Autofocus is that great time saver that is found in one
form or another on most cameras today. In most cases, it helps
improve the quality of the pictures we take.
In this edition of HowStuffWorks,
you will learn about the two most common forms of autofocus,
and find out how to determine which type of autofocus your
camera uses. You will also learn some valuable tips about
preventing the main causes of blurred pictures when using an
What is Autofocus?
really could be called power-focus, as it often uses a
computer to run a miniature motor that
focuses the lens for you. Focusing is the moving of the lens
in and out until the sharpest possible image of the subject is
projected onto the film.
Depending on the distance of the subject from the camera, the
lens has to be a certain distance from the film to form a
In most modern cameras, autofocus is one of a suite of
automatic features that work together to make picture-taking
as easy as possible. These features include:
- Automatic film advance
- Automatic flash
- Automatic exposure
There are two types of autofocus systems: active and
passive. Some cameras may have a combination of both
types, depending on the price of the camera. In general, less
expensive point-and-shoot cameras use an active system, while
more expensive SLR
(single-lens reflex) cameras with interchangeable lenses use
the passive system.
In 1986, the Polaroid
Corporation used a form of sound navigation ranging (SONAR),
like a submarine
uses underwater, to bounce a sound wave off the
subject. The Polaroid camera used an ultra-high-frequency
sound emitter and then listened for the echo (see How Radar
Works for details). The Polaroid Spectra and later SX-70
models computed the amount of time it took for the reflected
ultrasonic sound wave to reach the camera and then adjusted
the lens position accordingly. This use of sound has its
limitations -- for example, if you try taking a picture from
inside a tour bus with the windows closed, the sound waves
will bounce off of the window instead of the subject and so
focus the lens incorrectly.
This Polaroid system is a classic active system. It
is called "active" because the camera emits something (in this
case, sound waves) in order to detect the distance of the
subject from the camera.
Active autofocus on today's cameras uses an infrared
signal instead of sound waves, and is great for subjects
within 20 feet (6 m) or so of the camera. Infrared systems use
a variety of techniques to judge the distance. Typical systems
For example, this
patent describes a system that reflects an infrared pulse
off the subject and looks at the intensity of the reflected
light to judge the distance. Infrared is active because
the autofocus system is always sending out invisible infrared
light energy in pulses when in focus mode.
- Amount of infrared light reflected from the
It is not hard to imagine a system in which the camera
sends out pulses of infrared light just like the Polaroid
camera sends out pulses of sound. The subject reflects the
invisible infrared light back to the camera, and the camera's
computes the time difference between the time the outbound
infrared light pulses are sent and the inbound infrared pulses
are received. Using this difference, the microprocessor
circuit tells the focus motor which way to move the lens and
how far to move it. This focus process repeats over and over
while the camera user presses the shutter release button down
half-way. The only difference between this system and the
ultrasound system is the speed of the pulse. Ultrasound waves
move at hundreds of miles per hour, while infrared waves move
at hundreds of thousands of miles per second.
Infrared sensing can have problems. For example:
- A source of infrared light from an open flame (birthday
cake candles, for instance) can confuse the infrared sensor.
- A black subject surface may absorb the outbound infrared
- The infrared beam can bounce off of something in front
of the subject rather than making it to the subject.
One advantage of an active autofocus system is that it
works in the dark, making flash photography much
On any camera using an infrared system, you can see both
the infrared emitter and the receiver on the front of the
camera, normally near the viewfinder.
To use infrared focusing effectively, be sure the emitter
and the sensor have a clear path to and from your subject, and
are not blocked by a nearby fence or bars at a zoo cage. If
your subject is not exactly in the middle, the beam can go
right past the subject and bounce off an undesired subject in
the distance, so be sure the subject is centered. Very
bright subjects or bright lights can make it difficult for the
camera to "see" the reflected infrared beam -- avoid these
subjects when possible.
patent, and this
patent each show a different form of infrared sensing.
commonly found on single-lens reflex (SLR) autofocus cameras,
determines the distance to the subject by computer
analysis of the image itself. The camera actually looks at
the scene and drives the lens back and forth searching for the
A typical autofocus sensor is a charge-coupled
device (CCD) that provides input to algorithms that
compute the contrast of the actual picture elements. The CCD
is typically a single strip of 100 or 200 pixels. Light from
the scene hits this strip and the microprocessor looks at the
values from each pixel. The following images help you
understand what the camera sees:
The microprocessor in the camera looks at the strip of
pixels and looks at the difference in intensity among the
adjacent pixels. If the scene is out of focus, adjacent pixels
have very similar intensities. The microprocessor moves the
lens, looks at the CCD's pixels again and sees if the
difference in intensity between adjacent pixels improved or
got worse. The microprocessor then searches for the point
where there is maximum intensity difference between
adjacent pixels -- that's the point of best focus. Look at the
difference in the pixels in the two red boxes above: In the
upper box, the difference in intensity between adjacent pixels
is very slight, while in the bottom box it is much greater.
That is what the microprocessor is looking for as it drives
the lens back and forth.
Passive autofocus must have light and image contrast
in order to do its job. The image needs to have some detail in
it that provides contrast. If you try to take a picture of a
blank wall or a large object of uniform color, the camera
cannot compare adjacent pixels so it cannot focus.
