Canon EOS mag

Technique

Flash factors

Reproduced from EOS magazine October-December 2012

lee beel

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.

Flash synchronisation and 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

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.

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.

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

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.

If you shoot at or below

the flash synchronisation

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.

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.

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.