Understanding exposure

18/04/2009 Understanding Exposure
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we are still painting with light, albeit using a much more advanced arsenal of tools.
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In this lesson, we will take you all the way back to the basics. We will explore the
fundamental, manual process that makes photography what it is, "painting with light." By the
end of this lesson, you'll be able to take full control over your camera settings including
shutter speed, aperture, and ISO. These basic camera settings fall under the umbrella of
exposure. Understanding exposure is the first and most important step to unleashing your full
photographic and artistic potential.

(Most images can be clicked for an enlarged view.)

Topics Covered:
How Cameras Work
Understanding the Digital SLR
The Aperture
Exploring Depth of Field
The Shutter Speed
The ISO Setting
The Light Meter
Tying It All Together

Equipment Used:

webphotoschool.com/…/index.html 1/18

Camera/Media

Olympus E-510

From the very first box cameras to be mass-produced in the 19th century all the way to the
most current digital cameras, the mechanics of exposure have remained relatively
unchanged. The most notable difference today is that the camera is now able to do most of
the "thinking" and arrive at various exposure settings automatically. Nevertheless, the camera
is not right 100% of the time. More importantly, the camera does not know, nor does it
care, about your personal vision for any given photograph. Therefore, in order to gain
control over one's photographic vision, one must understand how a camera works and how
to manually control the mechanical process of exposure.

How Cameras Work
First, let's explore the internal workings of a camera. Simply stated, the camera is really
nothing more than a lightproof box with a hole on one side and a light sensitive material on
the other side.

The very first camera actually existed long
before there was such a thing as film or light
sensitive material. It was called the Camera
Obscura (translated from Latin as "dark
chamber"). The mechanics of a Camera
Obscura are simple (figure 1).

It is a light-tight box with a tiny pinhole on
one side. Light enters through this tiny pin
hole and an upside down, laterally reversed
image of whatever is outside or in front of the
pin hole is projected onto the interior wall of
the dark box. It is really quite magical and it
Figure 1
really does work. However, without any film
or digital sensor, it is practically useless to
photographers.

The next logical step to the Camera Obscura is a basic, homemade pinhole camera. Anyone
who has taken a beginning photography course in the pre-digital age was probably taught
how to make one. A pinhole camera is a light-tight box with a hole on one side and a piece
of photographic paper on the inside. As with the Camera Obscura, light enters through the
pinhole and an image is projected onto the light-sensitive paper. By controlling the amount of
light entering through the hole and reaching the paper, one is able to control the exposure of
the recorded image.

If this concept is still a little unclear, don't worry. For now, just remember that a camera
need not have any dials or LCDs in order to be a camera. Rather, a camera is simply a dark
box.


Understanding the Digital SLR
Camera Obscuras and pinholes aside, you might still be asking yourself how all of this
relates to your brand new Canon 20D, Nikon D200, or Olympus EVOLT E-3. Well, the
truth of the matter is that every one of these modern digital cameras are build around the
same design concept as the basic dark box example shown above.

The Digital SLR is also a dark box with a hole on one side and a sensor on the other side.
SLR is an acronym for Single Lens Reflex. Single Lens Reflex means that the lens that you
are looking through when you look into the viewfinder is the same lens that is taking the
picture. There are other types of cameras where this is not the case, but for the purpose of
this lesson, we will direct our attention to the SLR as this is the most common type of
camera being used today.

The diagram on the right (figure 2) illustrates
the inner workings of an SLR (Single Lens
Reflex) camera. In a nutshell, here's how it
works. Light enters through a lens instead of a
pinhole. The lens is made up of numerous
glass elements, which work together to focus
the light coming in. Having a lens instead of a
pinhole makes it possible to let in a much
larger amount of light while keeping the
projected image in focus. After the light passes
through the lens it encounters a mirror, which
sits at a perfect 45-degree angle to the lens.
The mirror reflects the light upward into a
Figure 2
specially shaped prism. The prism bounces the
light around its various surfaces turning the
image right side up.

When you look inside the viewfinder of an SLR, you are seeing the image reflected inside of
the prism. Keep in mind that this entire mirror and prism mechanism is only there so that you
can see the image right side up in the viewfinder prior to taking the picture.

