Properly Exposed Photo
Note that there
are details throughout the photo.
Just about anyone can pick up a modern automatic camera and take
reasonably good, well exposed photos under optimal lighting conditions.
The electronics in modern cameras, especially those of expensive,
auto-focus, single-lens-reflex (SLR) cameras, can cope with a variety
of lighting conditions and users need merely set the camera to the
full auto mode and snap away. A good photographer, however, will
understand that some lighting conditions can fool the camera and
result in poor exposure when using the camera's automatic modes.
Understanding the basics of exposure and the effects of shutter
speed, aperture setting and ISO rating on the final photograph will
allow the photographer to over-ride the camera's recommendations
to achieve good exposures under adverse conditions that would most
likely fool the camera's automatic settings. This knowledge also
gives the photographer the creative edge necessary to produce specific
photographic effects in the resulting picture. It has been said
that knowledge is power. This is definitely true when applied to
the concepts of photography.
A modern camera is a very sophisticated instrument with a myriad
of controls that can seem baffling to a neophyte photographer. In
its simplest form, however, a camera is merely a light-tight box
that contains a light sensitive media with an opening that will
allow light to enter the box and strike (or expose) the material,
and a shutter to block the light from entering the camera until
the photographer is ready to expose the media. The opening (or aperture),
the shutter and the sensitivity of the light sensitive media are
the elements the photographer must consider for proper exposure.
Each of these elements can be manipulated to create specific photographic
effects that will be discussed later in this article.
Keep in mind, there is no "correct" or perfect exposure.
Exposure is a creative decision about what portions of the photograph
will contain the most detail, but in general terms photographers
refer to photos as being properly exposed, "underexposed"
and "overexposed." Underexposure results from allowing
too little light to hit the exposure media (film or a digital sensor),
and overexposure results from allowing too much light to hit the
media. The photographer can bias the exposure either one way or
the other, thus allowing for darker shadows (underexposure) or brighter
highlights (overexposure), but in doing so he or she risks losing
the details in the photograph. This results from the lack of dynamic
range in film or digital sensors.
Much of what is written about exposure in this article pertains
equally to both film and digital cameras since optics and exposure
principles are unchanged from one format to another. The term light
sensitive material or exposure media refers to film or to the digital
sensor. Film has a recommended light sensitivity rating determined
by the manufacturer, but a digital sensor has an adjustable sensitivity
than can be set by the photographer. There are limits to this sensitivity and you should always use the lowest sensitivity (ISO setting) for a the situation to avoid excessive noise (more on this later), but this is a great boon for photographers.
"Dynamic Range" and other terminology:
Photography seems to have its own language at times.
You'll often hear photographers talking about "stopping down
a lens", "two stops less exposure", "fast film",
"a slow lens", and "two stops of exposure latitude"
and "five stops of dynamic range." It's almost like you
need a dictionary to understand what they are talking about. One
key term is "stop" or "f-stop." A stop
represents either doubling or halving the amount of light hitting
the exposure media. It can also represent doubling or halving the
exposure media's sensitivity to light. "Stopping down a lens"
means reducing the size of the len's aperture to reduce the amount
of light through the lens. "One stop more exposure" when
referring to shutter speed means the shutter will be open twice
as long allowing twice as much light to enter the camera.
Exceeding Dynamic Range
Note the lack
of detail in the highlights and deep shadows!
"Dynamic Range" refers to amount of difference in light
between the darkest and brightest parts of the scene in which the
exposure media can produce good detail. More simply, it is the range
of subject brightness values over which a change in subject brightness
will create a change in image brightness. For example, slide film
and digital sensors are generally considered to have about 5 stops
of dynamic range. Print films usually have about 7 stops of dynamic
range. When the light falling on the scene exceeds the dynamic range
of the exposure media, the shadows may become completely black with
no details at all, or the highlights may become completely white (blown highlights),
or both. This is called a "high contrast" scene or lighting
Dealing with lighting in a scene is a topic for a future
article and usually involves adding fill light to soften the shadows or diffusing the light source to reduce the contrast. For now, it is enough to simply know that very strong sunlight
or bright lights in a dark environment create high contrast scenes.
This can be used with some creative effect by photographers with a strong mastery of light (or who are just plain lucky), but the results are
typically harsh and not always conducive to glamour or portrait photography.
Exposure and 18% Gray
Note the dull,
Most cameras contains a light meter which measures the amount of
light reflected from the scene. This light meter is set to assume
that the average of all the dark and light areas of the scene is
"18 % gray." For many scenes this will be appropriate.
