The three types
of focusing systems common in modern 35 mm cameras are fixed
focus, manual focus, and automatic focus. Fixed-focus lenses,
common to the simplest compact cameras, are preset to provide
sharp focus over a given distance range (typically from a
minimum of about 4 feet to infinity). Fixed-focus lenses
are fine for taking snapshots of family events or vacation
scenics, but they do limit your flexibility and image control.
The ability to focus your camera selectively allows you to
concentrate attention on a particular area of a scene.
Manual-Focusing Cameras
Many SLR cameras use manual focusing. To focus a manual SLR camera, you
look through the viewfinder while turning the focusing collar on the lens
until the area that you want to be sharp comes into focus. The viewfinder
in an SLR usually contains a split-image rangefinder, a microprism, and
a ground glass to help you in judging sharpest focus (see illustration).
A split-image rangefinder does what it says-it splits the image. With split-image
rangefinders, you look through the viewfinder at your subject and turn
the focusing ring on your camera lens until the split images in the viewfinder
line up. The subject is then in focus. Split-image finders work best when
there is a prominent vertical line (a telephone pole, for example) in your
composition to use as a focusing guide. One disadvantage of the split-image
rangefinder is that it sometimes blacks out when you use it with telephoto
lenses, obliterating part of the image area. If your camera has a microprism
area (see illustration) or a ground glass for focusing, you just turn the
focusing ring until the subject looks sharp in the viewfinder. The microprism
is a ring in the center of the viewfinder that exaggerates the unsharpness
of out-of-focus subjects and therefore makes correct focusing easier to
judge.
Each focusing aid works best in certain situations. For instance, you may
prefer split-image focusing when you're photographing a building because
there are straight lines to focus on. But if you're taking a picture of
a crowd of people where prominent lines are hard to find, the microprism
and ground-glass systems of focusing are easier and faster. In addition,
the ground glass offers the advantage of showing you what part of the picture
is in the plane of sharpest focus relative to other parts of the picture.
The ground glass also lets you preview depth of field over the entire image
area if your camera has the feature that permits this.
Practice focusing your camera until you do it so naturally that you
don't have to give it a second thought.
Auto-Focus Cameras
See Fig 1.2
Electrical contacts in the lens and camera enable the computer in an autofocus
SLR to set the autofocus and the aperture, and adjust shutter speed or
aperture according to the lens focal length.
Although the engineering behind an auto-focus camera is complex, using
one is easy. In both compact and SLR cameras, the viewfinder has a center
target to show what the camera will automatically focus on. To get sharp
pictures, aim your camera to superimpose this target on the main subject.
The obvious problem is that you won't always want to put your main subject
in the center of the frame. To circumvent this, most auto-focus cameras
have a focus lock. With it, you focus on your subject in the center of
the viewfinder and then lock the focus. Once you lock the focus, you
can recompose the scene.
Autofocus cameras focus on whatever is in the center of the viewfinder.
The background in this picture is sharp, the subject is unsharp.
Many autofocus cameras have a focus lock. Depressing the focus lock
will lock the focus of the camera.
Using the focus lock, focus on your subject, then... recompose your
picture and press the shutter release. Using this method, the 2 subjects
are now in focus.
With most auto-focus cameras, partially depressing the shutter button
activates the focus. As long as you do not release the button (or take
a picture) focus will remain locked on your chosen subject. Some cameras
have a separate button that locks the focus until a picture has been
taken. For the focus-lock technique to work, however, the camera-to-subject-distance
cannot change. If the distance does change, the subject will likely be
out of focus. Refocusing is a simple matter of re-centering your subject
and again partially pressing the shutter release button.
More sophisticated SLR auto-focus cameras may offer a choice of two
autofocus modes: single shot and servo. The single-shot mode is better
for photographing stationary subjects. The camera will not fire until
the camera has confirmed sharp focus. The servo mode works well for photographing
moving subjects-a girl on a bicycle, for instance. It will continue to
focus until the instant of exposure. One drawback of the servo mode is
that many cameras will fire even if the subject is not in sharp focus
(if the subject has continued to move as the shutter is tripped, for
example). Some very advanced SLR cameras are actually able to overcome
even this problem by measuring the speed of a moving subject. The camera
calculates how far the subject will move during the split second it takes
to make an exposure and adjusts the focus accordingly. Consult your manual
for advice on photographing moving subjects.
Some situations and subjects can cause focusing errors or failure. For
instance, the camera cannot automatically focus on subjects behind other
objects, such as a fence or a branch. If you are using a leafy tree limb
to frame a barn, you must be certain that the focus area in the viewfinder
is not seeing the limb or it will cause the lens to focus on the limb
and make your main subject, the barn, out of focus.
Because most auto-focus systems work by comparing the tonal contrasts
within a scene, they may balk in both low-light and low-contrast situations
such as a foggy scene.
Most cameras will beep or flash an indicator to indicate a focus problem.
In dimly lit situations, the camera may tell you to use an accessory
flash. Some flash units have infrared emitters that enable the camera
to focus automatically in almost total darkness, and the flash provides
enough light for picture taking. Brightly backlighted scenes also trouble
most SLR auto-focus systems because the contrast in the scene is intense
and changes rapidly if you move the camera even slightly.
In SLR auto-focus cameras, the sensors are behind the lens. As long
as you have an unobstructed view of your subject, the camera will focus.
In compact auto-focus cameras, however, the auto focus sensors are behind
windows on the front of the camera. It's very important not to block
these windows with a finger or camera strap, or focusing will be impaired.
Finally, some auto-focus SLR cameras have trouble focusing on subjects
that contain predominantly horizontal lines-clapboard shingles on a house,
for example. You can get around this difficulty by turning the camera
on a slight angle, locking in the focus, and then recomposing the scene.
Focus Assist
Most SLR auto-focus cameras have an override that allows you to focus
the lens manually. This can be an important feature if, for example,
you want to manipulate focus for a special effect or to focus on a subject
in a difficult situation-such as photographing a zoo animal through the
bars of a cage. Many of these cameras have a feature called "focus
assist" that will provide an audible or visible signal to tell when
the subject is in focus. |