As soon as you press the shutter release, the mirror pops up and a shutter curtain opens,
exposing the digital sensor or film. At this point, the light simply travels in a straight path
through the lens and if you've focused the lens correctly, a sharp image is projected onto the
light sensitive material (CCD or film). This is where the concept and technique of controlling
exposure comes into play.

One of the primary skills in photography is properly determining how much light reaches the
sensor, how long the sensor is exposed to the light, and finally how sensitive the sensor is to
begin with. This is what we mean when we compare photography to "painting with light." As
you read on, we will revisit each of these points in detail.

The 45-degree mirror described above is an essential part of every SLR, film or digital.
Here are two illustrations that show the mirror inside of an Olympus EVOLT E-510 (figures
3 & 4). For demonstration purposes, the lens has been removed. When you remove a lens
from an SLR body, you will see where the mirror is positioned directly behind the lens
(figure 3). When the shutter release is pressed, the mirror instantly moves up, out of the way,

exposing the digital sensor (figure 4).

Figure 3 Figure 4

Now that you've got a grasp on some of the basic parts that make a digital SLR or any
other camera work, we can begin a more in-depth discussion of exposure and its various
elements. As mentioned above, exposure is comprised of three major factors, all of which
work together to control how light or dark the recorded image will be.

The three factors that effect exposure are:

1. The amount of light coming in through the lens. This is controlled by the aperture
setting.
2. The length of time that digital sensor is allowed to be exposed to the light coming in
through the lens. This is controlled by the shutter speed setting.
3. The sensitivity level of the digital sensor or film. This is controlled by the ISO setting.
In the case of traditional film, each individual roll of film has an ISO rating.

The Aperture
Inside every lens there is an adjustable opening called the aperture. The aperture controls
how much light is allowed to enter through the lens. The larger the aperture opening, the
more light can enter through to the digital sensor. The smaller the opening, the less light can
enter.

The relative size of the aperture is measured in f-stops. The range of f-stop settings available
on the camera depends on each lens. With older 35mm film cameras, the aperture
adjustment setting was usually located on the lens, and not the camera. Each lens would
have an aperture ring right behind the focus ring. With most of today's digital SLRs,
however, the aperture adjustment setting is located on the camera body. But regardless of
how the aperture is controlled on whatever camera you are using, the range of f-stop
settings have essentially remained the same.


Most lenses have the following standard range of f-stop settings available:

[f/2.8, f/4, f/5.6, f/8, f/11, and f/16]

For the purpose of this tutorial, we will not delve into the complex equation from which
these f-stop numbers are derived. Much more vital to one's understanding of exposure are
the following aperture related facts:

The smaller the f-stop number, the larger the aperture opening inside the lens, and the
more light is allowed to enter.
The larger the f-stop number, the smaller the opening, the less light can enter.
The list of apertures given above is the standard list expressed in full stop increments.
A "full stop" increment can be expressed as either twice or half the amount of light as
the f-stop before it. In other words, an aperture setting of f/5.6 will allow twice as
much light in through the lens as f/8 or half as much light in as f/4. This will make more
sense later, when we discuss how the shutter speed interacts with the aperture.
Many current Digital SLRs will allow the aperture to be set in 1/3-stop increments.
This allows for f-stops like f/9 and f/10, which allow more precise exposure control,
but this can be confusing for the absolute novice.
Some lenses will allow for additional stops at the widest and smallest aperture
settings. For example, some lenses will allow apertures as wide as f/2, f/1.8, or even
f/1.4. These lenses are considered fast lenses because they let more light in, which
makes them useful for low light situations. We will explore this concept later in the
lesson.

The following five images (figure 5) are illustrations of 5 full-stop increments in aperture,
ranging from f/11 at the smallest lens opening to f/2.8 at the largest lens opening. The
window towards the bottom of each image shows a picture of an actual aperture inside of a
lens, shot through the back of a 35mm film camera. For each shot, the shutter speed and the
ISO setting were kept constant in order to demonstrate the effect of the aperture on the
exposure of this shot.

figure_override figure_override

figure_override figure_override figure_override

Figure 5



As you can see, a larger lens opening (represented by a smaller f-stop number) lets in more
light, which causes the image to be lighter. A smaller aperture lets in less light, which causes
the image to be darker. In this example, the third aperture setting (f/8) resulted in the most
accurate overall exposure for this particular lighting situation, shutter speed, and ISO setting.
At this point, it is important to remember that the shutter speed and ISO settings were kept
unaltered throughout. As we will soon demonstrate, each of these other exposure settings
play an equal role in determining exactly how much light is recorded.

