Auto focus in photography

One important technique to understand in photography, especially when you’re starting out, is the concept of focus. If you don’t focus properly, you will end up with blurry photos even when all your other camera settings are correct. Focusing can be easy or difficult depending on your subject, like a nonmoving landscape versus a fast-moving bird in flight. This guide covers everything you need to know in order to focus properly and capture sharp images.

What Is Autofocus?

Autofocus (AF) is the feature of a camera that tries to ensure that your chosen subject is sharp within the photo.  Sensors detect how far away the subject is from the camera, and this information is relayed to the lens, which then uses an electric motor to adjust the focal distance of the lens.  Most point and shoot cameras are autofocus only, but all DSLRs and mirrorless cameras have the option to disable AF if desired.

The first autofocus cameras were developed in the late 1970’s and their speed and accuracy has improved greatly since that time.  These days cameras have incredibly complex, dedicated AF sensors that analyze the scene through the viewfinder and predict what it is that you are trying to focus on.  Once they have locked onto a subject, focus can be maintained on a moving target which makes it particularly useful for sports photography.

Modern cameras use what is called passive AF, where the AF sensor analyzes the image coming in through the lens using one of two methods; contrast detection or phase detection. 

Autofocus Sensors

A camera's autofocus sensor(s) are the real engine behind achieving accurate focus, and are laid out in various arrays across your image's field of view. Each sensor measures relative focus by assessing changes in contrast at its respective point in the image — where maximal contrast is assumed to correspond to maximal sharpness.

The process of autofocusing generally works as follows:

(1) An autofocus processor (AFP) makes a small change in the focusing distance.

(2) AFP reads the AF sensor to assess whether and by how much focus has improved.

(3) Using the information from (2), the AFP sets the lens to a new focusing distance.

(4) The AFP may iteratively repeat steps 2-3 until satisfactory focus has been achieved.

This entire process is usually completed within a fraction of a second. For difficult subjects, the camera may fail to achieve satisfactory focus and will give up on repeating the above sequence, resulting in failed autofocus. This is the dreaded "focus hunting" scenario where the camera focuses back and forth repeatedly without achieving focus lock. This does not, however, mean that focus is not possible for the chosen subject.

Lens Quality and Autofocus

While manual focus works equally on most cameras and lenses, autofocus does not. The Autofocus quality is different on most cameras and lenses, and it’s often related to the product price.

If you compare Autofocus of a cheap camera and an expensive one, you may be surprised to see exactly how big the difference might be. In most cases, high quality cameras and lenses are able to focus faster and more accurately.

For a landscape photographer, the Autofocus speed is not especially important as most of the scenes we photograph aren’t moving quickly. Our cameras are on the tripod and we have the time to carefully compose each image. What is most important for us is the fine tuning of the focus. We want the image to be as sharp as possible and focus has an important role in this.

The “problem” with lenses that have an older Autofocus motor is that they aren’t always able to achieve the best focus. When viewing the image on a smartphone or small computer screen you might not see a huge difference but once you begin zooming in on the image you will see how certain areas, if not the entire picture, are out of focus and soft.

For wildlife photographers, or any other photographers that photograph moving subjects, a quick Autofocus is more important. When you’re photographing a lion running away, it’s essential that the focus is quick so that you’re able to capture that brief second.

When I was just getting started with photography a friend of mine taught me an important lesson that I tell others today: It’s better to invest in an expensive lens than an expensive camera. Lenses will stay with you for years but camera bodies will be changed more often. Lens technology advances more slowly than cameras.

Understanding Autofocus

Now that we have a basic understanding of how the lens works to focus the light onto the sensor or film, we can talk about the magic of autofocus. As technology advanced, camera companies figured out how to motorize the camera body and lenses to move the focusing elements or focusing group toward or away from the sensor or film. A vast majority of today’s cameras do not have autofocus motors inside the camera body, but rely on tiny motors built into the lenses, which are controlled from the camera itself.

Not really rocket science, right? But, how does the camera know when the subject is in focus? When we focus a lens manually, we look through a viewfinder or at an LCD screen and verify, with our eyes, if the subject looks sharp. Many viewfinders in the days of film had useful split-screen microprisms at the center that assisted with manual focusing. The autofocus camera needs to calculate focus electronically as the lens moves to and from the sensor or film. And, luckily for us, especially if you do not have perfect vision, it can now do this extremely fast and accurately.

Active versus Passive

You won’t see Active AF systems much these days, but let us give a nod to the technology. Active AF systems were around in the early days of autofocus technology and relied on the camera transmitting an ultrasonic or infrared signal toward the subject. The subject would reflect the sound or light back to the camera’s focus sensor and by crunching the time it took to receive the return versus the speed of sound or speed of light, the camera would know how far away the subject was. It actually sounds pretty cool and high tech, right? This is, basically, sonar and radar in a camera. Sonar and radar are cool. So is Active AF.

Before you get all excited about having pioneering technology on your camera, if you have what is known as an AF-assist lamp on your camera, its use is not an Active AF system—it merely augments lighting in a dark scene to assist the passive system.

Passive AF is the choice of the vast majority of today’s cameras. In the Passive AF world we have two different systems: Phase Detection and Contrast Detection. We will wrap up this intriguing article by describing how each system works, again, keeping it relatively simple.