Understanding how autofocus cameras operate is key to capturing sharp and clear photos every time.

Autofocus is a time-saving feature found in many modern cameras, often improving the sharpness and quality of the photos we take.

In this article, we will cover the two most common types of autofocus and how to identify which one your camera uses. Plus, we'll offer some tips on avoiding the common causes of blurry photos when using autofocus.

What is Autofocus?

Autofocus (AF) could be better described as power-focus, utilizing a computer that controls a small motor to adjust the lens for you. This process involves moving the lens to find the clearest image of your subject, ensuring it is focused correctly on the film. The lens's position relative to the film depends on how far the subject is from the camera, which is necessary to create a sharp image.

In most modern cameras, autofocus is part of a broader set of automated features that simplify the photography process. These features are designed to make taking pictures as straightforward as possible, and they include:

Autofocus systems come in two types: active and passive. Some cameras feature a mix of both systems, often depending on the camera's cost. Generally, budget-friendly point-and-shoot models use an active system, while higher-end SLR (single-lens reflex) cameras with interchangeable lenses use the passive system.

Active Autofocus

In 1986, the Polaroid Corporation employed a version of sonar, much like a submarine uses underwater, to send a sound wave out and bounce it off the subject. The camera then listened for the echo. The Polaroid Spectra and later SX-70 models measured how long it took the ultrasonic sound wave to return and adjusted the lens based on that data. However, this system has limitations—if you're inside a tour bus with the windows closed, the sound waves could bounce off the glass instead of your subject, leading to incorrect focus.

This Polaroid system is an example of a active autofocus. It's called 'active' because the camera sends something (in this case, sound waves) to measure the distance to the subject.

Modern active autofocus cameras use an infrared signal rather than sound waves, making them effective for subjects within 20 feet (6 meters). Infrared autofocus systems assess the distance through various methods, including:

For instance, this patent outlines a system that reflects an infrared light pulse off a subject and measures the intensity of the reflected light to determine the distance. Infrared is considered active because the autofocus system continually emits invisible infrared light pulses whenever the camera is in focus mode.

Imagine a system where the camera emits infrared light pulses, much like the Polaroid camera uses sound pulses. The subject reflects the infrared light back to the camera, and the camera's microprocessor calculates the time difference between when the infrared pulses are sent and when they are received. Using this time difference, the microprocessor directs the focus motor on how far and in which direction to move the lens. This process continues as long as the shutter release button is pressed halfway. The only major difference between this system and the ultrasound system is the speed of the pulse. Ultrasound waves travel at hundreds of miles per hour, while infrared waves travel at hundreds of thousands of miles per second.

Infrared sensing does have its drawbacks. For example:

A key advantage of an active autofocus system is its ability to function in low-light conditions, making flash photography much more efficient and convenient.

On any camera equipped with an infrared system, both the infrared emitter and receiver are typically located on the front of the camera, usually near the viewfinder.

To use infrared focusing effectively, make sure that the emitter and sensor have an unobstructed line of sight to your subject. Avoid blocking the path with nearby fences or zoo cage bars. If the subject is not perfectly centered, the infrared beam might pass by the subject and reflect off something else in the distance, so always ensure the subject is centered. Very bright subjects or strong light sources can interfere with the camera's ability to detect the reflected infrared beam, so it's best to avoid these when possible.

This patent, this patent, and this patent each demonstrate a unique method of infrared sensing.

Passive Autofocus

Passive autofocus, which is commonly used in single-lens reflex (SLR) autofocus cameras, measures the subject's distance through computer analysis of the image itself. The camera analyzes the scene and moves the lens back and forth to find the sharpest focus.

A typical autofocus sensor is a charge-coupled device (CCD) that feeds data into algorithms which calculate the contrast of individual picture elements. The CCD usually consists of a single strip of 100 to 200 pixels. Light from the scene strikes this strip, and the microprocessor evaluates the pixel values. The following images will help illustrate what the camera detects:

The camera's microprocessor examines the pixel strip and evaluates the intensity differences between neighboring pixels. If the scene is blurry, adjacent pixels will have very similar intensities. The processor moves the lens, rechecks the CCD's pixels, and assesses whether the intensity difference between pixels has improved or worsened. It then searches for the point where there is the maximum intensity difference between adjacent pixels – which indicates the sharpest focus. For example, in the two red boxes shown above: the top box shows minimal intensity variation between adjacent pixels, while the bottom box shows a much higher difference. This is the focus point the microprocessor seeks as it adjusts the lens.

Passive autofocus requires both light and image contrast to function properly. The image needs to contain distinct details that offer contrast. If you try to focus on a blank wall or a large object of a uniform color, the camera won't be able to compare adjacent pixels, thus it can't focus.

Unlike active autofocus, passive autofocus doesn't have a subject distance limitation. It also works well through windows, as the system 'sees' the subject through the glass just as you would.

Passive autofocus systems typically respond better to vertical detail. When the camera is held horizontally, the autofocus system may struggle to focus on objects like a boat on the horizon but will have no trouble with vertical elements like a flagpole. In horizontal mode, focus on the vertical edge of a subject. When the camera is held vertically, focus on horizontal features.

More advanced, higher-end camera models feature both vertical and horizontal sensors to resolve this issue. However, it remains the responsibility of the photographer to prevent the camera's sensors from being confused by objects of a uniform color.

To understand the coverage of your camera's autofocus system, look through the viewfinder at a small image or a light switch on a blank wall. Move the camera horizontally and observe where the autofocus starts to lose track.

How Can I Identify Which Autofocus System My Camera Uses?

Examine the type of camera you own:

Here’s a simple test to identify the autofocus system in your camera (some cameras may have both types of autofocus):

Is Autofocus Always Reliable and Faster?

It’s ultimately up to the user to determine if the subject is in focus. The camera simply aids in making that decision. The two most common reasons for blurry images taken with autofocus cameras are:

Your eye is a fast autofocus system! Try this simple test: Hold your hand up close to your face and focus on it, then quickly shift your focus to something far away. The distant object will appear sharp, while your hand will not. When you look back at your hand, it will be sharp, while the distant object will blur out of your peripheral vision. Your camera is not as fast or precise as your eye, so it often needs your assistance.

Focus Lock: The Secret to Perfect Autofocus Shots

It’s possible to trick the autofocus system. If two people are positioned centrally in the frame, the focus area (the region between the two brackets) may fail to focus correctly. Why does this happen? The autofocus system may focus on the background, seeing the area between the subjects as the landscape.

The solution is to move your subjects slightly off-center and activate the focus-lock feature. Typically, the focus-lock works by pressing the shutter button halfway and holding it while you adjust the composition. The process goes as follows:

You can also apply the above technique in the vertical direction, for instance, when photographing mountains or a shoreline.

When Should I Rely on Manual Focus?

Manual focus rings are still found on most SLR cameras. If you're photographing an animal behind bars at a zoo, the autofocus might mistakenly focus on the bars instead of the animal. For most consumer-grade autofocus cameras, consider using manual focus in these cases:

Autofocus in Video Cameras

In a video camera, autofocus is a passive system that primarily works by focusing on the center of the image. While it’s convenient for quick shooting, autofocus in video cameras has a few drawbacks:

Autofocus video cameras work best in well-lit environments. When filming in low light, it's a good idea to switch to manual focus.