Last week I covered autofocus for stationary subjects – today it’s all about AF for moving subjects!
Just a little recap below on how autofocus works and the different types…
Modern SLRs have a variety of focus options, but there are really only three focus options: single shot, continuous or manual.
Single shot AF is designed for stationary subjects. It is the default mode in which the camera focuses when you half-press the shutter down, and the focus remains locked for as long as you half-press the shutter. This allows you to recompose the image while keeping the point in focus. This was the mode I discussed last week.
The second option, continuous autofocus, is the best option for moving subjects. The camera continuously adjusts focus to track moving subjects even when the shutter is half-pressed. It also has a predictive tool that allows it to track the anticipated path of the moving object to ensure all This will be discussed in more depth in this article.
Many cameras also have an extra, Automatic AF mode. This is not a dedicated focusing system, but a mode where the camera selects whether to use single or continuous AF.
How Autofocus Works
Light passes through the lens, and is reflected off the mirror onto a focusing screen, where it can then be passed into the viewfinder, a little light bounces though the semi-silvered surface of the mirror, and is reflected into the AF module. Here, the dedicated AF sensor measures contrast in the image, working by the rinciple that a sharp image has higher contrast than an unsharp one. When the highest contrast ha been detected, the camera considers the scene to be in focus, and lets you know, usually by some indication on the screen, or even a beep.
Using Continuous AF
Press the AF button and turn the main dial to select the AF mode on Canon or Nikon, and choose AI Servo/ Continuous AF and choose your drive mode (for sport or action use high speed continuous bursts). If your camera or lens has optical stabilization, turn on ‘Panning mode’, or else the camera will try to stabilize all movement, including that resulting from purposeful movement such as panning.
If you have a sony, use the Fn button to access you quick shooting menu, and choose your AF setting from there.
In continuous AF mode, the camera should track a moving subject and aim to keep it in focus at all times. Of course, this does have it’s limitations, because if the background is cluttered the camera may become confused, and the number of AF points also plays a role in it’s success.
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