There is no distance-to-subject limitation with passive
autofocus like there is with the infrared beam of an active
autofocus system. Passive autofocus also works fine through a
window, since the system "sees" the subject through the window
just like you do.
Passive autofocus systems usually react to vertical
detail. When you hold the camera in the horizontal
position, the passive autofocus system will have a hard time
with a boat on the horizon but no problem with a flagpole or
any other vertical detail. If you are holding the camera in
the usual horizontal mode, focus on the vertical edge of the
face. If you are holding the camera in the vertical mode,
focus on a horizontal detail.
Newer, more expensive camera designs have combinations of
vertical and horizontal sensors to solve this problem. But
it's still the camera user's job to keep the camera's sensors
from being confused on objects of uniform color.
You can see how much area your camera's autofocus sensors
cover by looking through the viewfinder at a small picture or
a light switch on a blank wall. Move the camera from left to
right and see at which point the autofocus system becomes
How Do I Know Which Autofocus System My Camera
Look at the type of camera you have:
Here's a quick
test to tell which autofocus system is in use in your camera
(some cameras may have both systems):
- If it is an under-$50 point-and-shoot camera or one of
the single-use, disposable cameras, it is definitely a
fixed-focus camera with no focusing system of any
kind. This type of lens has its focus set at the factory,
and it typically works best with a subject distance of about
8 feet. Four feet is about as close as you can get to the
subject with a fixed-focus camera. When you look through a
fixed-focus camera, you typically do not see the square
brackets or circles found in an autofocus camera. However,
you may see a "flash ready" indicator.
cameras with interchangeable lenses typically use the
passive autofocus system.
- Cameras without interchangeable lenses typically use
active infrared, and you can see the emitter and the
sensor on the front of the camera.
- Go outdoors and aim the viewfinder at an area of the sky
with no clouds, power lines or tree limbs. Press the shutter
button halfway down.
- If you get a "focus okay" indication, it's an active
- If you get a "focus not okay" indication, it's a passive
autofocus system. The CCD cannot find any contrast in a blue
sky, so it gives up.
Is Autofocus Always Accurate and Faster?
is really up to the person using the camera to determine if
the subject is in focus. The camera merely assists you in
making this decision. The two main causes of blurred pictures
taken via autofocus cameras are:
- Mistakenly focusing on the background
- Moving the camera while pressing the shutter
has a fast autofocus! Try this simple experiment: Hold your
hand up near your face and focus on it, and then quickly look
at something past your hand in the distance. The distant item
will be clear, and your hand will not be as clear. Look back
at your hand. It will be clear, while out of the corner of
your eye the same distant item will not be as clear. Your
camera is not nearly this quick or this precise, so you often
have to help it.
Focus Lock: The Key to Great Autofocus
The camera user can often fool the
autofocus system. A pose of two people centered in the picture
may be unclear if the focus area (the area between the two
square brackets) is in the middle of the two people. Why? The
camera's autofocus system actually focuses on the landscape in
the background, which is what it "sees" between the two
The solution is to move your subjects off-center and use
the focus-lock feature of your camera. Typically, focus lock
works by depressing the shutter button part-way and holding it
while you compose the picture. The steps are:
- Compose the picture so that the subject is either in the
left third or the right third of the picture. (This makes
for pleasing pictures.) You will come back to this position.
- Move the camera right or left so the square brackets in
the center of the viewfinder are over the actual subject.
- Press and hold the shutter button halfway down so the
camera focuses on the subject. Keep your finger on the
- Slowly move your camera back to where you composed the
picture in step 1. Press (squeeze) the shutter button all
the way down. It may take some practice to do it right, but
the results will be great!
You may also use the above procedure in the vertical
direction, say when taking a picture with mountains or the
shore in the background.
When Should I Use Manual Focus?
focus rings are still available on most SLR cameras. When
taking a picture of an animal behind bars in a zoo, the
autofocus camera might focus on the cage bars instead of the
animal. On most consumer-grade autofocus cameras, use manual
- You have a zoom lens on an active autofocus camera, and
your subject is more than 25 feet away.
- You have a passive autofocus camera and the subject has
little or no detail, like a white shirt with no tie.
- You have a passive autofocus camera and the subject is
not well lit or very bright and more than 25 feet away.
Autofocus Video Cameras
Autofocus in a video
camera is a passive system that also uses the central
portion of the image. Though very convenient for fast
shooting, autofocus has some problems:
Autofocus video cameras work
best in bright light. Switch to manual focus in low light.
- It can be slow to respond.
- It may search back and forth, vainly seeking a subject
to focus on.
- It has trouble in low light levels.
- It mis-focuses when the intended subject is not in the
center of the image.
- It changes focus when something passes between the
subject and the lens.
How to "See" Infrared
With Your CamcorderYou can
sometimes "see" infrared via this simple
experiment, using a camcorder with a TV
monitor attached. Point the camera toward a TV remote
control. Push some buttons on the TV remote
control and the camera should "see" invisible
infrared light from the remote control. Camcorders
typically use CCD imaging chips. These chips are
sensitive to infrared light. That's why your camera
shows a white spot where the remote's infrared source is
located. A "spy" can take pictures in complete darkness
if they illuminate the scene with bright infrared light.
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