If, however, the scene contains many dark areas you want to keep
dark, or many bright areas you want to keep bright, you probably
won't get what you want if you rely completely on your camera's
automatic exposure. For example, if the scene is dark the light
meter is going to tell your camera to let in more light which will
make your shadows too light and will "blow out" your highlights
(overexposure). Conversely, if you have a brightly lit scene, such
as sunlit snow, the automatic camera settings will try to reduce
the amount of light hitting the scene producing a photo of gray
snow and completely black shadows (underexposure).
Modern cameras have exposure algorithms programmed into their electronics
that help them deal with these exposure problems, but a good photographer
should anticipate these exposure problems and adjust the camera
settings accordingly. This may mean bracketing exposures to ensure
that one of several exposures will produced the desired effects.
For example, for a dark scene the photographer may override the
camera settings to shoot an additional photo at one stop below the
recommended exposure setting and another at 2 stops below the recommended
exposure. For a high contrast scene the photographer may shoot the
scene at the recommended exposure plus one stop below the recommended
exposure and one stop above the recommended exposure. By bracketing
the photographer can improve the chances that at least one shot
will have acceptible exposure. Additionally, improved software has made combining individually exposed images into a single High Dynamic Range (HDR) image a relatively painless experience, especially when the photographer takes care to use a tripod to keep the images aligned.
Note the blown
out highlights (no details)
Lens Aperture and Depth of Field
Photographic exposure is determined by three variables: the quantity
of light hitting the media (determined by the lens aperture), the
length of time the light is allowed to hit the media (shutter speed),
and the sensitivity of the media to light (ISO rating). The lens
aperture, used in conjunction with the other two variables, not
only helps produce the proper exposure, but gives the photographer
control over the photographic Depth of Field (dof).
The term depth of field refers to the range of acceptable focus
in front of and behind the point of focus. Shallow depth of
field refers to a very narrow range of acceptable focus and
wide or deep depth of field means that most, if not all,
of the photo will be very sharp. So, the question arises, why would
anyone want a shallow depth of field? Wouldn't an expert photographer
want the entire photograph to be sharply in focus? The answer is
subject isolation. When the photographer wants to isolate
a sharply focused subject from the background (or less often from
the foreground), a shallow depth of field is the answer. By opening
the lens aperture, the photographer can decrease depth of field
to narrow the range of focus. Of course, shutter speed and ISO would
have to be appropriately adjusted to compensate for exposure, and
that will affect other creative aspects of the shot.
Shallow Depth of Field - Canon 135 f/2 L @ f/2
In the example Shallow Depth of Field photograph, the busy background
is out of focus minimizing the distraction from the main subject
(who is distracting enough all on her own). In the Medium Depth
of Field example, a smaller aperture was used to ensure the entire
motorcyle was rendered in sharp focus.
Depth of field is also affected by the focal length of the lens
used and the subject to lens distance as well as a few other more
esoteric factors, but in general the longer the focal length of
the lens and the shorter the lens to subject distance, the shallower
the depth of field. For most amateur photographers, the aperture
will be the most important variable in determining the the depth
of field, and this variable strongly impacts exposure whereas the
focal length and lens to subject distance have no discernible impact
Lens aperture is measured is f-stops. An f-stop
(or f-number) is the ratio of the focal length of the lens
to the diameter of the aperture (or opening) in the lens and is
expressed as a decimal number. So a 50mm lens with an effective
aperture diameter of 25mm would have an f-stop of 2.0.
For most lenses the range of variable f-stops goes from
a maxium (largest opening) of f1.4 to a minumum (smallest
opening) of about f22.0. Some lenses have a smaller or
larger range of f-stops. The f-stop values represent
a doubling or halving of light between each stop follow the following
Aperture (f-stop)Value Sequence
Medium Depth of Field
A small aperture
creates more DOF
Starting at f/1.4, each subsequent f-stop value
produces a smaller effective aperture (remember this is a ratio)
and less light (1/2 the amount of the previous stop) is allowed
to enter the camera. Also remember, the larger the f-stop number,
the smaller the effective aperture of the lens and less light enters
the camera (which requires a slower shutter speed or more sensitive
media to compensate). Finally, keep in mind, the larger the aperture (smaller
f-stop number), the shallower the depth of field, all other variables
being equal. A fast lens has a large maximum aperture (f2.8
and below are considered fast) and stopping down a lens
means setting a higher f-stop (smaller aperture).
