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 article, 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 autofocus camera.
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 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:
- 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 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 beam.
- 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 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.
How Do I Know Which Autofocus System My Camera Has?
Look at the type of camera you have:
- 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.
- SLR 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.
Here's a quick test to tell which autofocus system is in use in your camera (some cameras may have both systems):
- 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 autofocus system.
- 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?
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:
- Mistakenly focusing on the background
- Moving the camera while pressing the shutter button
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.
Focus Lock: The Key to Great Autofocus Pictures
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:
- 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 button.
- 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?
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:
- 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:
- 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.
Autofocus video cameras work best in bright light. Switch to manual focus in low light.