Automatic focus (AF) does the focusing for you. In the simplest designs, you push
the shutter-release button and the lens snaps the image into focus. The camera
adjusts the lens to focus sharply on whatever object is at the center of the viewfinder
within the focusing brackets. This type of autofocus works well in situations where
the main subject is—and stays—in the middle of the picture. The camera will display
a confirmation light when it has focused, but the light does not assure that the
picture will be sharp overall.
If your subject is not within the focusing brackets, you can use autofocus lock to
make it sharp:
1. Frame the subject within the focusing brackets.
2. To temporarily lock in the focus, press the shutter release button halfway down.
3. Keeping the shutter partially pressed, reframe the scene (move the subject out
of the center), then press the shutter release button all the way down.
Wide-area focus systems provide more elaborate electronics. The viewfinder displays
several focusing brackets. By rotating a dial or thumbwheel on the camera, you select a bracket that covers the subject you want to be sharp. When you press the shutter,the camera focuses on the selected area. This feature allows you to maintain your framing without autofocus locking and reframing each picture. It also allows you toshoot fast-moving subjects that are not in the center of the frame
the shutter-release button and the lens snaps the image into focus. The camera
adjusts the lens to focus sharply on whatever object is at the center of the viewfinder
within the focusing brackets. This type of autofocus works well in situations where
the main subject is—and stays—in the middle of the picture. The camera will display
a confirmation light when it has focused, but the light does not assure that the
picture will be sharp overall.
If your subject is not within the focusing brackets, you can use autofocus lock to
make it sharp:
1. Frame the subject within the focusing brackets.
2. To temporarily lock in the focus, press the shutter release button halfway down.
3. Keeping the shutter partially pressed, reframe the scene (move the subject out
of the center), then press the shutter release button all the way down.
Wide-area focus systems provide more elaborate electronics. The viewfinder displays
several focusing brackets. By rotating a dial or thumbwheel on the camera, you select a bracket that covers the subject you want to be sharp. When you press the shutter,the camera focuses on the selected area. This feature allows you to maintain your framing without autofocus locking and reframing each picture. It also allows you toshoot fast-moving subjects that are not in the center of the frame
Some wide-area autofocus systems use the light reflected off your eyeball to determine
where you are looking in the viewfinder, and therefore what aspect of the picture
to bring into sharp focus. Other systems automatically select the nearest subject
in the picture and focus there.
where you are looking in the viewfinder, and therefore what aspect of the picture
to bring into sharp focus. Other systems automatically select the nearest subject
in the picture and focus there.
Types of Autofocus
Your camera might let you select among manual, single-shot, and continuous focus.
With single-shot autofocus, sometimes called focus priority, the camera will not let
you take a picture until it has focused. After the lens locks in the focus, you must
either expose the picture or let up on the shutter-release button. This assures you of
a high percentage of sharp pictures, but can prevent you from capturing a critical
moment.
With continuous focus, the camera constantly searches for the correct focus when
you partially depress the shutter button. The camera revises its focus as the subject
moves closer or farther away. Continuous focus allows you to make an exposure
whenever you like. This approach works well for shooting action but does not guarantee
a sharp picture every time.
With single-shot autofocus, sometimes called focus priority, the camera will not let
you take a picture until it has focused. After the lens locks in the focus, you must
either expose the picture or let up on the shutter-release button. This assures you of
a high percentage of sharp pictures, but can prevent you from capturing a critical
moment.
With continuous focus, the camera constantly searches for the correct focus when
you partially depress the shutter button. The camera revises its focus as the subject
moves closer or farther away. Continuous focus allows you to make an exposure
whenever you like. This approach works well for shooting action but does not guarantee
a sharp picture every time.
After you lock onto a subject, some cameras predict where the subject
is likely to be next, keeping the subject in focus even if it moves
across the frame. These focus tracking systems can
lock onto a subject, adjusting the focus as the
subject moves closer to, or farther from, the camera.
The system works especially well if the subject,
like a race car, is traveling at a constant
speed toward or away from the camera.
Some cameras have two autofocus systems.
Active autofocus sends out a beam of red light
that the camera uses to measure the distance to
the subject. Passive autofocus uses the contrast in the
scene to determine when the subject is sharp.
Autofocus systems are not infallible. Active autofocus
is limited to subjects fairly close to the camera.
