How does a camera focus?
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.
What is Autofocus?
Autofocus (AF) 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 clear
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
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
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
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
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 might use:
Amount of infrared light reflected from the subject
For example, this patent describes a system that reflects an
infrared pulse of light 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.
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 microprocessor 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 easier.
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.
Passive autofocus, 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 best
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 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
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 confused.