Digital Photography is a science as well as an art form, and like any scientific subject, it has its fair share of technical terms.
Without getting too immersed in jargon, the definitions below will
explain some of the more common terms.
Exposure in digital photography
simply means the amount of light that is allowed to reach the
sensor. More light produces a brighter image, less
light a darker one. Too
much light will lead to the photo being overexposed with bright
areas where no details can be seen, whereas with too little light,
dark areas will be pure black. The ideal exposure gives the sensor enough light to detect details
in the shadowed areas of the image without being dazzled by too much
light in the highlights.
The amount of light reaching the sensor depends on the length
of time the shutter is kept open and the size of
the aperture through which the light passes.
The sensitivity (ISO value) controls how fast the
sensor reacts to the light it receives. The ideal exposure comes
from a proper balance between shutter speed, aperture and
sensitivity and thus any changes made to one require corresponding
changes to one or both of the others.
In practice, exposure is
usually controlled by changing shutter and aperture settings. If a
smaller aperture is chosen, the shutter must stay open for longer to
let in the same amount of light and when a faster shutter speed is
required, the aperture has to be widened accordingly. In circumstances where the
chosen combination of shutter speed and aperture would make the
image too dark or too bright, the ISO settings can be altered in compensation.
Although any of the three factors (shutter speed, aperture and ISO settings)
can be specified individually and the correct exposure obtained by
altering the others to suit, each has its own implications. As well
as controlling how much light is available to the sensor,
- SHUTTER SPEED controls whether moving objects are captured
sharply and whether the camera can be hand-held or needs
to be steadied on a tripod
- APERTURE SETTINGS determine how much of the image will be
sharply in focus (See also Depth of Field)
- INCREASED ISO VALUES can also increase the amount of digital noise in the image, possibly to unacceptable levels.
Digital cameras measure the levels of light
entering through the lens and calculate settings
that will achieve a standard, balanced exposure. Fully automatic
"point and shoot" cameras and mid-range or DSLR cameras set to
the basic capture mode are designed to choose an average, "one size fits all" combination of settings,
but the result can be disappointing if your subject falls outside
the average range for any reason. Fortunately, except for the
simplest models, most modern digital cameras offer features such as
a range of capture modes that allow you
to change at least some of the settings and choose how you want to
capture a particular image.
The shutter is opened by pressing the
shutter release button, usually located conveniently on top
of the camera body at the right hand side. While the shutter is
open, light can enter the camera and cause the sensor to react, and
the term "shutter speed" refers to the length of time the shutter is
open. Under normal circumstances, a single press on the release
button causes the shutter to open, remain open for a predetermined period and then
close again automatically (for exceptions, see
Shutter speeds are measured in fractions of a second - a setting of
1000 opens the shutter for 1/1000 of a second, 500 for 1/500,
250 for 1/250 and so on, and a setting of 1 (i.e. 1/1) opens
the shutter for 1 second. Blurred photos are often attributed to bad focus when the real cause
of the problem is movement, either of the camera or the subject.
With a steady hand, it should be possible to hold the camera still
for 1/250 of a second or so, but it is better to use a tripod than
risk blurring the shot by accidentally moving the camera while the
shutter is open.
Even if the camera is held perfectly still, the subject itself may
move during the shot (leaves on a windy day; birds in flight;
people, animals or vehicles moving past, etc). Movement can often be
"frozen in time" by
using a faster shutter speed, but you can also choose to accentuate
the appearance of motion by deliberately using a relatively slower
setting. See Shutter Speed Demonstration
for the results of experimenting with different speeds.
The aperture is the "hole" through which light enters the camera from
the outside world, via the lens. The size of the aperture is
controlled by a diaphragm consisting of overlapping metal plates that can widen and contract in much the
same way as the iris of the eye, to allow more or less light to
enter the camera. The actual amount of light that reaches the camera
depends on the ratio between the size of the aperture and the focal
length of the lens, but to avoid having to do complicated
calculations when adjusting the aperture, aperture settings are
simply expressed in
terms of "f" numbers.
Here is a typical sequence of settings:
Note that the lower numbers refer to wider
apertures, higher numbers to narrower apertures. The widest
available aperture (in this example, f/1) will be limited by the
diameter of the particular lens in use.
Dividing both shutter and aperture settings into steps known as "f stops" establishes a
relationship between them and makes calculations easier when aiming
for a balanced exposure. Each "f stop" in the sequence
below decreases the
size of the aperture by a factor of 2 from the previous one.
To see how this works in practice, suppose the correct exposure can be obtained by using a
shutter speed of 1/125 of a second and an aperture of f/16. If we
slow down the shutter speed by one stop to 1/60 and change the
aperture setting by 1 stop to f/22, we have doubled the length of time
the shutter will remain open and halved the size of the aperture,
allowing the same amount of light to reach the sensor as at the
original setting. Alternatively, we could use a shutter speed of
1/250 and an aperture of f/11 to get the same result.
Depth of Field
Both shutter and aperture settings do more than just
control the amount of light reaching the sensor. The
shutter speed is used to manage the blurring effect
caused by movement, while setting the aperture wider or narrower
will control how much of the
image appears to be sharply in focus.