Exploring Depth of Field
Every exposure setting (aperture, shutter speed, and ISO) has its own unique side effect,
which can be used by the photographer to achieve specific creative goals. The main side
effect of aperture is depth of field.

Depth of field is a photographic term that relates to how many things remain in focus along
an imaginary line drawn from the lens into infinity. The wider the aperture or the bigger the
lens opening, the less will be in focus. The smaller the lens opening, the more will remain in
focus across a longer distance range.

For example, imagine that you are photographing a portrait and the model is in front of a
distant mountain landscape. With an aperture setting of f/2.8, you either have the model or
the mountain in focus, but not both at the same time. However, with an aperture setting of
f/16 (or the smallest lens opening), both the model and the background will be in focus.

Here's another example (figure 6). This image
of a branch set against a distant background
of foliage is a composite of two shots taken at
different aperture extremes. The left portion
of this comparison shows the result of using
f/2.8 as the aperture setting. This wide
aperture setting caused the foreground to
sharp, while the background was rendered
completely out of focus. This is known as a
"shallow" or "limited" depth of field.

On the right, an aperture setting of f/22 was
used. There the background is almost as
Figure 6
sharp as the foreground. This is referred to as
"long" or "great" depth of field.

Another important thing to notice in the example above is the shutter speed setting, which is
dramatically different for the f/2.8 and the f/16 portions of the comparison. This will make
more sense when we discuss the shutter speed later on in this lesson.

For now, just keep in mind that by closing down the aperture from f/2.8 to f/16, you
significantly reduce the amount of light coming in through the lens. In order to keep the
overall exposure the same for both images, you have to compensate by using a longer
shutter speed. In other words, you have to let the light in for a longer duration in order to
have the same amount of light recorded on the digital sensor.



The image on the right (figure 7) is a classic
example of great depth of field being used to
keep the foreground and background
completely sharp. In this case, an aperture
setting of f/16 was used to maintain sharp
focus from the closest plants to the farthest
pine trees.

Figure 7

This next image (figure 8) is an example of
shallow depth of field. Here the photographer
used an aperture of f/2.8, while carefully
focusing on the single flower in the
foreground. This resulted in a completely
blurry background, which helps to isolate the
subject. If a smaller aperture had been used,
the flower might have been lost amidst a very
busy background. Using shallow depth of
field is a great way to keep the background
elements simple.
Figure 8

Before we move on, let’s review the key points that you need to understand about aperture.

The aperture is an adjustable opening inside of the lens, which controls or limits the
amount of light entering through the lens and reaching the film of digital sensor.
The aperture setting on the camera consists of a series of f-stops.
The larger the f-stop number, the smaller the opening in the lens.
The smaller the opening in the lens, the less light can come in and the darker the image
will be.
Depth of field is a side effect of aperture. The smaller the lens opening is, the greater
the depth of field.

The Shutter Speed
While the aperture setting is responsible for limiting the amount of light entering through the
lens, the shutter speed is responsible for limiting the length of time that the film or digital
sensor will be exposed to that light. Shutter speed is the second factor that helps to control
the total quantity of recordable light. In essence, the shutter speed is a direct counter-
balance to the aperture.

Just like apertures, shutter speeds are measured in full stop increments. Here is a list of


standard shutter speeds (measured in seconds or fractions of a second).

[BULB, 1 sec, ½, ¼, 1/8, 1/15, 1/30, 1/60, 1/125, 1/250, 1/500, and 1/1000]

It is worth noting that most modern cameras will allow you to set the shutter speed in third
stop increments. For the sake of keeping things simple, we will only concentrate on full stops
for this lesson.

Many cameras will also allow shutter speeds as fast as 1/4000th of a second and/or as slow
as 30 seconds. Most if not all SLR cameras offer an alternative shutter speed setting called
BULB. When the camera is set to BULB, the shutter will stay open as long as the shutter
release is pressed. This makes it possible for much longer, manually-timed shutter speeds
that can be in excess of several minutes or even hours in length.

As you can see from the list of standard, full-stop shutter speeds listed above, each
consecutive shutter speed (from left to right) is half the time of the preceding speed. 1/250th
of a second is half the time or half the amount of light reaching the sensor as 1/125th of a
second. We will explore this concept in more detail later.

