circled). The HSS icon will appear in the LCD panel if the selected shutter speed is faster than the normal flash synchronisation speed. The maximum working range of the Speedlite with this setting is indicated on the right of the LCD display (4 metres in this example). With Speedlites which do not have an HSS button, the function is set from the External Flash menu on the camera.

At shutter speeds faster than the flash synchronisation speed the top curtain has started moving by the time the bottom curtain is fully retracted (above) – part of the sensor is covered when the flash fires.

If the shutter is not in sync with the flash, part of the image will be unexposed (above). The lens creates an inverted image on the sensor and it is the top of this image which is affected – the bottom of the image we see.

32 Reproduced from EOS magazine October-December 2012

Reproduced from EOS magazine October-December 2012 33