Depth
of field
As a group we were set the task to gather some information on websites as source of information for our magazine concept I looked at this website in particular and found out about the history of the depth of field.
Depth of field refers to the range of distance that appears
acceptably sharp. It varies depending on camera type, aperture and focusing
distance. 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.
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.
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 are 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 focusing distance 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:
FOCAL
LENGTH AND DEPTH OF FIELD
Focal length has not been listed as
influencing depth of field, contrary to popular belief. Even though telephoto
lenses appear to create a much shallower depth of field, this is mainly because
they are often used to magnify the subject when one is unable to get closer. If
the subject occupies the same fraction of the image (constant magnification)
for both a telephoto and a wide angle lens, This would of course require you to
either get much closer with a wide angle lens.