In its basic form, the mechanism that makes the shutter speed work consists of two opaque
curtains which move in front of the sensor exposing it to light. When the shutter release is
pressed, the mirror pops up and out of the way. Then, the first curtain moves to the right,
revealing the sensor and exposing it. To end the exposure, the second curtain follows the
first and covers the sensor. Then the mirror can come back down and into position. When
shutter speeds are faster than 1/15th of a second or so, the second curtain will actually begin
moving before the first curtain has reached the end. This causes a small window or slit to
move laterally across the film. The shorter the exposure time, the narrower this window will
be.

Keep in mind that this description is based on a much older style of shutter found in older
35mm film cameras. Modern cameras feature more advanced versions of essentially the
same process.

The series of images shown below (figure 9) illustrate how the shutter speed affects the
exposure. For each of these examples, the aperture and the ISO settings have been kept
constant. The window below each image illustrates how the curtain shutter mechanism
moves across at different speeds.

figure_override figure_override

figure_override figure_override figure_override

Figure 9



From this series of illustrations, you should be able to understand that as the shutter speed
gets shorter, less light is allowed to reach the sensor. Less light results in a darker image.

In addition to controlling the length of the exposure and the total amount of light reaching the
sensor, the shutter speed is also responsible for either freezing movement or blurring
movement. Just as depth of field is a side effect of aperture, motion blur is a side effect of
shutter speed. This is where your creative decisions come into play.

When the shutter speed is slow (i.e. 1 full second), the shutter is open for a relatively long
time. During this long exposure, anything that is moving (subject and/or camera) will cause
motion blur in the image. If the exposure is really short (i.e. 1/1000th of a second) whatever
is moving will inevitably become frozen. It is almost like taking a thin slice out of time.

In some situations you might prefer to freeze the moving subject. Other times, you might
want to use a slow shutter speed to add motion blur to your shot as an aesthetic element.
Perhaps the most important point to remember is that the shutter speed must be fast enough
to freeze any camera shake that can occur by the movement of your hands and body while
holding the camera. If a fast enough shutter speed is not possible, then the camera must be
kept still using a tripod, monopod, or other camera support.

Rule of Thumb: To prevent motion blur due to camera shake when hand holding the
camera, make sure that the shutter speed is greater than the focal length of the lens. For
example, if you are using a 60mm lens, make sure that the shutter speed is 1/60th of a
second or faster to prevent camera shake. If your lens is a 25mm wide-angle lens, then you
can use a shutter speed as slow as 1/30th, or if you have really steady hands, maybe even
1/15th of a second.

Here is an example of intentional motion blur
(figure 10). The amount of available light in
this situation was extremely low, which
required a very slow shutter speed of about
15 seconds. The camera was mounted to a
tripod in order to prevent any motion blur due
to camera shake. During the 15-second
exposure, a large truck with bright headlights
drove past the camera. The truck was
rendered as several long streaks of light,
while the other parts of the image -- the road
and landscape in the background -- remained
Figure 10 absolutely sharp.

This next shot of a dog frozen in mid-stride is
an example of how a fast shutter speed can be
used to freeze action. Here, the photographer
used a shutter speed of 1/2000th of a second,
which resulted in every element, including the
tiny droplets of water splashing around the
dog, to be recorded as absolutely still and

tack sharp.

Figure 11

Keep in mind that in order to use a super fast shutter speed like 1/2000th of a second, there
must be enough available light to record a proper exposure in such a short amount of time.
In this example, the bright midday sun offered plenty of light to warrant the use of a fast
shutter speed. Whereas in the previous example taken at night, there was not nearly enough
light to use such a fast shutter speed.

The ISO Setting
The third factor that directly affects exposure is the ISO setting. ISO is an acronym for the
International Standards Organization. In relation to the photographic process, ISO is a
measurement of film or digital sensor sensitivity. In other words, the ISO rating tells us how
sensitive a film or a digital sensor is to light. Therefore, the ISO rating determines how much
light is needed for an accurate exposure.

In the days of film, every roll of film came with its own ISO (or ASA) rating, expressed in
full stop increments as follows.

