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| in collaboration with Alan
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The depth of field defines the
zone in which all elements show clearly from foreground to
background. The depth of field in an image is controlled
by three factors: the distance to the subject, the focal
length, and the aperture used to capture the image.
This article is an attempt at explaining a subject which
appears simple superficially, but is in fact quite complex,
and one for which explanation
often comes through the guise of formulae and equations. We thought we
would approach the topic in more general terms, and avoid peripheral ideas
such as depth of focus, the hyperfocal point of a lens and optical concepts
such as circles of confusion. Instead, we seek to explain the observable
difference between the depth of field of a digital camera that uses a small-size
CCD, and others that use larger imagers or film.
Before anything else, it is necessary to understand that the term
depth of field defines a somewhat "elastic" concept: acceptable sharpness.
The perception of sharpness can differ from one individual to another,
and when the expression depth of field is used, it really denotes that
area in an image that exhibits a sufficient sharpness to be considered
more or less in focus. The depth of field doesn't have be one of precise
sharpness, but instead that portion of the image in which things — or
people — remain recognizable for what they are.
For users of compact digital cameras, depth of field is a subject
of special interest because depth of field is more difficult to
control with a compact
digital camera than with earlier film cameras. The small imaging sensors
of compact cameras require the use of short focal lengths, and this in
turn gives these cameras an unusually long depth of field when compared
to 35mm cameras. Thus, intentionally getting a shallow depth of field is
more difficult.
The influence of subject distance on depth of field:
As a rule, depth of field decreases as the subject gets closer
to the camera. Meaning that as the point of focus gets closer to
the lens, the possible
extent of the depth of field diminishes. On the other hand, if the subject
is far enough away from the camera — and for compact digital cameras,
this need not be very far — the depth of field extends out to infinity.
Focal length and focal plane:
To understand the relationship between the focal length of a lens
and the perceived depth of field, some terms need to be defined.
The graphic below,
(based on a graphic © Olympus America), illustrates the meaning of
focal length and focal plane: |
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Focal length describes the length of the lens, as
measured in millimetres, from the focus point in the lens,
to the focal plane.
The focal plane is the area — in this case the area of the
sensor — that needs to be covered by the circle of light focused by
the lens.
The physical dimensions of the lens are determined by the need to cover
the entire focal plane. As the diagonal of the focal plane represents
its longest dimension, the diameter of the circle of light created on the
focal plane by the lens must exceed the diagonal measurement of
the focal plane. |
| Therefore, in the case of a 1/2-inch
sensor, to ensure complete coverage of the sensor, the circle
of light focused by the lens must be larger than the diagonal
of the sensor when it arrives at the focal plane. With a
35mm camera, that same coverage must be even bigger since
the area to be imaged is larger — hence the need for
a bigger and longer lens. |
As a corollary, the longer the lens' focal
length, the shorter the possible depth of field will be.
In other words, as the focal length of the lens increases,
the depth of field for any given aperture, becomes progressively
shorter as long as the position of the lens, relative to
the subject, remains unchanged. The reverse also holds true.
As the focal length gets shorter, the possible depth of field
becomes progressively longer.
Furthermore, the depth of field is usually uneven divided between the front
and the back of the focus point. For shallow depths of field, the division
is almost equal, with the far depth being a bit greater. When the near
depth extends one quarter of the way toward the camera, the far depth will
extend twice that depth behind the focus point. By the time the near depth
of field has reached half the distance to the camera, the far depth has
gone out to infinity and everything beyond the focus point will be in focus.
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 The
relationship between aperture and depth of field
Not only does the physical length of the lens impact the depth of field,
the depth of field is also impacted by the aperture used to capture
the image. The aperture of the lens is the opening that controls how much
light gets to the sensor. In a camera with a variable aperture, a mechanism
called a diaphragm, or iris, is used to restrict the amount of light that
travels through the lens. The diaphragm provides control over the exposure,
as well as the depth of field. |
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The diaphragm can be made as simple covers with holes that
provide different apertures, or composed of blades that are
adjusted incrementally to provide varying apertures.
The illustration below shows the relationship of aperture to depth of field:
(Note that the f-numbers shown below are just examples of the relationship,
and do not precisely correspond to the lens apertures shown.) |
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So, as the f-number representing the aperture
increases, for example progressing from f2 to f11, the
depth of field increases — which provides an easy
way to remember the relationship:
small f-numbers = less depth of field,
big f-numbers = more depth of field.
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The influence of focal length
on depth of field
As mentioned previously, short focal lengths have an intrinsically long
depth of field. Since compact digital cameras use short focal lengths,
obtaining a long depth of field — where elements from foreground to
background are clearly recognizable — isn't a problem, but obtaining
photos with a shallow depth of field is more difficult. In fact,
in wide angle (the shortest focal length), and at distances of more than
3 or 4 metres (9 to 12 feet) the depth of field of a compact digital is
very long, and the aperture's primary role is more to control the amount
of light reaching the sensor and therefore the exposure, than the depth
of field. Unlike with a 35mm, for the compact digital the subject needs
to be closer to the camera for the influence of the aperture to become
observable.
This is demonstrated in the images below, both of the same subject, photographed
from approximately 1 metre (3 feet) away at f1.8. The lenses are set so
as to have a similar focal length as is evidenced by the width of field. |
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This first photo is captured with a compact digital that
uses a 1/2-inch CCD. The lens' aperture is set to f1.8,
the maximum possible with the camera.
As the background is a relatively short distance behind the subject,
the large aperture only blurs it slightly. Details such as the pine needles
on the ground at the base of and behind the flower remain visible, and
readily recognizable.
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This second image is captured with a digital SLR camera
equipped with a much larger CCD which allows it to use standard
35mm lenses. As can be seen, the longer lens means that the
depth of field at f1.8 is much shorter.
The pine needles in the background are not recognizable as such, and the
outline of the rocks is very soft. |
If a long depth of field is "natural" for
the compact digital, it also means that purposefully obtaining
a shallow depth of field is bound to be more difficult.
Controlling the depth of field with a compact digital camera
Control over the depth of field means being able to select whether the
image will have a long, or a short depth of field. With most compact digital
cameras, this can be quite challenging, and in some cases almost impossible.
Yet, a short depth of field is very useful for some types of photos, particularly
portraits. With a portrait, the photographer generally wants to make the
subject appear detached from the background, as the latter can be distracting.
One way to mitigate — to some extent — the long depth of field
of a compact digital and obtain the appearance of a shallower depth of
field, is to place the subject at least a few meters in front of the background,
and use the optical zoom to frame the subject. This technique places
the background on the outer edge of the depth of field. |
The image at right, and the
one below make use of these two factors wile highlighting
the different depth of field created by the aperture.
The rhododendron in the background is approximately 3 metres (9 feet) behind
the zebra, and the zebra itself is about one metre (3 feet) from the camera.
By zooming a bit, and selecting a wide aperture of f2.2, the background
becomes suitably blurred. |
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The same subject captured
with an aperture of f11 shows the background clearly, and
the difference between the two images illustrates the advantage
of a shallow depth of field for portraiture. With a clearly
defined background the zebra's head looses its dominance,
as the busy background detracts attention.
Often, the use of the zoom is more important as a means to frame the subject
and avoid any potential wide angle distortion. |
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Although increasing the focal
length used for the image can also be seen as a means to
decrease the depth of field, much depend on the particular
camera used. While some will have an aperture range that
is sufficient to provide control over the depth of field,
others will exhibit little difference between an image captured
at f2.8 and one shot at f8.
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