HYPERFOCAL DISTANCE.
­The hyperfocal
distance of a lens is the distance from the optical center
of the lens to the nearest point in acceptably sharp focus
when the lens, at a given f/stop, is focused at infinity. In
other words, when a lens is focused at infinity, the
distance from the lens beyond which all objects are
rendered in acceptably sharp focus is the hyperfocal
distance. For example, when a 155mm lens is set at f/2.8
and focused at infinity, objects from 572 feet to infinity
are in acceptably sharp focus. The hyperfocal distance
therefore is 572 feet.
The following equation is used to find hyperfocal
distance:
F
2
H =
f x C
Where:
H = hyperfocal distance
F = focal length of lens
f
= f/stop setting
C = diameter of circle of confusion
F and C must be in the
millimeters, and so forth.
same units, inches,
NOTE: 1 inch is equal to 25.4mm.
Where:
F = 155mm (6.1 inches)
f
= 2.8
C = 0.05 (0.002 inches)
Then:
H =
6 . 1
2
2.8 x 0.002 = 6650 inches = 554 feet
Thus the hyperfocal distance for this lens set at f/2.8 is
554 feet.
Hyperfocal distance depends on the focal length of
the lens, the f/stop being used, and the permissible circle
of confusion. Hyperfocal distance is needed to use the
maximum depth of field of a lens. To find the depth of
field, you must first determine the hyperfocal distance.
By focusing a lens at its hyperfocal distance, you cause
the depth of field to be about one half of the hyperfocal
distance to infinity.
ND = H x D
H + D
DEPTH OF FIELD.
­Depth of field is the distance
from the nearest point of acceptably sharp focus to the
farthest point of acceptably sharp focus of a scene being
photographed Because most subjects exist in more than
one plane and have depth, it is important in photography
to have an area in which more than just a narrow, vertical
plane appears sharp. Depth of field depends on the focal
length of a lens, the lens f/top, the distance at which the
lens is focused, and the size of the circle of confusion.
Depth of field is greater with a short-focal-length
lens than with a long-focal-length lens. It increases as
the lens opening or aperture is decreased. When a lens
is focused on a short distance, the depth of field is also
short. When the distance is increased, the depth of field
increases. For this reason, it is important to focus more
accurately for pictures of nearby objects than for
distance objects. Accurate focus is also essential when
using a large lens opening. When enlargements are made
from a negative, focusing must be extremely accurate
because any unsharpness in the negative is greatly
magnified.
When a lens is focused at infinity, the hyperfocal
distance of that lens is defined as the near limit of the
depth of field, while infinity is the far distance. When
the lens is focused on the hyperfocal distance, the depth
of field is from about one half of that distance to infinity.
Many photographers actually waste depth of field
without even realizing it. When you want MAXIMUM
depth of field in your pictures, focus your lens on the
hyperfocal distance for the f/stop being used, NOT on
your subject which of course would be farther away than
the hyperfocal distance. When this is done, depth of field
runs from about one half of the hyperfocal distance to
infinity.
There are many times when you want to know how
much depth of field can be obtained with a given f/stop.
The image in the camera viewing system may be too dim
to see when the lens is stopped down. Under these
conditions, some method other than sight must be used
to determine depth of field. Depth of field can be worked
out mathematically.
The distance, as measured from the lens, to the
nearest point that is acceptably sharp (the near distance)
is as follows:
The distance, as measured from the lens, to the farthest
point that is acceptably sharp (the far distance) is as
follows:
1-25

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