Australian Camera — May-June 2017

for a period of time and then

closing it. These timed settings

are called the ‘speed’ and they

can vary from a mere fraction

of a second up to several hours

or even longer.

Short – or fast – shutter speeds

are typically used in very bright

conditions while slow shutters

speeds – a.k.a. long exposures –

are required when there’s very little

available light (i.e. at night). Fast

shutter speeds are considered to

be those from around 1/60 second

up to 1/8000 second while slow

speeds extend from 1/30 second

to around 30 seconds or perhaps

60 seconds. This is generally the

timed speed range available on a

digital SLR or mirrorless camera,

but there are provisions for making

much longer exposures that you

time yourself via ‘B’ (for bulb) or ‘T’

(for time) settings.

As with f-stops, the difference

between each shutter speed

setting – for example 1/30 second

and 1/60 second – is one stop, but

modern cameras offer intermediate

settings representing one-third

stop or half stop adjustments such

as 1/80 second (which is 1/60

second less one-third of a stop)

or 1/400 second (which is 1/250

second less two-thirds of a stop).

Remember that the faster shutter

speeds progressively reduce the

overall exposure with, for example,

the 1/500 second setting halving

the exposure from 1/250 second.

Timing

In a digital camera, exposure times

can be controlled via a physical

shutter or by switching the imaging

sensor on and off. The advantages

of the sensor-based shutter are

that it is completely silent and

eliminates any vibrations.

However, there can be some

issues when shooting fast-moving

subjects so a conventional shutter

mechanism is retained.

In D-SLRs and digital mirrorless

cameras, this is a focal plane (FP)

type shutter, so-called because it’s

located in the camera body just in

front of the image plane (i.e. the

receptor surface of the sensor).

An FP shutter employs thin metal

(or sometimes composite) blades

which run vertically and open and

close to control the exposure

timing. With very fast settings, this

opening will merely be a slit – of

varying widths – that scans across

the front of the sensor, but from

about 1/60 second and slower,

the shutter will open completely.

The fastest speed at which the

shutter is fully opened is also the

maximum speed for using fl ash –

and hence known as the fl ash sync

speed – because otherwise part of

the shutter would still be in front of

the sensor when the fl ash is fi red

(so that part of the image will be

signifi cantly underexposed).

Some fi xed-lens digital cameras

and some of the lenses available

for digital medium format camera

systems employ leaf-type shutters.

These are also known as between-

the-lens shutters or simply lens

shutters because, you guessed

it, they’re located within a lens

rather than in the camera body.

Consequently, leaf shutters are

circular in shape and work in much

the same way as the aperture

diaphragm, except they open

fully and it’s the time factor that

controls the exposure. This design

puts some limits on how fast they

can be opened and closed, but

fl ash sync is available at any and all

speeds (which is attractive to some

professionals such as wedding

photographers).

Exposure Values

Now we know that apertures

and shutter speeds both control

exposures, then obviously they

can be worked together which

is important because both

adjustments also have visual

implications.

Apertures affect the depth-of-

fi eld – the zones in front of and

behind the focusing point that

are still sharp – and the shutter

speed will affect how movement is

recorded in the image. Put simply,

a fast shutter speed ‘freezes’

movement while a slower speed

will create varying degrees of

blurring (simply because the

subject moves while the shutter

is open). There are obviously

creative implications here, so

the combination of aperture and

shutter speed that you select

does more than simply control the

exposure. It’s for this reason that

being in control of these settings –

rather than simply leaving it to the

camera – enables you to achieve

the visual outcomes that you

want or, indeed, which are better

matched to the subject or scene.

This is where understanding

exposure values comes in

handy. At any given light level

• as measured by the camera’s
  exposure meter – the combination
  of aperture and shutter speed
  which gives a ‘correct’ exposure

is collectively called the exposure

value (or EV for short).

But remember that apertures

and speeds work together so, if

you reduce one by a stop, but then

increase the other by a stop, the

EV remains exactly the same (i.e.

you’ve halved one and doubled the

other). You can do this across the

full ranges of apertures and speeds

available on your camera so the

following combinations actually

all deliver exactly the same

exposure value.

1/4000 second at f2.0

1/2000 second at f2.8

1/1000 second at f4.0

1/500 second at f5.6

1/250 second at f8.0

1/125 second at f11

1/60 second at f16

1/30 second at f22

1/15 second at f32

1/8 second at f45

You’ll again note that the

relationship within any exposure

value is simple; if you reduce one

by a stop, you have to increase the

other by a stop in order to maintain

the same amount of exposure.

So, for example, if you’re shooting

high-speed sports action, you

might use 1/2000 second at f2.8,

but if you’re shooting close-ups

and need maximum depth-of-fi eld,

you might use f32 at 1/15 second.

At any given light level, then,

there is always a choice of aperture

and shutter speed combinations

that you can use to obtain the

required exposure, and your

selection may often be dictated by

the nature of the subject itself.

Sensitivity

There is a third key adjustment

which is related to exposure

and that’s the sensitivity setting

which is expressed in ISO values

(primarily to it can be related

back to the old fi lm speeds). An

imaging sensor’s sensitivity is

determined primarily by the size

of the pixels, but also the design

EXPOSURE CONTROL

73

Focal plane shutter operation sequences – at slower speeds (top) the

shutter blades will open completely( top, 3), but at faster speeds the

frame is essentially scanned by a slit (lower 2, 3 & 4) so the shutter is

never fully open. This is why FP shutters have a maximum speed for fl ash

synchronisation, otherwise part of the frame will be obscured when the

fl ash fi res. The maximum fl ash sync speed on most D-SLRs and mirrorless

cameras is usually between 1/125 and 1/250 second.

Aperture settings control the amount of light reaching the imaging sensor.

As the diaphragm progressively closes down each aperture setting – which

represents one stop of adjustment – lets in half the amount of light as the

previous one. Note that larger f-numbers represent smaller apertures.

Aperture settings control the amount of light reaching the imaging sensor.

1 2 3 4

5

FP SHUTTER

APERTURES

CamMayJune17_072-074 BacktoBasics.indd 73 13/04/2017 12:16 AM