A Beginner’s Guide to Focusing: How Camera Autofocus Works

There’s a lot more to focusing than many novice photographers realize. When they delve into it, it can seem complex. However, it is easy to master once you have found your way around your camera.

There is much to take in, so I have divided the topic into two articles. This one will give you a brief and simplified understanding of how autofocus works, and the next will dive into which modes and methods you can use for differing circumstances.

Although I can give you pointers on what to look for, there is an enormous diversity between brands. Even variations in models from the same manufacturer make it impossible to write a definitive guide telling you which button to press, dial to turn, or menu position to visit, let alone the terminology the various brands use. Consequently, this is a generalization. Furthermore, I have grossly simplified some of it. However, it will provide you with a basic understanding of how your camera works and which focusing methods work best for you.

As you read this, I urge you to have your camera and its manual nearby. The instructions that come with cameras are notoriously bad, but they will highlight which button to press, dial to turn, or submenu to select so that you can find the relevant functions.

Focus Points

Focus points are the areas in your camera’s viewfinder or LCD where it can lock focus. The number available varies from camera to camera. Some older basic models may have three or five in the center of the viewfinder, while newer versions have dozens. Top-of-the-range models offer over one thousand that cover the entire frame.

Different cameras have different methods for activating and switching between focus points. Some use a joystick, while others require pressing a button. Meanwhile, some work by using the touch screen, and a 23few will give you a choice of all those methods.

A Single autofocus point allows you to select one spot in the frame manually. You place this point where you want to focus, such as on a subject’s eye. It also allows you to shoot past items in the foreground.

While it is an accurate way of choosing exactly where you want the camera to focus, it becomes difficult to use if you are attempting to align it with fast-moving subjects. In that case, using a group of focus points is more helpful. Meanwhile, you can also allow the camera to choose from all available points, in which case it will focus on whatever is closest to the camera.

On older and more basic cameras, the center point is recommended for precision, as it can be the most sensitive. With newer and more advanced cameras, all the focus points are likely equal in their ability to focus.

Types of Autofocus

Generally, DSLRs use two different types of focusing methods: phase detection and contrast detection. Meanwhile, some newer mirrorless cameras may use a combination of both.

Phase Detection Autofocus

Imagine cutting a photograph of, say, a fence post horizontally across the middle. You can slide the top and bottom parts in different directions, and the top half of the post won’t line up with its bottom. But you can position them so the top will be in phase with the bottom.

That is a very simplified analogy of how phase detection autofocus works. If the subject were not in focus, the camera would see the two halves of the subject misaligned. Furthermore, depending upon the direction of the misalignment, it knows whether the lens was focusing too near or too far.

Can you see the problem there? It’s all very fine for trying to marry up the two halves of a vertical subject. However, if you have a photograph of a wire running horizontally across the picture, it would be much more challenging to line that up unless, instead, you cut the photo from top to bottom.

The significant advantage of phase detection autofocus is its speed, as it knows whether it is focusing too close or too far, utilizing that misalignment.

On older, basic cameras, some phase detection focus points were available in either horizontal or vertical orientations. Each type would be better suited to subjects that ran perpendicular to it. In other words, a horizontal point would be great for focusing on that post, while a vertical point would work better on that wire.

Nevertheless, as cameras became more advanced, cross-type focus points were introduced. Initially, that was only in the center of the frame, but newer and more advanced cameras included more of them, and now all focus points can be cross-type, allowing far more flexibility.

Contrast Detection Autofocus

When your camera’s lens is out of focus, it loses contrast: whites, blacks, and all the tones in between become a mush. As the camera comes into focus, the contrast becomes more evident until you can differentiate the subjects from each other. That is how a DSLR focuses when you use the live view screen. It is more accurate but slower, as it doesn’t know whether it is focused too near or too far; it is just out of focus. It will also really struggle in low contrast situations, such as at dusk or in fog.

The technology has moved on. Contemporary higher-end models combine both systems at once with hybrid autofocus systems. They offer both speed and accuracy.

Moreover, while face and eye detection have been around for a long time, newer cameras have AI-based functions that will identify and focus on other subjects, such as birds, animals, and vehicles, and immediately lock focus onto them.

In Conclusion

That gives you a brief overview of autofocus. It’s worth looking up the type of autofocus your camera has. Find out how to move between different focus points and how to change the number of active points.

In the second article, we will explore the different autofocusing methods and consider which work best in various situations. We will also explore the hyperfocal distance, which will help you get the maximum depth of field in your photos.