Passive autofocus is limited by the brightness and
contrast of the subject. A camera might fail to focus
or miss the focus if a subject has very low contrast,
is in very dim light, or consists of a repetitive pattern
like window blinds or a complex pattern, like
a plaid. Some cameras employ both systems for
maximum sharpness under most conditions.
Read your camera’s instructions to know how its
autofocus mechanism operates and when you
would be better off focusing manually.
is likely to be next, keeping the subject in focus even if it moves
across the frame. These focus tracking systems can
lock onto a subject, adjusting the focus as the
subject moves closer to, or farther from, the camera.
The system works especially well if the subject,
like a race car, is traveling at a constant
speed toward or away from the camera.
Some cameras have two autofocus systems.
Active autofocus sends out a beam of red light
that the camera uses to measure the distance to
the subject. Passive autofocus uses the contrast in the
scene to determine when the subject is sharp.
Autofocus systems are not infallible. Active autofocus
is limited to subjects fairly close to the camera.
Passive autofocus is limited by the brightness and
contrast of the subject. A camera might fail to focus
or miss the focus if a subject has very low contrast,
is in very dim light, or consists of a repetitive pattern
like window blinds or a complex pattern, like
a plaid. Some cameras employ both systems for
maximum sharpness under most conditions.
Read your camera’s instructions to know how its
autofocus mechanism operates and when you
would be better off focusing manually.
Center-Weighted Autofocus Lock
Autofocus cameras often focus on the center of a scene. This can make an off-center
main subject out of focus if it is at a different distance from whatever is at the center. To focus properly on the subject of your picture if he/she/it is not in
the center of the image:
1. In autofocus mode, first focus by placing the autofocus brackets on the main
subject. With many cameras, you partially press the shutter button down.
Keep partial pressure on the release to lock focus (Figure 4.10).
2. Reframe your picture (see Figure 4.11) while keeping partial pressure on the
shutter release.
3. Push the shutter button all the way down to make the exposure.
Autofocus cameras often focus on the center of a scene. This can make an off-center
main subject out of focus if it is at a different distance from whatever is at the center. To focus properly on the subject of your picture if he/she/it is not in
the center of the image:
1. In autofocus mode, first focus by placing the autofocus brackets on the main
subject. With many cameras, you partially press the shutter button down.
Keep partial pressure on the release to lock focus (Figure 4.10).
2. Reframe your picture (see Figure 4.11) while keeping partial pressure on the
shutter release.
3. Push the shutter button all the way down to make the exposure.
Focus and Depth of Field
What exactly is sharpness, and how much can it be controlled? In theory, a lens can
only focus on a single distance at a time (the plane of critical focus) and objects at
all other distances will be less sharp. However, in most cases, part of the scene will
be acceptably sharp both in front of and behind the most sharply focused plane.
Objects will gradually become more and more out of focus the farther they are from
the most sharply focused area.
Depth of field is the part of a scene that appears
acceptably sharp in a photograph. Depth of field can
be shallow, with only a narrow band across the
scene appearing to be sharp, or it can be deep, with
everything sharp from nearest to farthest. To a
large extent, you can control how much of it will
be sharp. There are no definite endings to the
depth of field; objects gradually change from sharp
to soft the farther they are from the focused distance.
What exactly is sharpness, and how much can it be controlled? In theory, a lens can
only focus on a single distance at a time (the plane of critical focus) and objects at
all other distances will be less sharp. However, in most cases, part of the scene will
be acceptably sharp both in front of and behind the most sharply focused plane.
Objects will gradually become more and more out of focus the farther they are from
the most sharply focused area.
Depth of field is the part of a scene that appears
acceptably sharp in a photograph. Depth of field can
be shallow, with only a narrow band across the
scene appearing to be sharp, or it can be deep, with
everything sharp from nearest to farthest. To a
large extent, you can control how much of it will
be sharp. There are no definite endings to the
depth of field; objects gradually change from sharp
to soft the farther they are from the focused distance.
Controlling Depth of Field
Evaluating and controlling depth of field is more
important in some situations than in others:
■ If you are relatively far from the subject, the
depth of field (the distance between the nearest and farthest points in a scene
that appear sharp in a photograph) will be greater than if you are up close.