[25, 50, 100, 200, 400, 800, 1600, and 3200]

The higher the ISO, the more sensitive the film is to light. An ISO 200 film is twice as
sensitive (or 1 stop more sensitive) to light as an ISO 100 film and requires 1 stop less light
to achieve the same exposure.

The same principle applies to digital photography, with one major difference. The sensitivity
of a digital sensor is adjustable. This means that the same range of ISO ratings can be dialed
in as a setting right in the camera. No longer is it necessary to match the appropriate film to
any given lighting situation. Now we have the entire range of film speeds built in to the
camera and accessible on demand.

The side effect of ISO is grain or digital noise. The higher the ISO setting, the more noise is
present in the image. Digital noise lowers the overall clarity of the image. This can be a really
tough compromise, especially in situations where both a fast shutter speed and high image
quality are important.

The illustration below (figure 12) compares two extremely different ISO settings. Here, we
can see the effect of these settings on this composite shot of several, fast-moving billiard
balls.



In this scene, the light level was relatively low,
as it would be in a pool hall type of setting.
ISO 100 is a relatively low ISO setting,
which makes the sensor less sensitive to light.
This means that more light or a slower shutter
speed is needed for an accurate exposure.
The result in this case, however, is motion
blur.

When the camera is set to ISO 1600, less
light is needed for the same exposure, which
allows the use of a faster shutter speed. This
helps to freeze the billiard balls that are in
Figure 12
motion. However, notice that the higher ISO
setting here also caused more noise in the
image, which significantly obscures the fine
details in the subject and lowers the clarity of
the image. (Click on the image for an
enlarged view to see this effect more clearly.)

Before we move on to see how all three exposure factors, aperture, shutter speed, and ISO,
are used in relation to one another, let’s recap some of the most important information
we’ve covered so far. Here is a list of points that you should be clear on before proceeding
to the next portion of this tutorial.

Exposure is a process for controlling the amount of light that is allowed to record on
the digital sensor.
The main objective of making an exposure is to record just the right amount of light so
that the image is neither too dark (underexposed), nor too light (overexposed).
The aperture setting controls the amount of light coming in through the lens.
The shutter speed setting controls how long the sensor is exposed to the light coming
in through the lens.
The ISO setting controls how sensitive the digital sensor is to light.

With all of this in mind, one might ask: "How is it possible to know which shutter speed and
aperture settings to use for any given situation? Surely, you can’t just pick random exposure
settings out of a hat! There must be some method for arriving at an accurate exposure
without having to do a ton of trial and error!"

The Light Meter
Fortunately there is such a method. In the next and final portion of this lesson we will tie all
of this information together by exploring the most essential tool - aside from the camera -
that every photographer must know how to use: the light meter.

A light meter is simply that - a device that measures the amount of light in a scene. Light
meters come in several varieties and usually have several modes of operation. In general,
there are two main categories of light meters: incident and reflective. Incident light meters
measure light that is falling onto the subject, while reflective meters measure light that is
reflecting from the surface of a subject. Within these two categories, there are many different
types of light meters. Some are separate devices and some are built into the camera. Some
measure ambient or available light and some measure flash or studio strobe lights

measure ambient or available light and some measure flash or studio strobe lights.

In this lesson, we will concentrate on the type of light meter that is built into the camera.
Every modern SLR has a built in light meter, which is an essential tool for determining
proper exposure.

How does a light meter work? What exactly does it do?
Essentially, a light meter measures the strength of light and determines an appropriate shutter
speed and aperture combination based on the ISO setting selected. It is then up to you, the
photographer, to decide whether to use that specific aperture and shutter speed combination
suggested by the meter, or to adjust these settings to tailor to your own creative vision.

All light meters are calibrated to read the light as middle gray. Whether the subject is all
white, all black, or a mix of different tones, the light meter will average everything together
and offer up an exposure that will render the metered area(s) as a middle gray tone. This
might sound confusing at first, but we will clarify this point with some examples below.

Most modern digital SLR cameras offer several metering modes. The most common
metering modes are described and illustrated in the diagram below (figure 13).

Figure 13

The best way to explain how the light meter reading translates into an actual exposure is with
the following experiment. After reading through the next section, we recommend setting up a
i il i tf lf t if hi th lt

similar experiment for yourself to see if you can achieve the same results.

