Understanding Autofocus Points and Modes
Possibly the most misunderstood part of photography is autofocus. Many of us have figured out what works best for a particular situation, but lack a more in-depth understanding of what is going on and what each autofocus mode does. In this tutorial we’ll look at what autofocus points are, and what the benefits of each autofocus mode is.
How Does a Camera Focus?
When you half-press the shutter (or AF-On button; they both do the same thing) your camera focuses the scene for you. But how it does this is actually quite interesting. Without going into too many specifics, the camera looks for contrast in the image to determine sharpness. If the area you’re trying to focus on appears to lack contrast the camera will refocus the lens and reanalyse the scene, adjusting this lens’ focus until it determines high contrast and a subsequently sharp image. This is known as ‘Passive Autofocus’.
This is why your camera may appear to search for focus, moving backwards and forwards through the different focal planes. It’s particularly apparent when you try to focus on a white wall or blurred surface, as the camera finds it difficult (or impossible) to pick up the contrasting areas.
‘Active Autofocus’ is where the camera emits a beam to bounce off the subject, measuring the distance between the two points. It can then focus accurately. However, few cameras use this nowadays due to its limitations (such as only being able to be used on something 15-20 feet away). You’re more likely to see this in use when working with a flashgun that has AF Assist enabled. So, there’s no need to worry about Active AF.
What is an AF Point?
Autofocus (AF) points are what you use to determine where the camera will be focusing the image. When you look through your viewfinder, these are the rectangles or circles that you see.
There are different types of AF points: vertical or horizontal, and cross-type. Vertical AF points detect contrast on vertical lines, whereas cross-type analyse both vertical and horizontal lines together. As a result, cross-type AF points are more accurate and have a greater ability to retain focus, but they are more expensive. They are present in greater numbers in more expensive DSLR cameras, which is part of the reason why AF systems improve with the cost of a camera.
You might hear photographers raving about the number of AF points in the latest flagship camera to hit the market. But why does this matter? Well, an increased number of focus points makes it easier to track and maintain focus on a moving subject. This is particular useful for us as nature photographers, as action photography is something we engage in often.
You can’t necessarily see all the AF points when you look through the viewfinder. Some of them are small and crushed between the larger points you can select between, increasing the accuracy of the system. You don’t need to be too concerned about them, but knowing they are there to help you is useful. The camera can only focus where there is an AF point, and cameras with fewer points will have a harder time achieving and maintaining accurate focus as the gaps between each point are larger. You could think of the areas of the scene not covered by AF points as ‘dead zones’.
Your camera will come with a couple of autofocus modes, most likely. These determine how the camera should behave whilst autofocusing. They have different names between different manufacturers, but they do the same thing. For Nikon users, you have AF-Single, AF-Continuous, and AF-Auto modes.
- AF-S will lock focus onto the subject once it has been achieved, for as long as you keep the shutter half-pressed. If you want to refocus, you’ll have to release and depress the shutter again. (Best for a stationary subject).
- AF-C will maintain focus of the subject, adjusting as the subject moves. (Best for a moving subject, like a bird in flight).
- AF-A mode can be found on some newer Nikon cameras, which changes automatically between the previous two modes mentioned.
For Canon users, AF-S is known as One-Shot; AF-C is known as AI-Servo.
The AF-area modes determine how the AF points should be used to achieve focus. This is where it gets more complicated, so I hope you’re still with me! There are three main AF-area modes. These are: single-point AF, dynamic area AF (aka. expansion / zone autofocus for Canon), and 3D tracking.
Single-point AF gives you pin-point accuracy, allowing you to select just one AF point. This means you can tell the camera precisely where you want it to focus. It’s more suitable for stationary or slow-moving subjects, as it’s difficult to keep it aligned with something moving quickly.
Dynamic area AF allows you to choose between 9, 21 and 51-point placements. With each different option, the selected number of AF points are used in combination to achieve focus. Some newer cameras, such as the Nikon D500, offer an even greater number of point placements to select from.
3D tracking will attempt to find and follow a moving subject for you, adjusting the AF point in use as it goes. This is a bit of a temperamental option, working best when there are strong differences in colour and contrast between the subject and the background. For example, focusing on a dark bird against a blank sky would work well, but tracking it with a busier background would be difficult.
Which AF-Area Mode Should You Use?
You’re probably now wondering which grouping of AF points you should use.
Well, for anything stationary or slow moving (like an animal slowly foraging around an area), you should be using single-point AF. This allows you to ensure the focus is on the eyes of your subject, thanks to the accuracy of using just one point.
If you’re looking to photograph something moving more quickly, like a bird in flight, then you’ll be better off with one of the dynamic modes. You can do it with single-point, but it is much harder to keep on target. I stick with 9 or 21 points in dynamic area AF mode, meaning that should a bird move out of the selected point, the surrounding 9 or 21 points and working to maintain focus. Using 51 points can be a little counter productive, as it then slows down the autofocus system and brings more opportunities to confuse the camera. It’s better to work on your panning technique and maintain the bird more centrally, than to use more points and wobbly panning.
Your AF-Lock setting, adjustable from your camera’s menu, determines how long the camera holds its lock onto a subject once it has lost it. I usually keep this between 0 and 3 on the scale. 0 means it will refocus immediately, and the greater the number means the longer the delay. A delay is useful should a bird fly behind a tree, as you don’t want the camera to then lock onto the tree as you pan across it. However, a delay can be problematic should the camera lock onto something incorrectly and you want it to refocus on the bird quickly. It’s all about a balance.
You can read more about this and photographing birds in flight in our tutorial.
Advanced Autofocus Techniques
So now you’ve read up on autofocus points and modes, you might be interested in trying out something a little more advanced. It’s possible you’ve heard wildlife photographers raving about ‘back button focus’. This is where the action of focusing the image is stripped from the shutter button, and instead assigned to the AF-On button on the back of your camera. This allows you to reap the benefits of all the focus modes (AF-S, AF-C and manual) at once.
To learn more about back button focusing, be sure to read our tutorial.
That’s potentially a lot of new or confusing information for you. The main thing to remember is that there’s no real ‘one size fits all’ option here, and knowing which modes to go for comes with experience. But take away the recommendations from this tutorial and put them into practice. You’ll soon see if they work for you.
Don’t forget that your panning technique will be a big player in whether or not your photo is in focus for moving subjects, and it may not be the fault of your autofocus mode choice. Here’s some extra reading for you:
- How to Photograph Birds in Flight
- How to Photograph Fast-moving Mammals
- Back Button Focus: A Game Changer