When the lens is focused on an object at a certain distance from the
camera, either manually or using autofocus, anything at the same
distance (on the same plane) will also be in focus. Only one plane in an image can actually be in
perfect focus at any one time, but there is a limited area, extending
approximately one third in front and two thirds behind this plane, that still appears sharp to the human
eye. This area, where the image is still acceptably sharp, is called
the depth of field. Narrowing the aperture by setting the f stop to a higher number
will increase the depth of field, giving a greater area that still
appears sharp, while setting the f stop to a lower number will
There are various ways you can use this feature to improve your
photos. When shooting a landscape scene, you will probably want to
have as much of the image as possible appear in sharp focus, so you
can increase the depth of field by “stopping down” (narrowing) the
aperture by choosing a higher f stop number.
You may, on the other hand, want to feature a subject
close to the camera and distract attention from the rest of the
In this case you can set a wider aperture to narrow the depth of field
to the area immediately around the subject so that it stands out
sharply against a blurred background.
Depending on the current capture mode, the
camera may automatically alter the shutter speed to suit when you
change the aperture setting, but if not you may have to make the
necessary adjustment manually. See Depth of Field Demonstration for examples.
Sensitivity (ISO value)
ISO settings (50, 100, 200, 400, 800, 1600, etc, depending on the
individual camera), determine how quickly the electrical cells on
the sensor will react as light reaches them from the outside world.
Switching the ISO setting is the digital equivalent of changing a
film to one with a different ASA speed, with the advantage that it
can be done on an image by image basis.
Digital images are formed when light hitting
the sensor produces an electrical charge in each cell. An increase
in the ISO value
boosts the charge so that the sensor will react faster to
the available light. Although this can be useful by making it possible to use a faster shutter
speed or smaller aperture than would otherwise be required, it unfortunately often results in
loss of quality. The charge generated to increase sensitivity can cause
a phenomenon known as digital noise, where cells start to register
random specks of colour that interfere with the image. In black and
white film photography, a certain graininess could add character to
a photograph, but digital noise is actually a degeneration of the
image. While small amounts may be hardly noticeable in a small
print, an image badly affected by noise can look rather like a
multicoloured snowstorm and be completely ruined for all practical
Setting the sensitivity to 'auto' allows the camera to raise the ISO
as well as balancing the exposure by means of shutter and aperture
settings, but take care when using this setting. Noise often starts to
appear at settings above ISO 200, so the best way to avoid it is
simply to set your ISO manually to the lowest allowed by the camera
(ISO 200 or less) and only raise it as a last resort when the light
is very low and you need a fast shutter speed.
The physical size of the sensor is an important factor affecting the
amount of noise in an image. Sensors with a smaller surface area
need more of a boost and are more likely to be affected by noise, so
for the best quality images, choose a camera with as large a sensor
as you can afford. Note that it is the surface area of the sensor
that is in question, not the number of megapixels. It makes sense
that the tiny sensor in a mobile phone will not give the same
quality as the one in a full frame DSLR.
The purpose of the viewfinder is to give the photographer a preview of
the final picture in order to compose and in some cases to focus the
On a simple or compact camera, the viewfinder
takes the form of a reverse telescope, separate from the main lens.
The photographer may look directly through the viewfinder, or view
the image projected on to a small LCD monitor on the back of the
Depending on the camera’s design, the viewfinder may be offset slightly from the main lens.
This can cause problems when composing the picture, because the
image in the viewfinder is not seen from exactly the same angle as
the one seen by the lens, and the difference can be more noticeable
when the subject is quite close to the camera. The composition may
look perfect through the viewfinder, but appear to have moved up or
to one side in the final picture. The image on the monitor type of
viewfinder is likely to be more accurate, but lighting conditions
can sometimes make it difficult to see clearly.
Single lens reflex (SLR, DSLR)
An SLR camera uses a mirror to transfer the image as seen
by the main lens to the viewfinder, by-passing the aperture. When
the shutter is released, the mirror flips out of the way to allow
the light through the aperture and into the camera. Some cameras
also allow the mirror to be moved manually to give a preview through
the aperture of the actual image that will be captured, which is
useful for checking the depth of field.
The area of the subject that is captured in the image depends on the
focal length of the lens. A longer focal length magnifies the image
so that a smaller proportion of the scene fills the frame, giving
the impression of being closer to the subject. A camera may have a
fixed-length lens or be fitted with a zoom lens where it is possible
to vary the focal length. It may also be possible to remove the lens
from the camera and exchange it for another with a different range.
Lenses classed as ‘wide angle’ have a short focal length (on a
digital camera, a lens with a focal length of 18mm would be classed
as ‘wide angle’) and so give a large picture angle, resulting in a
wide field of view. In general, wide angle lenses tend to have a
greater depth of field and closer minimum focusing distances.
Telephoto lenses have a long focal length (on a digital camera, a
lens with a focal length of 300mm would be classed as ‘telephoto’).
The long focal length gives a small picture angle, with a narrower
field of view as a result.
Telephoto lenses can be used to draw attention to a specific
subject, where wide angle lenses can be used to show the vastness of
A zoom lens is one where the focal length can be altered (by
manually turning a ring on the lens or by operating a lever on the
camera) within certain limits. Examples of zoom lenses for digital
cameras would be those where the focal length can be altered between
18mm and 55mm or between 75mm and 300mm.
Digital zoom is
a method of enlarging an area of the scene by increasing the size of the pixels in
the image. This method often gives poor results and can cause a significant reduction in the quality
of images when printed.