As previously mentioned, the light meter is calibrated to read everything it sees as a middle
gray tone. To demonstrate this, we positioned our model, Mary, in an ordinary scene with
flat, overcast lighting. We asked Mary to hold three 8x10 cards: a white card, a black card,
and a Kodak 18% gray card.

Note: The Kodak 18% gray card is an inexpensive light metering tool, which can be
purchased at virtually any camera shop. It is called the 18% gray card because it reflects
18% of the light falling on it. In terms of light reflectance this is considered a perfect, middle
gray. Therefore, this gray card can be used as a metering guide in conjunction with a light
meter to arrive at accurate exposures in any lighting condition. We strongly recommend
getting your hands on one, as it will definitely come in handy in more situations than you
might realize.

To use the built-in meter in your camera, press the shutter release down halfway. This will
activate the meter and you will see the current shutter speed and aperture settings displayed
inside the viewfinder. Next to the shutter speed and aperture is an exposure indicator, which
usually has markings ranging from -3 to 0 to +3. Adjust the aperture or shutter speed until
the indicator reads 0. This is the exposure that the light meter deems accurate for that
particular amount of light.

When metering, remember that the ISO setting will also affect what the light meter will deem
accurate because the ISO setting determines how much light the sensor actually needs.
Remember to avoid shutter speeds that are slower than 1/30th of a second (depending on
the lens, as was discussed earlier) in order to prevent motion blur.

For the following examples, we used the Center-Weighed metering mode in order to limit
the metering area to just the center of the viewfinder. This mode excludes any light values
around the edges of the frame. We set the camera to ISO 200 and kept the aperture at its
widest setting of f/2.8.

First, we asked Mary to hold an 8x10" inch white card. To take the meter reading, the
photographer stepped in close, almost filling the frame with the white card. Then he
proceeded to meter the card (figure 14). The light meter suggested 1/500th of a second as
the correct shutter speed.

Remember that the light meter is designed to read everything as middle gray. The result,
then, is underexposure. 1/500th of a second at f/2.8 with ISO 200 rendered the white card
as a nice middle gray tone or about two stops darker than it actually was in real life. And of
course, this caused all of the other tones in the image to be two stops darker as well (figure
15).



Figure 14 Figure 15

Next, we repeated the same process using a black card (figure 16). This time, the meter
suggested an exposure of 1/30th of a second (same aperture, same ISO setting).

The result, as you might guess, is overexposure. The meter performed its function flawlessly,
turning the tone of the black card into a middle gray tone, which in turn recorded all of the
other tones in the image two stops lighter than they should be (figure 17).

Figure 16 Figure 17

Then, we conducted the experiment once more, this time using a middle gray card. The light
meter suggested 1/125th of a second as the shutter speed (figure 18). This resulted in an
accurately exposed image, with all tones rendered as close as possible to how they are
perceived in real life (figure 19).



Figure 18 Figure 19

With this concept of metering in mind, the next step is learning how to visually perceive
middle gray in whatever subject matter you are photographing, and then take your light
meter readings from that. For example, green grass is generally a good middle gray. Let’s
say you are photographing a big white house surrounded by a lush green lawn. If you take
the meter reading from the house, you will render the house as a middle gray and cause the
image to be underexposed. If you meter off the green grass, then you will be more likely to
achieve an accurate exposure.

This is primarily why an Auto exposure mode cannot always guarantee optimal exposures.
Of course, modern digital cameras feature highly advanced metering algorithms, which can
average the house and the grass together and arrive at a fairly accurate exposure.
Nevertheless, if the scene is predominantly dark or light, the camera will not intuit your
intention is to keep it that way and will try to compensate by making the image unnecessarily
lighter or darker.

Tying it All Together
Now that we’ve covered the basic exposure mechanisms of a camera, which work together
to record light onto a digital sensor or a piece of film, let’s consider how we can apply these
techniques to achieve a desired creative outcome in our images. As you already know now,
the shutter speed, aperture, and ISO settings work together to control the exposure. And
the light meter can measure a given lighting condition and offer up a combination of shutter
speed and aperture settings to use with a given ISO setting.

There is no such thing as a single "correct" exposure in any given situation. In fact, there are
always many different possible combinations of settings, which can produce the same
exposure. However, as you are already aware, each of the settings (aperture, shutter speed
and ISO) has their unique set of limitations and side effects. The combination of settings you
end up selecting is ideally driven by the look you want to achieve for your photograph.

