How Autofocus Cameras Work

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 image.

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:

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 might use:

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:

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.

This patent, this patent, and this patent each show a different form of infrared sensing.

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 focus.

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 confused.

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):

It 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:

Your eye 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.

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 people.

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:

You may also use the above procedure in the vertical direction, say when taking a picture with mountains or the shore in the background.

Manual 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 focus when:

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.