Today’s Question: My camera allows me to choose how many focus points I want to have available (1, 4, 8, etc.). When photographing a moving subject such as a flying bird, how many focus points to you recommend having active?
Tim’s Quick Answer: Generally speaking, with a moving subject my preference is to use a single focus point. I will then choose which focus point to select based on where in the frame I want the subject to appear. This enables me to focus where I want to, but also provides a “target” to help make sure I’m positioning the subject where I want it in the frame. But of course there are also scenarios when this approach might not provide the best solution.
More Detail: When you have multiple focus points active, the camera will decide which specific focus point will be used for establishing automatic focus for the scene you are capturing. As a general rule this will be the object closest to the lens that actually falls at one of the focus points that is active.
In many cases, of course, it may be perfectly acceptable (and preferable) to let the camera choose which focus point to use to establish automatic focus. In some cases you might want to enable the full range of focus points, which for many cameras could mean dozens of options. And with the sophisticated autofocus in many cameras, you may find that the camera does a great job of selecting the right focus point for many photos.
That said, I prefer to exercise as much control as possible when it comes to establishing a focus point, especially when depth of field is a concern. For bird photography, for example, I would want to make sure that the focus point is on the eye of the bird if at all possible, which can make working with a single focus point all the more important.
If you know you’ll have more than enough depth of field for a given photographic scenario, I think it is reasonable to let the camera select the focus point for you. But when depth of field is limited, or you are otherwise concerned about exactly where the focus is set, I think using a single focus point makes more sense. And, as noted above, enabling a single focus point also provides you with a “target” to use within the viewfinder for positioning and tracking your key subject.