Basic Photography Course

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

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Basic Photography Course

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DOFMaster for Windows On-line Depth of Field Calculator DOFMaster for Mobile Devices On-line Depth of Field Table Hyperfocal Distance Chart Articles FAQ Recommended Books Support Contact Links Home As an Amazon Associate I earn from qualifying purchases.	contents <- previous page next page -> 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 contents Basic Photography Course <- previous page next page ->	As an Amazon Associate I earn from qualifying purchases.
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