Shutter Speed and Motion Blur
The second exposure variable is shutter speed. Shutter speed is
a much more simple concept to understand. The shutter speed setting
simply determines how long the exposure media is exposed to the
light entering the camera. Shutter speed is typically expressed
as a fraction of a second, and like aperture, the standard increments
refer to doubling or halving the amount of light allowed to hit
the exposure media. There is a much larger range of shutter speeds
available than there are for apertures, especially on modern cameras,
but typical settings are 1/60th, 1/125th, 1/250th, 1/500th and 1/1000th
of a second. Some cameras can go has high as 1/8000th of a second
and most cameras can easily do 30 second exposures. Some cameras
also have a Bulb or B setting that will hold the shutter
open for as long as the photographer presses the shutter button.
This is useful for very long exposures in very low light.
The longer the shutter is held open, the greater the chance is
that something (either the subject or the camera) will move during
the exposure. If the camera moves significantly during the shot,
the resulting photograph will look blurry and almost as if it is
out of focus. Since no photographer can hold a camera completely
still, long exposures are more likely to cause this phenomena called
"camera shake." So for long exposures a photographer should
use a tripod to hold the camera still. Longer focal length lenses
(which magnify the subject image) are more prone to this problem
than wide angle lenses (which reduce the subject image). A good
general rule to use for hand-held photography is to set your shutter
speed to no slower than the reciprocal of the lens
focal length. So, for example, if your lens has a focal length of
50mm, you would use a shutter speed of 1/60th of a second (the closest
setting to 1/50). With good technique an experienced photographer
may be able to shoot 1 stop slower, say 1/30th, but the above rule
is a good place to start for beginners.
As with aperture, shutter speed can be used with creative effect.
Shooting a moving subject, such as flowing water, with a slow shutter
speed can create a silky look to the water or create a sense of
movement for an otherwise static subject. A fast shutter speed can
freeze action and stop a bird in mid-flight. In glamour photography,
unless striving for an unusual effect, the shutter speed will be
determined more by consistency with the aperture setting or by the
focal length rule than by anything else.
Motion Blur of Water
The silky look
of the water resulted from a slow shutter speed on a tripod
Creating a Sense of Motion with Shutter
The shutter speed
for this shot was fast enough to capture the individual drops
of water, but slow enough to create an elongation of the droplet.
ISO Ratings and Exposure
High ISO Photo
ISO stands for International Organization for Standards and an ISO rating refers to the standard sensitivity rating of film.
Digital cameras also use ISO ratings, but in the digital realm this
really refers to the electronic gain or amplification of the digital
sensor and is a setting on the camera itself.
The ISO values a photographer will most likely encounter
are 100 ISO, 200 ISO, 400 ISO, and 800 ISO. Some films and digital SLR
cameras can go as high as 3200 ISO and as low as 50 ISO. As with
aperture and shutter speed, these standard ISO values represent
a doubling or halving of light sensitivity. For example, 200 ISO
film is twice as light sensitive as 100 ISO film and half as light
sensitive as 400 ISO film.
As with the other exposure variables, there are advantages
and disadvantages inherent in the ISO setting. Very slow ISO films
as settings allow for virtually noiseless (digital) or grainless
(film) images, but require correspondingly slower shutter speeds
and/or larger aperture combinations. Slow ISO ratings are best for
finely detailed images or brightly lit conditions. Fast films (high
ISO) can allow a photogapher to shoot hand-held in lower light or
allow the use of smaller apertures to get greater depth of field,
but at a loss of fine image detail and an increase in grain or noise.
So the choice of ISO settings and film is as much a creative decision
for the photographer as is the choice of shutter speed and aperture
The example photograph was shot at 3200 ISO using
a professional digital SLR camera with the only light coming from a set of candles below the model. You can easily see the noise inherent in the photo at this high
ISO level. Sometimes noise, (but more often. film grain is used for special creative effects. This photo could not have
been taken at a lower ISO rating, but there was certainly a trade
off in detail and in noise.
Putting It All Together
Contolling exposure and the creative elements associated
with each element of exposure are what help to differentiate the
photographer as a artist rather than merely the user of a recording
device. Composition and lighting are also very important and will
be discussed in future articles, but understanding exposure is the
starting point for any photographer seeking to do more than simply
take pictures. A photographer who understands the difference of
making an exposure with a 50mm lens at 100 ISO, f/2.0, and
1/60th of a second or making the same exposure with a 100mm lens
at 800 ISO, f/4.0, and 1/125th of second has much more creative
control of the final result than snapshooter who does not.
This article presents only a fraction of the information
avalible about photographic exposure. Other recommended sources