For example, let’s say you metered a scene made up of predominantly green foliage. With
the ISO set to 200, the light meter suggested an exposure of 1/500th of a second at f/5.6,
which turned out to be an accurate exposure.

Based on this reading, we automatically know that the following list of aperture and shutter
speed combinations will produce an identical exposure with an ISO setting of 200:

1/30 @ f/22

1/30 @ f/22
1/60 @ f/16
1/125 @ f/11
1/250 @ f/8
1/500 @ f/5.6
1/1000 @ f/4
1/2000 @ f/2.8

Remember that each change in shutter speed or aperture represents either half or twice the
amount of light. This is called a full stop increment. So if we change the shutter speed from
1/500 to 1/2000, then we are letting in two stops less light. To compensate for this, we
would need to open the aperture two stops from f/5.6 to f/2.8.

If we were to change the ISO setting from 200 to 100, then we would be making the digital
sensor one stop less sensitive. This would mean that we would need one stop more light to
produce the same exposure. Therefore 1/500 @ f/5.6 with ISO 200 is equal to 1/250 @
f/5.6 with ISO 100.

At this point, the preferred combination becomes creative decision. If your priority is to
freeze action, then 1/2000 @ f/2.8 is your best bet. If you want to maximize depth of field,
then go with 1/30th @ f/22.

Here are a couple more examples of how exposure decisions are made with purely creative
goals in mind.

The image below (figure 20) was taken about 1 hour after sunset. The light was rapidly
escaping. In fact, when this image was taken, there was barely any light left in the sky at all.
Believe it or not, this is often the best time to photograph. The last trace of light after sunset
can be extremely unique and surreal.

Because of the dim lighting conditions, an
extra long shutter speed had to be used in
order to record the tiny bit of light that was
illuminating this landscape. The camera was
mounted to a tripod to prevent motion blur.
The ISO setting was set to 100 to ensure
maximum image quality. The aperture was set
to f/16 to ensure that the cliff wall in the
foreground and the cliffs in the distant
background would both be in focus. After
taking a few light meter readings from
different surfaces in the composition, the
photographer was able to calculate a shutter
Figure 20
speed of 4 minutes, which required the use of
the BULB setting and a stopwatch (figure
20).

In this case, the photographer could have altered the ISO setting and the aperture setting to
gain as many as 10 stops of extra light. However, this would have compromised his creative
vision for this image. The severe motion blur, caused by the long exposure, turned the
crashing waves into a soft mist, while the tripod ensured that everything else in the shot that

was motionless would be rendered sharp.

Here is a different example, which shows how
to handle low light situations without using a
tripod. This dog portrait was also shot after
sunset with a limited amount of available light.
To pull this off, the photographer used a high
ISO setting of 1600. At ISO 1600, the light
meter suggested an exposure of 1/60th @
f/2.8, just fast enough to hand hold the camera
and prevent any motion blur. Of course, the
side effect of using such a high ISO setting is
digital noise and a reduction in image clarity.
However, for this image, the compromise was
worth it and the extra noise, in our opinion,
Figure 21
adds a kind of impressionistic feeling that
seems to work well for this photograph.

These examples make it clear that photography is really a world of compromises. The
camera does not see things the same way that the human eye perceives. Sometimes, the
camera can see things that we can’t perceive. Most of the time, it is the photographer’s job
to manipulate the camera into seeing things a specific way. This is essentially the foundation
of creative image-making.

Hopefully, you can see why the autopilot settings on your camera are really not as smart as
they may appear. When you set your camera to the Program mode, for example, the
computer in the camera will use the built-in light meter to determine optimal shutter speed
and aperture settings based on averages and “common scenarios” that have been
preprogrammed by the manufacturer. In reality, the camera does not know whether you
want shallow depth of field, great depth of field, frozen action, or motion blur. The only
“computer” that can make these decisions is your brain.

In conclusion, we suggest reviewing the topics in this lesson a few times. When it comes to
manual exposure, practice is the only way to improve your proficiency. So, if you truly want
to take photography into your own hands, and develop your own personal vision, then you
must, at all costs, avoid the Program mode whenever possible. With enough practice, you
will be able to manually set the aperture and shutter speed with more accuracy and more
vision than any digital SLR can do on its own.

And remember to practice and have fun! That’s what photography is all about!


Understanding exposure

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