302.101
Figure 4-14.–When the lens is focused at 20 feet and set at f/22, the
depth of field ranges from about 10 feet to infinity
in sharp focus. In figure 4-14, when the lens is set at f/22
and focused at infinity, the depth of field ranges from
about 20 feet (the hyperfocal distance) to infinity;
however, when you change the lens focus to 20 feet, the
depth of field ranges from about 10 feet to infinity.
The lenses of modern SLR cameras stay at their
maximum aperture until the shutter is tripped. These
lenses provide a bright image in the viewfinder to focus.
As a result, when you look through the viewfinder, you
only see the depth of field for the maximum aperture and
not the working f/stop. Most SLR cameras have a
depth-of-field preview button to compensate for this.
When you press it, the aperture closes down to the set
f/stop. Although the viewfinder becomes darker, you
can see the actual depth of field at the selected aperture.
Image Sharpness
The outer edges of a lens are least likely to produce
a well-defined or aberration-free image; therefore,
proper use of the diaphragm, aperture, or f/stop can
improve image sharpness by blocking off light rays that
would otherwise pass through the outside edges of a
lens.
There is a limit to how far the aperture can be
stopped down and still increase image sharpness. When
the aperture is very small, it causes diffraction of light
rays striking the edge of the diaphragm. Diffraction
results in a loss of image sharpness. This loss of image
sharpness is especially noticeable in copy work
Physical limitations in the design of lenses make it
impossible to manufacture a lens of uniform quality
from the center to the edges; therefore, to obtain the best
quality with most lenses, you can eliminate the edges of
the lens from being used by closing down the aperture
about two f/stops from wide open This recommended
adjustment is called the optimum or critical aperture.
The optimum aperture for a particular lens refers to the
f/stop that renders the best image definition.
When a lens is stopped down below the optimum
aperture, there is an actual decrease in overall image
sharpness due to diffraction. Although the depth of field
increases when a lens is stopped down below the
optimum aperture, image sharpness decreases;
therefore, increased depth of field should not be
confused with image sharpness. For example, the image
formed by a pinhole camera has extraordinary depth of
field but lacks image sharpness. When the lens aperture
is closed down to the size of a pinhole, it behaves like
one. This is an important factor for subjects in a flat
plane (such as copying) where depth of field is not
needed.
SHUTTER
A camera shutter controls both the exact instant
when the film is exposed to light and the duration of that
exposure. The shutter is used in conjunction with the
diaphragm to control the exposure of the film. The most
important function of the shutter is that it limits the time
that light is allowed to pass through the lens and act on
the film. There are two types of camera shutters: leaf
and focal plane.
Leaf Shutter
The blades of this type of shutter are usually located
between or near the lens elements and close to the
diaphragm. It is sometimes called a between-the-lens
shutter; however, a more correct term for this type of
shutter is a leaf or diaphragm shutter.
Leaf shutters have several blades made of thin
spring steel. When the shutter is closed, these blades, or
leaves, are at rest and overlap each other. This prevents
light from reaching the film. When the shutter release
button is pressed, the blades move apart or open quickly
and allow light to pass and expose the film. They remain
open for the duration of the preset exposure time before
springing shut again (fig. 4-15).
4-12
