Depth of field refers to the range of distance that appears
acceptably sharp. It varies depending on camera type, aperture and
focusing distance, although print size and viewing distance can also
influence our perception of depth of field. This tutorial is designed to
give a better intuitive and technical understanding for photography,
and provides a
depth of field calculator to show how it varies with your camera settings.
The depth of field does not abruptly change from sharp to unsharp,
but instead occurs as a gradual transition. In fact, everything
immediately in front of or in back of the focusing distance begins to
lose sharpness — even if this is not perceived by our eyes or by the
resolution of the camera.
CIRCLE OF CONFUSION
Since there is no critical point of transition, a more rigorous term called the "
circle of confusion"
is used to define how much a point needs to be blurred in order to be
perceived as unsharp. When the circle of confusion becomes perceptible
to our eyes, this region is said to be outside the depth of field and
thus no longer "acceptably sharp." The circle of confusion above has
been exaggerated for clarity; in reality this would be only a tiny
fraction of the camera sensor's area.
When does the circle of confusion become perceptible to our eyes? An
acceptably sharp circle of confusion is loosely defined as one which
would go unnoticed when enlarged to a standard 8x10 inch print, and
observed from a standard viewing distance of about 1 foot.
At this viewing distance and print size, camera manufactures assume a
circle of confusion is negligible if no larger than 0.01 inches (when
enlarged). As a result, camera manufacturers use the 0.01 inch standard
when providing lens depth of field markers (shown below for f/22 on a
50mm lens). In reality, a person with 20-20 vision or better can
distinguish features 1/3 this size or smaller, and so the circle of
confusion has to be even smaller than this to achieve acceptable
sharpness throughout.
A different maximum circle of confusion also applies for each print
size and viewing distance combination. In the earlier example of blurred
dots, the circle of confusion is actually smaller than the resolution
of your screen for the two dots on either side of the focal point, and
so these are considered within the depth of field. Alternatively, the
depth of field can be based on when the circle of confusion becomes
larger than the size of your digital camera's pixels.
Note that depth of field only sets a maximum value for the circle of
confusion, and does not describe what happens to regions once they
become out of focus. These regions also called "bokeh," from Japanese
(pronounced bo-ké). Two images with identical depth of field may have
significantly different bokeh, as this depends on the shape of the lens
diaphragm. In reality, the circle of confusion is usually not actually a
circle, but is only approximated as such when it is very small. When it
becomes large, most lenses will render it as a polygonal shape with 5-8
sides.
CONTROLLING DEPTH OF FIELD
Although print size and viewing distance influence how large the circle of confusion
appears
to our eyes, aperture and focal distance are the two main factors that
determine how big the circle of confusion will be on your camera's
sensor. Larger apertures (smaller F-stop number) and closer focusing
distances produce a shallower depth of field. The following test
maintains the same focus distance, but changes the aperture setting:
note: images taken with a 200 mm lens (320 mm field of view on a 35 mm camera)
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