Evaluating and controlling depth of field is more
important in some situations than in others:
■ If you are relatively far from the subject, the
depth of field (the distance between the nearest and farthest points in a scene
that appear sharp in a photograph) will be greater than if you are up close.
■ If you are using a short focal-length lens, you will have more depth of field
than with a long lens.
■ If the important parts of the scene are more or less on the same plane left to
right, they are all likely to appear sharp as long as you have focused on one
of them.
But when you photograph a scene up close, with a long lens, or with important
parts of the subject both near and far, you might want to increase the depth of field
so that parts of the scene in front of and behind the point on which you focused will
also be sharp
than with a long lens.
■ If the important parts of the scene are more or less on the same plane left to
right, they are all likely to appear sharp as long as you have focused on one
of them.
But when you photograph a scene up close, with a long lens, or with important
parts of the subject both near and far, you might want to increase the depth of field
so that parts of the scene in front of and behind the point on which you focused will
also be sharp
Sometimes, though, you will want to blur a distracting background that draws
attention from the main subject. You can accomplish this by decreasing the depth of
field.
You can use the aperture to control depth of
field. To increase the depth of field so that more
of a scene in front of and behind your subject is
sharp, setting the lens to a small aperture is
almost always the first choice. Select f/16 or f/22,
for example, instead of f/2.8 or f/4. You might
have to use a slower shutter speed to maintain
the same exposure, however. A slow shutter
speed can be a problem if you are photographing
moving objects or shooting in low light.
attention from the main subject. You can accomplish this by decreasing the depth of
field.
You can use the aperture to control depth of
field. To increase the depth of field so that more
of a scene in front of and behind your subject is
sharp, setting the lens to a small aperture is
almost always the first choice. Select f/16 or f/22,
for example, instead of f/2.8 or f/4. You might
have to use a slower shutter speed to maintain
the same exposure, however. A slow shutter
speed can be a problem if you are photographing
moving objects or shooting in low light.
To decrease the depth of field and make less of the scene in front of and behind the
subject sharp, use a wider aperture, such as f/2.8 or f/4.
There are other ways to control depth of field. You can increase the depth of field by
changing to a shorter focal-length lens or stepping back from the subject, although
both of those choices will change the picture in other ways as well.
To decrease the depth of field and make less of the scene in front of and behind your
subject sharp, you can use a longer lens or move closer to the subject. These alternatives
will also change the composition of the picture.
subject sharp, use a wider aperture, such as f/2.8 or f/4.
There are other ways to control depth of field. You can increase the depth of field by
changing to a shorter focal-length lens or stepping back from the subject, although
both of those choices will change the picture in other ways as well.
To decrease the depth of field and make less of the scene in front of and behind your
subject sharp, you can use a longer lens or move closer to the subject. These alternatives
will also change the composition of the picture.
Lens Focal Length, Aperture, and Light
Why does a lens of longer focal length produce less depth of field than a shorter lens
at the same f-stop (aperture)? The answer relates to the diameter of the aperture
opening. The relative aperture (the same f-stop setting for lenses of different focal
lengths) is a larger opening on a longer lens than it is on a shorter lens
Why does a lens of longer focal length produce less depth of field than a shorter lens
at the same f-stop (aperture)? The answer relates to the diameter of the aperture
opening. The relative aperture (the same f-stop setting for lenses of different focal
lengths) is a larger opening on a longer lens than it is on a shorter lens
Both lenses were set at f/4, let in the same amount of light, even though the actual
opening in the 50mm lens is physically smaller than the opening in the longer
300mm lens.
The longer the focal length, the less light that reaches the film (or CCD chip on a
digital camera), therefore a long lens will form a dimmer image than a short lens
unless more light is admitted by the aperture.
The sizes of the aperture openings are determined so that at a given f-stop number
the same amount of light reaches the film, no matter what the focal length of the
lens. The f-stop number, also called the relative aperture, equals the focal length of
the lens divided by the aperture diameter.
opening in the 50mm lens is physically smaller than the opening in the longer
300mm lens.
The longer the focal length, the less light that reaches the film (or CCD chip on a
digital camera), therefore a long lens will form a dimmer image than a short lens
unless more light is admitted by the aperture.
The sizes of the aperture openings are determined so that at a given f-stop number
the same amount of light reaches the film, no matter what the focal length of the
lens. The f-stop number, also called the relative aperture, equals the focal length of
the lens divided by the aperture diameter.
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