Auto Focusing – Something Sounds Automatic but Not Really

Auto Focusing – Something Sounds Automatic but Not Really

You may ask, ”What? I have never thought about there are skills in auto focusing!” But the fact is that auto focusing is also a chapter you have to learn. DSLR have much shallower depth of field by comparing to point-and-shoot, which means not everything would be sharp. You need to tell the camera where to focus. How to make the camera focus that point, how to focus a moving object, etc are what you need to know. You must have the correct focus to get a sharp image. And that is why I am saying that auto focusing is not really too automatic.

Focus Points

Before talking about different auto focusing modes, let’s dig into what is focus point first. Focus points are some tiny squares and dots you can see when you look into the viewfinder. Entry level DSLR cameras normally have 9 to 11 focus points which generally fulfill the basic needs. A professional level DSLR camera, such as Nikon D4, has complex AF system with lots of focus points. Nikon D4 have 51 focus points.

auto focusing compare

The number of focus points is important because they allow you to focus on a particular area of an image. They are more accurate in tracking subjects. It is extremely useful in wildlife and sport photography. Focus points are also used in spot metering. Head back to my tutorial on metering mode for more detail.

Factor Affecting Auto Focusing

Auto focus of a camera relies on light that passes through the lens. In low light condition, you may find that your camera fails to focus. Auto focus is a detection of contrast. You will also have difficulty in focusing plain colour subject or low contrast stuff. To improve auto focusing, you can simply plug a flash onto the hot shoe and turn on “AF Assist” function. This function helps the camera to focus by changing the contrast detection into infra-red focusing mode. The speedlight will emit infra-red to the subject. While the infra-red bounce back, the flash will know how far is the subject is and tell the camera where to focus. This, however, only works great for stationary subject within 15 to 20 feet.

Focus Mode

AF Mode Selector on Nikon D700Nowadays cameras have several focus mode equipped to tackle various situations. Focusing a still object, a running man or a flying bird requires different focus modes. You only have to focus once for a still object. However, it is different as a running man is always changing the distance between the camera and himself.

Single Area Focus Mode

Single area focus mode is known as “AF-S” in Nikon family while “One Shot AF” in Canon family. In this mode, the camera focuses once when you half-pressed the shutter or press the dedicated AF button (not all model have this button). The focus is locked while you hold half-pressing the shutter. In the meantime if the subject moves around, the focus would not change. You are required to focus successfully before release the shutter.

Continuous / AI Servo Focus Mode

“Continuous/ AF-C” and “AI Servo” are the names Nikon and Canon gave this mode respectively. It is great to track a moving object topic, such as sport, flying birds and other non-stationary stuff. By analyzing the subject movement and predicting where the subject will be, the camera can place the focus at the predicted point. This mode is great because it automatically readjusts the focus for a moving subject while you continuously half-press the shutter or hold the AF button. In the professional DSLR model, this mode is highly configurable, such as choosing how many focus points to use in tracking.

Single / Continuous Hybrid Mode

This is designed for beginners. “AF-A” is Nikon’s name while “AI Focus AF” is Canon’s. This is basically a hybrid which automatically switches between single and continuous. The camera will first detect if the subject is stationary or moving, then choose the corresponding focusing mode. It works well for most situations but I suggest you not to choose this mode because the response is slower than the other two modes.

AF-Area Selection

AF Area selector on Nikon D700Many DSLR have the “AF-area selection” which makes thing confusing. This is not focusing modes which tell the camera how to focus. AF-Area selection is used to tell the camera “where to focus”. Entry and middle grade DSLR normally provide a AF-area selection in the camera menu while professional level cameras, such as Nikon D700, D3s, D300s, have a dedicated selector on the back.

Single-Point AF-Area

Again, Canon and Nikon give different names to the selection which means the same thing. They are “Manual AF Point” and “Single Point” respectively. You can understand this easily by the name. The camera uses only one focus point which you choose to focus. I usually use single point in portrait, macro and still life shooting which require a more accurate focus.

Dynamic AF-Area

You still choose one focus point in “dynamic”(Nikon) or “AF point expansion”(Canon) selection. The difference is that when the subject moves around, the camera automatically utilizes the surrounding focus points to track the subject and keep it in focus. You have to set how many surrounding focus point to be used and therefore you are required to track the subject by panning the camera and make sure the subject is close to the initial chosen AF point. This selection is great for photographing fast moving objects like flying birds , running people, etc. Nikon has introduced 3D-tracking to new models. In 3D-tracking, the camera can track the subject even when it moves away from the initial chosen point significantly. 3D-tracking somehow is more responsive than traditional dynamic AF.

Auto AF Area

The “Auto-Area AF” in Nikon and “Automatic AF Point Selection” in Canon refer to the point-and-shoot method. This mode is fully automatic and now most new camera models are optimized this with face detection function. If no face is detected, it would change the focus to the largest object in the viewfinder. This mode is great to capture landscape in order to save time for focus point selection.

To conclude, the most confusing in auto focusing is the different between focus mode and AF-Area selection. Once you have made them clear, you can handle auto focusing great and would never miss the shot. Again, please share this post to your friend by clicking the share buttons down below. Next topic is about composition.